Volume I, Section 5

5 NATURAL AND INDUCED ENVIRONMENTS

{A} For a description of the notations, see Acceleration Regimes.

This section contains the following topics:

5.1  Atmosphere
5.2  Microgravity
5.3  Acceleration
5.4  Acoustics
5.5  Vibration
5.6  Deleted
5.7  Radiation
5.8  Thermal Environment
5.9  Combined Environmental Effects

See the video clips associated with this section.

5.1 ATMOSPHERE

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5.1.1 Introduction

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This section concerns the appropriate design of spacecraft cabin atmospheres. The atmospheric design considerations subsection includes data on safe atmospheric compositions and pressures, human physiological response to these atmospheric environments, and the effects on humans of atmospheres with undesirable, or unsafe properties. The atmosphere design requirements subsection includes general design goals, atmospheric composition and pressure limits, monitoring and control of the cabin atmosphere, and limits on contaminants and toxins.

Cabin ambient conditions of temperature, humidity, and airflow are covered in Paragraph 5.8, Thermal Environment. EVA pressure suit atmosphere is covered in Paragraph 14.2.2.9, EVA Suit Pressure Design Considerations, and Paragraph 14.2.3.9, EVA Suit Pressure Design Requirements.

5.1.2 Atmosphere Design Considerations

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Crewmembers in the system must be provided with an environment to enable them to survive and function as a system component in space. An artificial atmosphere of suitable composition and pressure is the most immediate need. It supplies the oxygen their blood must absorb and the pressure their body fluids require.

Humans are accustomed to breathing an atmosphere that contains 21% oxygen by volume at sea-level. Figure 5.1.2-1 shows the composition of a sea-level-equivalent atmosphere. Oxygen partial pressure must be maintained above 152 mm Hg (3 psia) for normal functioning of average crewmembers. A crewmember unacclimatized to high altitude cannot survive for extended periods at total pressures lower than 417 mm Hg (8 psia). By breathing pure oxygen, they can survive at a total pressure of about 152 mm Hg (3 psia). At a total pressure of 760 mm Hg (14.7 psia) the oxygen supply is similarly inadequate when the concentration of oxygen is below about 11%. Too little oxygen (hypoxia) induces sleepiness, headache, the inability to perform simple tasks, and loss of consciousness. Too much oxygen (hyperoxia) can also be harmful. Prolonged breathing of pure oxygen at sea level pressure (and perhaps even at lower pressures) can eventually produce inflammation of the lungs, respiratory disturbances, various heart symptoms, blindness, and loss of consciousness.

Figure 5.1.2-2 shows human performance limits versus total pressure and oxygen concentration. Maximum range in total pressure varies from 760 mm Hg (14.7 psia) with an N2-02 mix, to approximately 190 mm Hg (3.6 psia) 100% 02. The lower pressure limits are determined primarily by the requirements for maintaining alveolar oxygen levels. At 190 mm Hg (3.6 psia) total pressure in a 100% oxygen atmosphere, the O2 partial pressure is near normoxic or 103 mm HgPO2 (2.0 psia) in the alveoli (the remainder of the lung being filled with CO2 and water vapor). If pressures less than 190 mm Hg (3.6 psia) are considered, the crew will need pressure suits and 100% oxygen under pressure. Using pressures of less than 760 mm Hg (14.7 psia) may provide protection from bends in the event of decompression (both accidental and intentional) because partial pressure of N2 (pN2) decreases in proportion to barometric pressure (pB). This aspect of atmosphere selection assumes considerable importance if a crewmember must leave the vehicle and perform tasks in space.

Figure 5.1.2-1 Standard Sea - Level Atmosphere

Parameter	Standard Sea-Level Atmosphere Values
	kPa	psia	mmHg
Total Pressure	101.36	14.70	760.0
Oxygen partial pressure	21.37	3.04	151.3
Nitrogen partial pressure	78.60	11.44	591.7
Water vapor partial pressure	1.38	0.2	10.7
CO2 partial pressure	0.04	0.0058	0.3
Note: Water vapor partial pressure at 74 deg. F and 50% relative humidity

Reference: 198, p. 27; NASA-STD-3000 55

Figure 5.1.2-2 Relationship Between Percentage of Oxygen in Atmosphere of Space Vehicle and Total Pressure of that Atmosphere

Reference: 6, Figure 3.9, p. 37; NASA-STD-3000 56

Considering crew needs only, space cabin pressure is sufficient in the range between 190 and 380 mm Hg (3.75 - 7.3 psia). In this range, crewmembers need not wear protective equipment, and as long as the amount of oxygen in the vehicle cabin provides an alveolar partial pressure of O2 (pO2) of at least 103 mm Hg (1.9 psia), the blood will have an oxygen level equivalent to that at sea level.

The use of low ambient pressure and 100% oxygen at the above pressure is attractive to the designer because it saves weight, simplifies engineering and monitoring, and reduces atmospheric gas leak rates. However, there is considerable fire hazard involved in using 100% oxygen in the pressure range of 190 to 380 mm Hg (3.75 - 7.3 psia). At 190 mm Hg (3.75 psia), 100% oxygen is essential to maintain the required alveolar PO2. At 380 mm Hg (7.3 psia), the oxygen can be diluted with an inert gas, such as nitrogen or helium, to about 50% of the total pressure, the inert gas acting as a retardant in case of fire. The burning time is approximately doubled by going from 100% oxygen at 190 mm Hg (2.75 psia) to a nitrogen-diluted atmosphere at 380 mm Hg (7.3 psia).

(Refer to Paragraph 6.6, Fire Hazards, for more details on fire protection and control.)

The current design approach for a manned spacecraft atmosphere specifies a pressure of 760 mm Hg (14.7 psia). This environment offers an advantage in that chemical or biological experiments performed in the cabin will not have the added complication of dealing with a nonstandard pressure or composition.

The following paragraphs give additional details on the atmospheric design consideration factors previously discussed.

5.1.2.1 Safe Atmospheres Design Considerations

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Humans can survive in a wide range of atmospheric compositions and pressures. Atmospheres deemed sufficient for human survival are constrained by the following considerations:

a. There must be sufficient total pressure to prevent the vaporization of body fluids.

b. There must be free oxygen at sufficient partial pressure for adequate respiration.

c. Oxygen partial pressure must not be so great as to induce oxygen toxicity.

d. For long durations (in excess of two weeks) some physiologically inert gas must be provided to prevent atelactasis.

e. All other atmospheric constituents must be physiologically inert or of low enough concentration to preclude toxic effects.

f. The breathing atmosphere composition should have minimal flame/explosive hazard.

More restrictive limits may be applied to atmospheric parameters to ensure crew health. Crew comfort and efficiency may be enhanced by imposing yet tighter constraints.

Paragraphs 5.1.2.1.1, Gas Composition and Pressure Design Considerations, and 5.1.2.1.4, Human Responses to the Diluent Gas Environment Design Considerations, address the selection of atmospheric parameters and discuss crew health and comfort impacts.

5.1.2.1.1 Gas Composition and Pressure Design Considerations

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Figure 5.1.2.1.1-1 shows optimum sea-level atmospheric parameters. Although an Earth-like atmosphere is not the only possibility, it is useful to consider such an atmosphere as a baseline.

The most basic requirements applicable to any spacecraft cabin atmosphere are that it provide:

a. Free oxygen of a suitable partial pressure for metabolic use.

b. A cabin absolute pressure sufficient to prevent vaporization of body fluids (ebullism), which occurs at approximately 45 mm Hg (0.9 psia) at 37° C (95° F).

Earth's atmosphere provides a physiologically inert gas, nitrogen, which comprises 78% of Earth's air by volume. Best candidate atmospheres will contain one or more of the. following physiologically inert diluent gases: nitrogen, helium, neon, argon, krypton, xenon, or hydrogen. The diluent gas can serve several functions:

Figure 5.1.2.1.1-1 Range of Permissible Cabin Atmosphere Absolute Pressure vs. Oxygen Concentration

Reference: 20, pp. 2, 42, 55; 111, p. 863; 198, p. 27; 264, Table 11.2, para. 11.1.1/4; 268, p. 5; NASA-STD-3000 57

1. It can be used to increase cabin total pressure without necessarily increasing oxygen partial pressure - this is important in vehicles that operate in a high absolute pressure ambient, e.g., a diving bell or a hypothetical manned lander on Venus.

2. In the event of a closed pocket of gas occurring in the crewmember's body, collapse of the pocket may occur. This may occur in the middle ear if the middle ear is not periodically ventilated (ear clearing). It can also occur in small segments of the lungs during high stress, since the oxygen and carbon dioxide present in the pocket are absorbed rapidly. A diluent gas added to the mixture will be absorbed more slowly, and will help prevent such a collapse.

3. Experiments, particularly in life sciences, may be sensitive to atmospheric parameters. A choice of Earth normal atmosphere [i.e., 760 mm Hg (14.7 psia) and 79% nitrogen, 21% oxygen, plus minor constituents] would typically provide a better laboratory test environment than pure oxygen. Normal atmosphere would allow use of standard laboratory equipment.

4. Except for hydrogen, the diluent gas(es) in the cabin atmosphere will act as a suppressant in case of fire.

5.1.2.1.2 Gas Pressure Design Considerations

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Figure 5.1.2.1.1-1 plots absolute cabin pressure as a function of percent oxygen in the mixture. Shown in the figure are:

a. Normoxic line which will typically represent the optimum pressure concentration combination.

b. Upper and lower pressure limits imposed by danger of oxygen toxicity and hypoxia, respectively.

c. Upper pressure limits imposed by danger of toxicity of various diluent gases.

Carbon dioxide will be present as a byproduct of respiration. Figure 5.1.2.1.2-1 plots alveolar partial pressure of CO2 against alveolar partial pressure of 02. Shown on the graph are relations between CO2-02 composition and human performance response. The normal 36 mm Hg (0.7 psia) alveolar partial pressure corresponds to approximately 3 mm Hg (0.006 psia) CO2 cabin partial pressure.

5.1.2.1.3 Mission Related Design Considerations

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Various flight regimes may influence the choices a designer makes in selecting an atmosphere. Some of the possible considerations are:

a. Prelaunch - Contamination from ambient atmosphere during boarding may influence pressurization or depressurization schedule. Low pressure cabins may require oxygen prebreathe.

b. Launch - The possibility of oxygen atelectasis during high-g stress, with a 100% O2 atmosphere, suggests including a diluent gas in the mixture. The shallow breathing that may result from high-g loading may dictate a higher oxygen concentration or an increased ventilation rate.

c. Short Flights - Greater ranges of atmospheric parameters (e.g., CO2 partial pressure and pure oxygen atmospheres) may be tolerated in short flights as the detrimental effects of these are time dependent.

d. Long Flights - For longer flights, tolerance to irritating or toxic substances are reduced, trace contaminants become more important, and crew comfort of greater concern.

e. Entry/Landing - Same as launch, plus consideration of ambient conditions if landing in extraterrestrial environment.

f. Post Landing - If on Earth, reintroducing ambient atmosphere on an appropriate schedule may be desirable while waiting for debarkation. If not on Earth, ambient conditions, EVA operations, experiments, etc., may influence atmosphere design.

g. Hyperbaric Treatment - The rate of pressurization during hyperbaric treatment should not result in a differential pressure across a crewmember's chest in excess of 80 mm Hg (1.5 psi), or in excess of 40 mm Hg (0.75 psi) for a period of longer than five seconds. Decompression scheduling and gas composition changes in the chamber depend on atmospheric composition. O₂ toxicity is the main concern in hyperbaric therapy regimes. See Reference 280 for specific protocols.

Figure 5.1.2.1.2-1 Relationship of Alveolar O₂ and CO₂ Composition to Performance

Reference: 29, pp. 2, 42, 55; 111, p. 863; 264, Table 11.2, para. 11.1.1/4; 268, p. 5; NASA-STD-3000 58

5.1.2.1.4 Human Response to the Diluent Gas Environment Design Considerations

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A choice of atmospheric composition that contains a diluent gas other than nitrogen may have associated side effects for a spacecraft crew. Paragraphs 5.1.2.1.4.1 through 5.1.2.1.4.3 discuss metabolic, thermal, and vocal factors that may impact crew performance.

Figure 5.1.2.1.4-1 lists some physical properties of selected inert gases.

Figure 5.1.2.1.4-1 Physical Properties of Inert Gases

Element	Helium	Nitrogen	Neon	Argon	Krypton	Xenon
Symbol	He	N2	Ne	A	Kr	Xe
Atomic number	2	7	10	18	36	54
Molecular weight	4.00	28.00	20.18	39.94	83.80	131.3
Density at 101.3 kPa (1 atm) and 0°C ( 32°F )
Kg/m3	0.1784	1.251	0.9004	1.784	3.708	5.851
(lb/ft3)	(0.011)	(0.078)	(0.056)	(0.111)	(0.231)	(0.365)
Viscosity at 0 deg C ( 32°F ) and 101.3 kPa (1 atm)
Pascal-second	19.4x10-5	17.5x10-5	31.1x10-5	22.2x10-5	25x10-5	22.6x10-5
(Centipoise)	(0.194)	(0.175)	(0.31)	(0.222)	(0.250)	(0.226)
Thermal conductivity at 0°C ( 32°F), 101.3 kPa (1 atm)
Kcal/M.hr °C	0.1252	0.0209	0.0407	0.0145	0.0077	0.0045
(BTU/ft. hr. °F)	(0.0840)	(0.0140)	(0.0273)	(0.0097)	(0.0052)	(0.0030)
Bunsen solubility coefficients:
In water at 38°C	0.0086	0.013	0.0097	0.026	0.045	0.085
In olive oil at 38°C	0.015	0.061	0.019	0.14	0.43	1.7
In human fat at 37°C	?	0.062	0.020	?	0.041	1.6
Oil, water solubility ratio	1.74	4.69	1.95	5.38	9.56	20.0

Reference: 42, Table 2-1, p. 53; NASA-STD-3000 59

5.1.2.1.4.1 Metabolic Factors Design Considerations

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All gases considered for the role of diluent in an atmosphere must be physiologically inert, i.e., relatively little metabolic response to the diluent under normal conditions. At higher pressures, however, the diluent gases exhibit toxic effects. Figure 5.1.2.1.1-1 shows approximate upper pressure limits for use of various diluents. At pressures above these limits, the diluent can act as a depressant. Standard hyperbaric treatment protocols only require total pressures up to 4560 mm Hg (88.2 psia).

5.1.2.1.4.2 Thermal Factors Design Considerations

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With the exception of helium, the diluent gases considered for use in cabin atmospheres do not present difficulties with thermal regulation significantly different from nitrogen.

The thermal conductivity of helium is six times that of nitrogen. For this reason, experience has shown that air temperatures must be maintained at least 2 to 3 C (4 to 5 F) higher than normal for subjects at rest.

Figure 5.1.2.1.4.2-1 shows thermal conductivity for several diluent gas atmospheres against diluent gas concentration.

(Refer to Paragraph 5.8, Thermal Environment, for more details on the atmospheric thermal environment.)

Figure 5.1.2.1.4.2-1 Thermal Conductivity of Binary Gas Mixtures Containing O₂ at 30 deg C

Reference: 92, Figure 2-18, p. 58; NASA-STD-3000 60

5.1.2.1.4.3 Vocal Factors Design Considerations

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The low density of helium-oxygen mixtures induces an increase in the frequencies of the human voice. At high-percentage mixtures of helium, substantial problems with speech intelligibility may be encountered. In these circumstances, partial mixes with nitrogen or neon added to the heliox (helium-oxygen) mixture will be of benefit. Electronic processing has also been used to improve communication clarity.

5.1.2.2 Dangers Associated with Unsafe Atmospheres Design Considerations

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Paragraphs 5.1.2.2.1 through 5.1.2.2.3 discuss the effects on humans of atmospheres that are unsuitable for crew health and comfort. Included are the effects of excess and insufficient O₂ and CO₂ , insufficient and excess total pressure, and contaminants and toxicity.

5.1.2.2.1 Adaptive Physiological Responses Design Considerations

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Adaptive physiological responses to unsafe atmospheres are considered to be those changes that take place in the body physiology to adapt to the outside stimulus, as opposed to nonadaptive responses, that indicate physical damage.

5.1.2.2.1.1 Hypoxia Design Considerations

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The condition of insufficient oxygen to support normal physiological functioning is called hypoxia. The only oxygen stored by the body is found in the hemoglobin in the blood stream, and some amount in the myoglobin of red muscle tissues. Muscles can function temporarily without oxygen but build up toxic fatigue products that limit their activity.

The central nervous system, including the brain and eyes are particularly sensitive to oxygen deficiency, and cannot function without oxygen. Acute impairment of brain function occurs within 13 seconds whenever the alveolar oxygen tension drops below about 33 mm Hg (0.5 psia).

Man can acclimatize to hypoxia (adaptive response) at relatively low altitudes (pressures) but not above 5500 m (18000 ft) or 379 mm Hg (7.34 psia) and normal air composition.

Figure 5.1.2.2.1.1-1 shows atmospheric pressure/ composition combinations where hypoxia is likely to occur. Figure 5.1.2.2.1.1-2 lists physiological effects relative to lack of oxygen.

Figure 5.1.2.2.1.1-1 Hypoxia Danger Zone

Note: The effects of falling oxygen pressure is insidious, as it dulls the brain and prevents realization of danger.

Reference: 111, p. 863; 92, p. 38; NASA-STD-3000 62

Figure 5.1.2.2.1.1-2 Effects of Insufficient Oxygen

Oxygen partial pressure mm Hg (psia)	Effect
160 (3.1)	Normal sea level atmosphere level
137 (2.7)	Accepted limit of alertness. Loss of night vision. Earliest symptoms is dilation of the pupils.
114 (2.2)	Performance seriously impaired. Hallucinations, excitation, apathy.
100 (1.9)	Physical coordination impaired, emotionally upset, paralysis, loss of memory.
84 (1.6)	Eventual irreversible unconsciousness.
0-46 (0-0.89)	Anoxia -- near-immediate unconsciousness, convulsions, paralysis. Death in 90 to 180 seconds.
Note: The effects of falling oxygen pressure is insidious, as it dulls the brain and prevents realization of danger.

Reference: 92, P. 38; 111, p. 863; NASA-STD-3000 64

5.1.2.2.1.2 Night Vision Abnormalities Design Considerations

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The visual functions of a human are particularly sensitive to hypoxia. The retina is the most O₂ sensitive tissue in the body. Figure 5.1.2.2.1.2-1 shows some thresholds of visual determination.

(Refer to Paragraph 4.2, Vision, for other vision design considerations and requirements).

Figure 5.1.2.2.1.2-1 Impairment of Visual Functions Produced by Hypoxia

Reference: 11, pp. 10-50; NASA-STD-3000 63

5.1.2.2.1.3 Oxygen Toxicity (Hyperoxia) Design Considerations

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For the purpose of this document, the condition of oxygen toxicity (hyperoxia) will be considered as associated with oxygen partial pressures between sea level normal, 160 mm Hg (3.1 psia), and the 310 mm Hg (6 psia) limit. Figure 5.1.2.2.1.3-1 shows times to onset of symptoms. As shown in the figure, the symptoms in this region will generally be respiratory. At pressures of oxygen at around 253 mm Hg (5 psia), changes in red blood cell fragility and cell wall permeability have been reported at long periods of exposure.

Figure 5.1.2.2.1.3-1 Approximate Time of Appearance of Hyperoxic Symptoms as a Function of Oxygen Partial Pressure

Reference: 92, Figure 2-10, p. 46; NASA-STD-3000 64

5.1.2.2.1.4 Chronic CO2 Toxicity Design Considerations

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Long-term exposures to CO₂ concentrations in the range of 1-1.5% will generally not produce significant changes in blood pressure, pulse, or temperature (chronic CO₂ toxicity). Such exposures have been noted to produce significant alterations such as respiratory acidosis, increased carbonate retention in bone tissue, increased cortical adrenal activity, and decreased cardiovascular function. No outward apparent symptoms would be expected at this concentration.

At CO₂ concentrations of about 3%, crewmembers will typically exhibit increased motor activity, excitement, euphoria, mental acuity and sleeplessness for about a day, followed by headache, mental depression and cloudiness, decreased memory and attentiveness, and decreased appetite. Typically, after the third day, there will be some return to normal.

Generally, subjects have felt normal after a week of breathing normal air again, and the characteristics have returned to normal after a month.

5.1.2.2.1.5 Acute CO₂ Toxicity Design Considerations

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Figure 5.1.2.2.1.5-1 shows effects of increased CO₂ concentration on respiration volume, rate, and pulse rate. It has been noted that individuals with a relatively large tidal volume and slow respiratory rate show less respiratory and sympathetic nervous system responses while breathing low concentrations of CO₂ .

Figure 5.1.2.2.1.5-2 shows CO₂ partial pressure rate increase, with loss of CO₂ removal function, for space modules of various volumes and configurations. Limits are defined as operational, 90-day degraded, 28-day emergency, and critical. Other effects of increased CO₂ concentration (in the range of 3-7%) are:

a. Reduced body temperature, typically 0.5° - 1.5° C (1° - 3° F).

b. Increased urine production (up to three times normal rate).

c. Reduced aerobic capacity (13% - 15% reduction).

Acute CO₂ toxicity symptoms include dyspnea, fatigue, impaired concentration, dizziness, faintness, flushing and sweating of face, visual disturbances, and headache. Exposure to 10% or greater concentrations can cause nausea, vomiting, chills, visual and auditory hallucinations, burning of the eyes, extreme dyspnea, and loss of consciousness. Without therapeutic support, respiratory depression, convulsion, shock, and death may result from CO₂ concentrations above 10%.

Figure 5.1.2.2.1.5-1 Effects of Increased Carbon Dioxide Inhalation

Note: Subjects at rest, hatched areas represent deviations from the mean.

Reference: 6, p. 41; 11, pp. 10-64; 21, p. 3; NASA-STD-3000 65

Figure 5.1.2.2.1.5-2 Carbon Dioxide Partial Pressure Increase without Carbon Dioxide Removal

Cases Cited
Curve	No. Mod./vol. (1)	Number of crew	Notes:
1	1 smod	8	1 Smod=1 short module=81.3 m3 (2873 ft3)
2	1 smod	4
3	1 lmod	8	1 lmod=1 long module=176.7 m3 (6244 ft3)
4	1/2 spst	8	1 spst=1 space station=548.7 m3 (19,387 ft3)
5	1 lmod	4
6	1/2 spst	4
7	1 spst	8
8	1 spst	4
Assumptions: 1.  1.0 Kb (2.2 lb) CO2/person day metabolic rate 2.  CO2 leakage overboard negligible 3.  Delta configuration volumes   a.  81.3 m3 (2,873 ft3) short module   b. 176.7 m3 (6244 ft3) long module   c.  72.8 m3 (2,572 ft3) logistics module   d.  21.7 m3 (768 ft3) tunnel   e.  8.4 m3 (296 ft3) interface module   f.  548.7 m3 (19,387 ft3) space station 4.  Equipment volume equals 10% of volume 5.  3 mm (0.58 psia) atmosphere pressure

Reference: 322; NASA-STD-3000 66

5.1.2.2.1.6 CO2 Withdrawal Design Considerations

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CO₂ withdrawal symptoms can be experienced after the cessation of certain exposures to CO₂ and can result in even greater functional impairment than the exposure itself. Headaches of varying severity are common. Withdrawal from more acute exposures may cause dizziness. Symptoms are more marked during acute exposures to 5-10% CO₂ than during chronic exposures to CO₂ concentrations below 3%. In the extreme case, profound hypertension and grave cardiac arrhythmias may occur. It has been observed that subjects recover from CO₂ exposure better when breathing oxygen than when breathing air.

5.1.2.2.1.7 Dysbarism Sickness Design Considerations

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Any diluent gas present in the cabin atmosphere will establish an equilibrium concentration in body tissues. If the ambient pressure is lessened, a certain amount of the diluent will come out of solution. If the pressure change (dysbarism) takes place slowly enough, the diluent can be transported away normally by the bloodstream. However, if the pressure change is rapid enough through a large differential, the diluent may come out of solution rapidly and form gas bubbles. A major consequence of this phenomenon is known as decompression sickness.

5.1.2.2.1.7.1 Evolved Gas Dysbarism Design Considerations

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The varied symptoms and pathological physiology of the effects of dysbarism can be divided into several categories under decompression sickness: bends, chokes, skin manifestations, circulatory collapse, and neurological disorders. The relative incidence of the different symptoms varies with the type and partial pressure of the gas at equilibrium, the level of exercise, and final pressure. Figure 5.1.2.2.1.7.1-1 shows the percent of exposed subjects per minute experiencing new symptoms (bends of grade 2 or > and chokes) at given times after exposure to 38,000 ft. at rest from previous sea level conditions with no preheating. The curve is thought to reflect the size history of a typical gas bubble in the sensitive tissue for these specific conditions.

Tests with EVA representative decompression, work rates, and prebreathe show a considerably delayed incidence of decompression sickness with symptoms occurring as late as the sixth hour in a 6-hour decompression.

Bends, the most common symptom, is manifested by pain in the locomotor system. This pain usually begins in the tissue around joints and extends distally along the bone shaft. Pain tends to occur in joints that are being flexed. The pain is deep and poorly localized with periods of waxing and waning. Relief is obtained by relaxation of the part of application of external pressures to the overlaying tissues. Symptoms may spontaneously disappear.

The next most common symptom complex is chokes. Chokes refer to a syndrome of chest pain, cough, and respiratory distress. It usually requires longer altitude exposure than that required for bends. It commences with a burning pain under the breast bone during deep inspiration which is relieved by shallow breathing and gradually becomes more severe and constant. Paroxysms of coughing become more frequent and are followed by cyanosis, anxiety, syncope, and shock.

Figure 5.1.2.2.1.7.1-1 Typical Time Course of Appearance of New Symptoms of Decompression Sickness in Absence of Prebreathing

Reference: 11, p. 12-3; NASA-STD-3000 67

Skin lesions, causing itching and a red blotchy rash, usually occur only after prolonged altitude exposure and are associated with, or presage, more serious manifestations of decompression sickness. About 10% of those cases going on to neurocirculatory collapse present previous skin changes. It appears that passage of emboli to the skin is the most probable mechanism.

Cardiovascular symptoms are varied: fainting, low blood pressure, coronary occlusions, heart arrhythmias, and shock have all been seen. Severe and progressive peripheral vascular collapse may develop after one to five hours at altitude. This reaction may, or may not have been preceded by fainting. Signs and symptoms of shock with or without neurological findings are seen. Delirium and coma are more common when neurological findings are present. All fatalities following altitude exposure are preceded by this picture of delayed shock. It usually develops in subjects who have experienced severe chokes, but may be preceded by few or no symptoms. The types of neurologic symptoms run the gamut of almost every acute neurologic disorder. Confusion, visual impairment, and headaches are the most common.

The precise conditions under which a particular individual will develop symptoms of decompression sickness are impossible to predict. In general, for an atmosphere using nitrogen as a diluent, if the supersaturation ratio R = pN2/pB exceeds about 1.22, there will be a risk of decompression sickness. (In the above equation, pN2 is the nitrogen tension in the subject tissue, and pB is the total barometric pressure). For other diluent gases, the critical R value will be different. The tissue tension of the diluent gas at any particular time will depend on the initial and final equilibrium tensions, the solubility of the diluent in the subject tissues, and the rate and duration of decompression. As an example, hyperbaric decompression sickness (DCS) from a normal sea-level atmosphere will occur typically only after decompression below 490 mm Hg (9.5 psia). This onset of DCS pressure will vary some depending on the susceptibility of an individual as influenced by any of the following factors:

a. Body Build - Obesity increases susceptibility to decompression sickness. It is less clear if the percent of body fat within a normal range affects the incidence of decompression sickness.

b. Temperature - Very cold conditions can increase the incidence of decompression sickness significantly.

c. Previous Exposures to Low Pressure - Repeated exposures to hyperbaric conditions may, or may not increase susceptibility to decompression sickness depending on the nature of the exposure. When EVA-type exposures have been repeated over a three-day period, there was no increase in susceptibility.

d. Barometric Compression Prior to Decompression - Any exposure to compressed air breathing occurring less than 24 hours prior to decompression will increase susceptibility.

e. Age - A threefold increase in incidence of decompression sickness has been observed in going from 19 to 25-year old to 40 to 45-year old age groups. In other studies, age has not been a factor for the crewmember population used in these studies.

f. Sex - Valid, conclusive studies are lacking, but it appears that women present a greater risk to decompression sickness than men do.

g. Exercise - Physical exertion can increase the incidence of decompression sickness by up to 40%.

h. Injury - Perfusional changes in an injured area particularly in a joint, may create an increased susceptibility to decompression sickness.

Decompression sickness can be prevented by denitrogenation prior to decompression. This is accomplished by breathing pure oxygen, which reduces alveolar nitrogen pressure and allows nitrogen to come out of the tissues of the body. Figure 5.1.2.2.1.7.1-2 shows nitrogen eliminated over time while breathing pure oxygen.

Figure 5.1.2.2.1.7.1-2 Nitrogen Elimination Curve

Reference: 78, p. 1-292; NASA-STD-3000 68

Ear problems associated with change in barometric pressure are tabulated in Figure 5.1.2.2.1.7.2-1.

Figure 5.1.2.2.1.7.2-1 Type of ear Problems Encountered During Change in Barometric Pressure

Differential pressure mmHg (psia)	Symptom
0 (0)	No sensation
3-5 (.06-0.10)	Feeling of fullness in ears
10-15 (0.19-0.29)	More fullness, lessened sound intensity
15-30 (0.19-0.29)	Fullness, discomfort, tinnitus in ears: ears pop as air leaves middle ear; desire to clear ears - if accomplished, symptoms stop
30-60 (0.58-1.16)	Increasing pain, tinnitus, and dizziness and nausea
60-80 (1.16-1.54)	Severe and radiating pain, dizziness and nausea
~100 (~1.93)	Voluntary ear clearing becomes difficult or impossible.
200+ (3.87+)	Eardrum ruptures

Reference: 92, Table 1-3, p. 14;NASA-STD-3000 70

5.1.2.2.1.7.2 Trapped Gas Dysbarism Design Considerations

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If the glottis is held closed during a decompression, or if the air passageway to the lungs is restricted or blocked (e.g., when using an oxygen mask) it is possible for the trapped gas in the alveoli to expand the alveoli past their elastic limit. A differential pressure between the alveoli and the ambient of 50 to 100 mm Hg (1.0 to 2.0 psi) may be sufficient to force gas into extra-alveolar space. Such an accident will likely result in arterial gas embolism, mediastinal and subcutaneous emphysema, and/or pneumothorax.

The effects of gas embolism are similar to those of decompression sickness, but occur immediately in contrast to the delay observed in decompression sickness, and the signs may clear rapidly, leaving a clinical picture similar to stroke.

Mediastinal emphysema, subcutaneous emphysema, and pneumothorax refer to conditions of accumulations of gas in the mediastinal cavity between the pleurae, under the skin, and in the pleurae, respectively.

Typically, equalization of pressure between air pockets in the body and the ambient will take place without conscious effort, except when there is a blockage in the air passages, as discussed above, or if the rate of depressurization is such that equalization cannot take place rapidly enough to produce effects. If this occurs, the most likely injuries will occur to the ear and sinuses, which have the smallest and most easily blocked passages to the ambient.

Occasionally, toothaches are reported due to changes in barometric pressure, usually occurring in teeth that are filled or have cavities, thereby allowing an air bubble to be trapped within.

5.1.2.2.1.7.3 Toxic Gaseous Contaminants Design Considerations

{A}

The effects of carbon monoxide (CO) toxicity and blood levels of carboxyhemoglobin after exposure to different atmospheric concentrations of carbon monoxide are shown in Figure 5.1.2.2.1.7.3-1. Carbon monoxide is particularly dangerous because it is odorless and colorless and symptoms of toxicity are not readily noticeable. CO is produced by crew metabolism, materials offgassing, and materials thermodegradation or combustion. Normally, the onboard ECLSS ambient temperature catalytic oxidizer (ATCO) does a good job of keeping CO at safe, low levels. CO would be a toxicological concern, however, if the platinum catalyst in the ATCO system were to be poisoned by another chemical or if a fire or smoldering combustion on board produced more carbon monoxide that the ATCO system could handle.

Figure 5.1.2.2.1.7.3-1 Effects of Carbon Monoxide Exposure

Atmospheric carbon monoxide, volumes per million	Carbon monoxide in blood, % Carboxyhemoglobin	Effects
0 - 60	0-10	None subjectively noticeable, but initial visual and psychomotor impairment is revealed in objective tests.
60 -120	10 - 20	Tightness across forehead, slight headache, flushed complexion.
120 - 180	20 - 30	Headache with throbbing in temples, breathlessness from any exertion.
180 - 240	30 - 40	Severe headache, weakness, dizziness, dimness of vision, nausea, and vomiting with possibility of collapse.
240 - 300	40 - 50	All preceding symptoms with increased pulse rate and respiration and greater possibility of collapse.
300 - 360	50 - 60	Loss of consciousness, with increased or irregular respiration, rapid pulse, and possibility of coma with convulsions.
360 - 480	60 - 80	Coma, convulsions, depressed heart action, respiratory failure, and possibility of death.

Reference: 111, p. 865; NASA-STD-3000 71

Ozone may possibly be produced by electric motors, welding, or ultraviolet light rays in the onboard lighting system. It is especially prone to be produced if there is electric arcing. Exposure to ozone at concentrations of 0.3 to 0.8 ppm causes irritation of the nose and bronchi. Exposure to 0.94 ppm also causes sleepiness and headache. Exposure to 1.5 ppm has been described as intolerable. The lethal concentration in 50% of rats and mice exposed (the LC50) is around 6 ppm. The animals died of shock with edema and hemorrhage in the lungs and bronchi. Higher animals, such as dogs and monkeys, appear to be somewhat more resistant. Persons who have been continuously exposed to ozone develop some tolerance to its effects.

Materials of construction are a source of gaseous contamination due to offgassing to the cabin atmosphere. Offgassing requirements and test procedures are detailed in Reference 24. Compounds for which seven-day maximum allowable concentrations have been established in manned spacecraft are also listed in Reference 24.

5.1.2.3 Atmosphere Monitoring Design Considerations

{A}

5.1.2.3.1 Atmosphere Toxicological Monitoring Design Considerations

{A}

Long tours of duty for crewmembers, closed Environmental Control Life Support Systems (ECLSS), and the possibility for potentially hazardous internal operations make the following three types of analyzers necessary for the toxicological monitoring of spacecraft air during long-duration manned space flights:

a. Regular Monitoring - The continuous generation of contaminants by the offgassing of nonmetallic materials in the vehicle and the potential for chemical spills or leaks necessitate regular monitoring for volatile organics.

b. Compound-Specific Analyzer - Other potential contaminants, such as low volatility organics and inorganics, metals, acid gases, and other gases such as carbon monoxide and nitrous oxide, may require dedicated, compound specific analyzers.

c. Particulate Monitoring - Based on Shuttle program experience, problems with airborne particulate matter can be anticipated and warrant an inflight particulate monitoring capability.

5.1.2.3.2 Atmosphere Microbiological Monitoring Design Considerations

{A}

Epidemiological principles and previous spaceflight studies indicate a high probability of cross-contamination among crewmembers and between crewmembers and space module during long confinements. The ability to monitor, identify, and characterize the microbial flora of the space module is essential due to continual habitation, relatively crowded conditions, and possible altered host/microorganism interactions. The following three design considerations are of particular importance for microbiological monitoring of space module air.

a. Traditionally, ECLSS systems (because of design constraints) have a limited capacity for removing biological agents from the air.

b. The unique properties of microgravity affect the distribution of microbial agents in the space module environment. On Earth, gravity is an important physical force in reducing the presence of aerosols in the air and thus, helps contain the spread of some infectious diseases. While large particles and droplets containing microorganisms are removed from the air in minutes in a 1-G environment, these aerosols may remain suspended indefinitely in microgravity.

c. The immunological status of crewmembers may be compromised due to physiological effects associated with stress and long periods of habitation in a microgravity environment.

5.1.2.3.3 Baro-Thermal Monitoring Design Considerations

{A}

The collection and analysis of atmospheric data (including barometric compositional, and thermal balance information) within the space habitat environmental system are necessary for monitoring human and environmental interactions. The environments to be monitored include those in the habitation modules, the airlocks, and the space suits. This information will be used to characterize the crew's environment relative to the environmental limits established to ensure crew health and safety. The derived information will form a database for analysis of any effect of these controlled environment factors on physiological responses of the crew to the microgravity environment.

The sensors required to monitor the environmental parameters described above are in large components of the cabin and suit environmental control system and in most uses will not require development of separate sensing systems.

5.1.3 Long Term Mission Atmosphere Design Requirements

{A}

Paragraphs 5.1.3.1 through 5.1.3.5 present requirements which are directly applicable to the design of respirable atmospheres for space module cabins for long term missions. These requirements may not be entirely applicable to short term space systems (e.g.; the STS Program or the Spaceplane. Included are atmosphere composition and pressure, monitoring and control of atmospheric parameters, and contaminants and toxicity.

5.1.3.1 Atmosphere Composition and Pressure Design Requirements

{A}

The following design considerations shall apply to the composition and pressure of the space module cabin atmosphere:

a. Internal Environment - An internal environment shall be provided adequate to support and maintain crew comfort, convenience, health, and well being throughout all operational phases in accordance with the requirements given in Figure 5.1.3.1-1. Concentrations of atmospheric contaminants in habitable areas of the space module shall not exceed the spacecraft maximum allowable concentrations (SMACs) as specified for various exposure periods in Figure 5.1.3.1-1 Spacecraft Maximum Allowable Concentrations. If no FCSIS SMAC is currently documented for a particular compound of interest, the 7 day SMAC values specified in NHB 8060.1B Appendix D apply. The SMAC values documented in the FCSIS Vol, I, Rev. A, take precedence over the SMAC values listed in NHB 8060.1B. Because ambient pressures on SSMB will be less than 14.7 psi, SMAC values in mg/m3 rather than ppm shall apply.

(Refer to Paragraph 5.8.3.1, Temperature, Humidity, and Ventilation Design Requirements, for other atmosphere related design requirements.)

Figure 5.1.3.1-1 Requirements for Space Module Respirable Atmosphere

(a) Respirable Atmosphere Requirements (Customary units)

Parameter	Units	Operational	90-day degraded (1)	28-day emergency
CO2 partial press	mmHg	3.0 max	7.6 max	12 max
Temperature (7)	deg. F	65 - 80	65 - 80	60-85
Dew point (2)	deg. F	40 - 60	35 - 70	35-70
Ventilation	ft/min	15 - 40	10 -100	10-200
O2 partial pressure (3)	psia	2.83 - 3.35	2.4 - 3.45	2.3 - 3.45
Total pressure	psia	14.5 - 14.9	14.5 - 14.9	14.5 - 14.9
Diluent gas		N2	N2	N2
Trace contaminants (6)	ppm	TBD	TBD	TBD
Micro-organisms	CFU/m3 (4)	500 (5)	750 (5)	1000 (5)
Particulates > 0.5 micron	counts/ft3	100,000 max	TBD	TBD

(b) Respirable Atmosphere Requirements (Sl Units)

Parameter	Units	Operational	90-day degraded (1)	28-day emergency
CO2 partial press	N/m2	400 max	1013 max	1600 max
Temperature (7)	deg. K	291.5-299.9	288.8-302.6	288.8-305.4
Dew point (2)	deg. K	277.6-288.7	273.9-294.3	273.9-294.3
Ventilation	m/sec	.076-203	.051-508	.050-1.016
O2 partial pressure (3)	kP2	19.5-23.1	16.5-23.8	15.9 -23.8
Total pressure	kP2	100-101.4	100-101.4	100-101.4
Diluent gas		N2	N2	N2
Trace contaminants (6)	mg/m3	TBD	TBD	TBD
Micro-organisms	CFU/m3 (4)	500 (5)	750 (5)	1000 (5)
Particulates > 0.5 micron	counts/m3	3,530,000 max	TBD	TBD

Notes:

(1) Degraded levels meet fail operational criteria.

(2) Relative humidity shall not exceed 70% in the operational mode or 75% in the degraded mode or 75% in the degraded or emergency mode and shall not be less than 25%.

(3) In no case shall the O2 partial pressure below 15.9 kP2 (2.3psia) or the O2 concentration exceed 23.8 percent of the total pressure at 14.7 psia.

(4) CFU - Colony Forming Units.

(5) These values reflect a limited base. No widely sanctioned standards are available.

(6) Will be based on NHB 8060.1B, (J8400003).

(7) In the operational mode temperature will be selectable to 1.1 °C ( 2 °F) throughout the range.

Reference: 37, Figure 20101-A, B; 92, pp. 2, 42, 55; 111, p. 863; 198, p. 27; 264, Table 11.2, Paragraph 11.1.1/4; 268, p. 5; 278 p. 321, Table 2.4-1; 323, Table C-4-lX, p. C-4-45; 324, Table 2-9; NASA-STD-3000 72

b. Atmospheric Revitalization - An atmospheric revitalization system shall continuously regenerate the module atmosphere as required to provide a safe and habitable environment for the crew. This system will be referred to as the Environmental Control Life Support System (ECLSS).

c. Atmosphere Control and Supply:

1. Atmospheric pressure and composition control functions shall provide a method of regulating and monitoring the total pressure and the major constituent partial pressures of gases in the module atmospheres.

2. The total pressure of the module shall be maintained at the pressure levels defined in Figure 5.1.3.1-1.

3. The controls shall be provided and shall be operable by a crewmember in a shirt sleeve environment or by a remote operator. For specific requirements for pressure suit operations for repressurization see Paragraph 9.2.3.2.1 b.

4. Normally, the controls shall operate autonomously with limited or no crew intervention necessary.

d. ECLSS Design Requirements :

1. The systems of the ECLSS will provide atmospheric pressure and composition control, module temperature and humidity control, atmospheric revitalization, water management, EVA support, and fire and contamination monitoring and control.

2. The ECLSS shall accommodate whatever phased evolutionary growth is anticipated for the space module.

3. The ECLSS shall embody regenerative concepts to minimize the use of expendables.

e. Hyperbaric Treatment - Where altitude decompression sickness may occur as a result of operational activity or contingency operation, access to a hyperbaric treatment facility is required.

5.1.3.2 Atmosphere Monitoring Design Requirements

{A}

Atmospheric monitoring instruments shall require as little crew time as possible for operation and maintenance .

5.1.3.3 Atmosphere Toxicology Monitoring Design Requirements

{A}

Design requirements for monitoring of volatile organics, airborne particulate matter, and compound-specific monitoring are as follows:

a. Monitoring Volatile Organics - The monitoring of volatile organics shall be accomplished.

1. Total hydrocarbon analyzers shall be used to monitor the overall organic concentration in the air of all habitable areas. These analyzers shall provide real time indication of total organic contamination in the air. These monitors shall be equipped with audible and visible alarms to alert crewmembers when contaminant concentrations exceed maximum acceptable levels.

2. An additional monitor shall be available to identify and quantify target organic compounds and take measurements at regularly scheduled intervals.

b. Compound-Specific Monitoring :

1. Compound specific monitors shall be located near equipment, chemical operations, and processing activities which are potential sources of chemical contamination of the space module. These monitors shall be used to monitor for specific chemical contaminants in the air. These analyzers shall have continuous real-time monitoring capabilities.

2. These monitors shall be equipped with audible and visible alarms to alert crewmembers when concentrations exceed maximum acceptable levels.

c. Particle Monitor - A monitor shall be provided to determine nonspecific particulate loading in the air on a real-time basis. Respirable particles in the 0.5-100 micron range per unit volume of air shall be measured.

5.1.3.4 Atmosphere Microbiological Monitoring & Control Design Considerations

{A}

The microbiological monitoring and control design considerations are as follows.

a. Limits - The limits given in Figure 5.1.3.4-1 shall be observed.

Figure 5.1.3.4-1 Atmosphere Microbiological Limits and Monitoring Requirements

Sample source	Monitoring Requirements		Postflight	Acceptability Limit
	Preflight	Inflight
Air	Prior to closeout	Weekly intervals and within 12 hrs of crew exchange	-	Levels of airborne microorganisms may not exceed 500 CFU per cubic meter
Notes: CFU - Colony forming units

Reference: 278, Figure C-2-6; NASA-STD-3000 73 with Updates

b. Air Sampler - Monitoring of the air will be conducted. An air sampler shall monitor air throughout the habitable areas of the facility. It shall be capable of monitoring for the presence of bacteria, yeast and molds.

5.1.3.4.1 Microbial Decontamination Design Requirements

{A}

Decontamination is required when acceptability limits are exceeded or sensory factors indicate microbial contamination has occurred. Decontamination procedures, antimicrobial agents, and supporting equipment shall be provided to counteract and control all contamination events.

5.1.3.4.2 Verification Design Requirements

{A}

Following decontamination measures, the previously contaminated entity shall be re-tested and verified to be within acceptability limits.

5.1.3.4.3 Cross Contamination Design Requirements

{A}

The following cross contamination design considerations shall apply:

a. Bioisolation Facilities - All procedures involving the maintenance and experimentation activities of biological specimens will be conducted in bioisolation facilities that prevent microbial (bacteria, fungi, and parasites) cross- contamination between crewmembers and biological specimens.

b. Pathogen Free Animals - All experimental animals to be utilized aboard the space module shall meet the specific pathogen free criteria established for the applicable project.

5.1.3.5 Baro-Thermal Monitoring Design Requirements

{A}

To insure that all environmental control systems are functioning properly, the Environmental Monitoring System shall monitor and record atmospheric parameters from each habitable element.

5.2 MICROGRAVITY

{O}

5.2.1 Introduction

{O}

This section provides a short description of the design considerations for the microgravity environment. The physiological effects of microgravity are stressed. More detailed data related to microgravity are found in Paragraph 3.3.4, Neutral Body Posture; Section 4.0, Human Performance Capabilities; Paragraph 5.3, Acceleration; Section 8.0, Architecture; Section 9.0, Workstations; and Section 11.0, Hardware and Equipment.

The term microgravity denotes the acceleration regime commonly referred to as weightlessness, zero gravity, or null gravity. It is almost impossible to achieve a pure zero-g environment due to the orbital mechanics. The only place on a spacecraft (in a stable orbit) that is at zero-g is at the spacecraft's center of mass.

For the purposes of man-system integration, any acceleration level below 1E-4 G's is, for all practical purposes, zero-gravity, or weightlessness, or microgravity. The physiological and performance effects are the same at any acceleration level at, or below, this approximate g-level.

5.2.2 Microgravity and Its Counterparts Design Considerations

{O}

This section addresses the physiological effects and the changes in eating, sleeping, and mobility design considerations associated with the microgravity environment.

5.2.2.1 Physiological Effects of Microgravity

{O}

This section addresses some of the physiological effects of microgravity. There is much more detail available on each of the selected topics that can be found in the references cited for this paragraph (refer to Appendix B in Volume 2).

Duration of microgravity exposure is a major factor in determining the detrimental effects described below. The longest US mission to date was the Skylab 4 mission of 84 days. There is a significant body of literature on the biomedical effects observed on this and the other Skylab missions. The Soviets have had crewmembers on orbit for a maximum of over 300 days. The biomedical data from these long- duration Soviet missions have not been widely published.

In general, it takes the body about three days to adjust to the microgravity environment. Most crewmembers become accustomed to working and living in space within a few hours and their performance improves throughout the mission. Most of the adverse biomedical effects are reversed within a matter of hours to weeks following return to Earth.

a. Calcium Loss - One of the biggest concerns during long-term microgravity exposure is the calcium loss from the bones. During the Skylab missions, this loss was not excessive. The Soviets indicate that the rate of calcium loss slows after four or five months. Calcium loss (which is similar to osteoporosis and is referred to as bone mass loss or bone demineralization) will limit the length of time crewmembers can remain in microgravity. At this time dietary mineral supplements are not known to be effective in preventing bone mass loss.

b. Fluid Shifts, Skeletal Changes, and Muscle Mass Loss - Other physiological effects are due to fluid shifts and decompression of the spine. The muscle mass of the lower body and, in particular the calves, becomes smaller due to disuse atrophy. Exercise can help reduce this tendency.

The body length increases due to spinal lengthening and straightening. The discs between the vertebrae expand (similar to what happens when sleeping) but do not recompress because of the lack of gravitational compression forces. There is also an upward shift of the internal organs causing a reduced waist measurement. These considerations should be taken into account when sizing space clothing.

There is a microgravity neutral body posture that results from a balancing of muscular forces acting on the various body joints in the weightless environment (see Figure 3.3.4.3-1). This neutral body posture causes some peculiar performance effects. For example, it is difficult to work at waist level as is done on Earth as the arms must be continually forced down to the waist level to do work at the table top level. It is also difficult to bend forward as this requires significant effort by the abdominal muscles. It is difficult to stand erect or sit in an upright (1-G) manner. Putting on shoes and socks becomes a significant chore if tried to be done as it is on Earth.

Fluid shifts occur that redistribute body fluids toward the upper body. This is due to the lack of gravity effects that normally distribute the fluids toward the lower body. The most visible effect of fluid shift is seen in the face and neck. The face becomes swollen and the veins in the forehead and neck appear distended.

c. Vestibular Alterations - Another system adversely affected by the microgravity environment is the vestibular complex. Two categories of vestibular side effects result from microgravity. One category includes a variety of vestibular reflex phenomena such as postural and movement illusions, vertigo, and dizziness. The second category is space motion sickness. These two categories of response are believed to be closely tied; motion sickness often follows vertigo and postural illusions. There is evidence to suggest that as vestibular reflex phenomena disappear with adaptation, the rise of motion sickness subsides.

Conflicting stimulation of the visual, vestibular, and proprioceptive systems can produce deficiencies in sensory-motor coordination, including control of posture.

Space motion sickness exhibits symptoms resembling Earth motion sickness. These symptoms range from stomach awareness and nausea to repeated vomiting. Symptoms also include pallor and sweating.

Space sickness has been a recurring problem in the history of manned space flight. While this syndrome appears to decline within three to five days, in some cases, the degree of illness has hindered work capacity and disrupted the scheduling of important mission activities. Nine of the 25 Apollo astronauts suffered some degree of sickness, while five of the nine Skylab crewmembers experienced symptoms. Soviet cosmonauts have reported similar experiences. There were suggestions that the 1971 Soyuz 10 flight may have been prematurely ended due to space motion sickness.

Errors in interpreting the visual environment can occur during space motion sickness. Errors in the perception of lights are common. Fatigue may cause a loss of binocular vision. Movement illusions are marked by perceived rotation or by changes in perceived linear acceleration.

(Refer to Paragraph 4.5, Vestibular System, for detailed discussion of the effects of microgravity on the vestibular system; this includes more information on space motion sickness.)

(Refer to Paragraph 4.6, Kinesthesia, for a discussion of the effects of microgravity on kinesthesia.)

(Refer to Paragraph 4.8, Motor Skills, for a discussion of the effects of microgravity on coordination.)

5.2.2.2 Sleeping, Eating, and Mobility Changes in Microgravity

{O}

In the microgravity environment, sleeping, eating and mobility are all affected in some measure.

a. Sleep - It is difficult to isolate weightlessness as a factor influencing the quality of sleep. Sleep disturbances have been common during space flight, but these appear to be much more profoundly affected by operational factors (thruster firings, fan noise, crew mobility) than by the microgravity environment alone. (Refer to Paragraph 7.2.4, Sleep, for more detailed discussion.)

b. Eating in Weightlessness - Space diets have consisted of freeze-dehydrated, intermediate moisture, thermo stabilized, and limited irradiated food. The freeze-dehydrated food is reconstituted inflight with either hot or cold water. Drinks and snacks are also provided. Initially, these foods were eaten through tubes incorporated into the food container, but it has been found that eating foods can be accomplished using conventional utensils. As food technology has improved, the food and food service utensils have approached the traditional practices on Earth.

(Refer to Paragraph 7.2.2, Nutrition, for details on nutrition.)

(Refer to Paragraph 10.5, Galley and Wardroom, for galley design considerations and requirements.)

c. Mobility - The absence of gravity has been found to be a bonus for locomotion in space. Once accustomed to movement in microgravity, mobility is accomplished with minimal effort. Acrobatic maneuvers, such as rolling, tumbling, and spinning, are done with ease.

(Refer to Paragraph 11.8 Mobility Aids, for mobility design considerations and requirements.)

5.2.3 Microgravity Design Requirements

{O}

The microgravity design requirements are given in other paragraphs where the applicable acceleration regime has been coded {O} (O = orbital). Refer to Appendix G in Volume 2 for a complete listing of all paragraph's acceleration regime applicabilities.

(Refer to Paragraph 1.4.3.3, Acceleration Regime Applicability, for an explanation of the acceleration regime coding used in this document.)

5.3 ACCELERATION

{A}

5.3.1 Introduction

{A}

This section addresses the design considerations and requirements for linear, rotational, and impact accelerations.

The documents and database include a coding for each paragraph that designates which acceleration regime(s) are applicable to the data.

(Refer to Paragraph 1.4.3.3, Acceleration Regimes Applicability, for an explanation of this coding.)

Figure 5.3.1-1 shows the coordinate system nomenclature that is used in this document. This system is based on the direction a body organ (e.g., the heart) would be displaced by acceleration. Table II in this figure (and in particular, system 4, which is based on displacement of body fluids) explains the most commonly employed terms.

Figure 5.3.1-1 Acceleration Environment Coordinate System Used in NASA-STD-3000

Reference: 101, p. 9; NASA-STD-3000 436

Figure 5.3.1-1 Acceleration Environment Coordinate System Used in NASA-STD-3000 (cont)

LINEAR MOTION	Direction of Acceleration		Inertial Resultant of Body Acceleration
	Acting Force	Acceleration Description	Reaction Force	Verticular Description
Forward	+ax	Forward accel.	+Gx	Eye Balls In
Backward	-ax	Backward accel.	-Gx	Eye Balls Out
Upward	-az	Headward accel.	-Gz	Eye Balls Down
Downward	+az	Foorward accel.	+Gz	Eye Balls Up
To Right	+ay	R. Lateral accel.	+Gy	Eye Balls Left
To Left	-ay	L. Lateral accel.	-Gy	Eye Balls Right
ANGULAR MOTION
Roll Right	+p		-Rx	Cartwheel
Roll Left	-p		+Rx
Pitch Up	+q		-Ry	Somersault
Pitch Down	-q		+Ry
Yaw Right	+r		+Rz	Pirouette
Yaw Left	-r		-Rz
Footnotes: Large letter, G, used as unit to express inertial resultant to whole body acceleration in multiples of the magnitude of the acceleration of gravity. Acceleration of gravity, go, = 980,665 cm/sec2 or 32.1739 ft/sec2.

Reference: 380 With Updates; NASA-STD-3000 435

(Refer to Paragraph 5.5, Vibrations, for the related acceleration environment of vibrations.)

(Refer to Paragraph 4.5, Vestibular System, for a description of the human vestibular system that is pertinent to discussion of acceleration environments.)

(Refer to Paragraph 5.2, Microgravity, for the special considerations and requirements for the microgravity environment.)

5.3.2 Acceleration Design Considerations

{A}

This section describes the acceleration environments and the human responses to these environments.

5.3.2.1 Acceleration Environments

{A}

This section contains the descriptions of the linear, rotational, and impact acceleration environments that can be encountered during space vehicle operations of launch, on-orbit, transorbit, planetary, entry, and aborts.

5.3.2.1.1 Linear Acceleration Environments

{A}

For space systems, sustained linear acceleration environments include the following:

a. Low Accelerations Experienced in Transorbital Flight - Approximately 10-6 to 10-3 G's (omnidirectional)

b. Low G-Levels Found on the Moon and Mars - Approximately 0.17 to 0.4 G's.

c. 1-G Level On Earth.

d. Multi-g's Experienced During Launch, Entry, and Abort Operations:

1. Approximately 1 to 6 +Gx during launch and entry (pre-Shuttle).

2. Approximately 1 to 2 +Gx during stage separation.

(Space Shuttle range is 1 to 3 +Gx during launch with a 4+Gx spike at booster ignition and 1/2 +Gx during separation maneuvers.)

5.3.2.1.2 Rotational Acceleration Environments

{A}

In the space environments, rotational accelerations are encountered during one of the following flight events:

a. Orbital Maneuvers - Approximately ± 0.8 to ± 1.46 deg/sec2 (omnidirectional).

b. Launch/Entry/Abort Maneuvers - Approximately ± 10 deg/sec2 (omnidirectional).

5.3.2.1.3 Impact Acceleration Environments

{A}

Impact accelerations are abrupt onset, short duration, high magnitude acceleration/deceleration events. It is generally considered that impact involves the occurrence of forces of less than one second duration. Some impact conditions to which space crewmembers may be exposed include: thruster firing, ejection seat/ejection capsule firings, escape device deployment, flight instability, air turbulence, and crash landings:

Aircraft ejection seat firings - up to 17 +Gz

Crash landings - from 10 to greater than 100 G's (omnidirectional)

Orbiter crew compartment design loads for crash landings are 20 Gx and 10 +Gz.

Violent maneuvers - approx. 2-6 G's (omnidirectional).

Parachute opening shock - approx. 10 +Gz.

5.3.2.2 Human Responses to Linear Acceleration

{A}

This section describes the human responses to linear accelerations. This includes the factors that affect human tolerance to linear accelerations and the general and specific human responses.

5.3.2.2.1 Factors Affecting Human Acceleration Tolerance

{A}

Linear acceleration tolerance depends on many factors. The following is a brief summary:

a. Magnitude of the applied force.

b. Duration of the applied force.

c. Rate of onset and decline of the applied force.

d. Direction of the g vector.

e. Types of g-protection devices and body restraints.

f. The coupling between the crewmember and the vehicle via seats, couches, etc.

g. Body positioning, including the specific back, head and leg angles.

h. Environmental conditions such as temperature and lighting.

i. Age of the crewmember.

j. Emotional/motivational factors such as competitive attitude, fear, anxiety, self-confidence, confidence in equipment, and willingness to tolerate discomfort and pain.

k. Previous acceleration training, techniques of breathing, straining, and muscular control.

l. Human Physical condition.

m. Extent of Microgravity Adaptation and Body Fluid Shift.

n. Dietary Habits, particularly with respect to the quantities of Fruits, Fibers, and Fluids ingested.

5.3.2.2.2 Subjective Effects of Linear Accelerations

{A}

The following is a summary description of the combined human responses to specific linear acceleration vectors and magnitudes.

In operational situations it is unusual, if not impossible, for acceleration to remain precisely constant. Accelerations may be accompanied by complex oscillations and vibrations. For purposes of the following discussion, it is simpler to consider the response to sustained linear accelerations in one direction:

a. Upward Acceleration Effects ( + Gz) (In Seated Posture)

1 Gz

Equivalent to the erect or seated terrestrial posture

2 Gz

Increased weight; increased pressure on buttocks; drooping of face and body tissue

2 1/2 Gz

Difficult to raise oneself

3 - 4 Gz

Impossible to raise oneself; difficult to raise arms and legs; movement at right angles impossible; progressive dimming of vision after 3-4 seconds; progressive tunneling of vision

4 1/2 - 6 Gz

Diminution of vision; progressive blackout after about 5 seconds; hearing and then consciousness lost if exposure continued; mild to severe convulsions in about 50% of the subjects during or following unconsciousness, frequently with bizarre dreams; occasionally paresthesias, confused states, and rarely, gustatory sensations; no incontinence; pain not common, but tension and congestion of lower limbs with cramps and tingling; inspiration difficult; loss of orientation of time and space for up to 15 seconds post-acceleration

b. Downward Acceleration Effects (- Gz)

-Gz - Unpleasant, but tolerable, facial suffusion and congestion

-2 to -3 Gz - Severe facial congestion; throbbing headache; ori-gressive blurring, , or graying, or occasionally reddening of vision after 5 seconds; congestion disappears slowly; may leave petechial hemorrhages, edematous eye-lids

-5 Gz - Five seconds is limit of tolerance rarely reached by most subjects

c. Forward Acceleration Effects ( + Gx)

2 - 3 Gx - Increased weight and abdominal pressure; progressive slight difficulty in focusing and slight spatial disorientation, each subsiding with experience; 2 Gx tolerable for at least 24 hours; 4 Gx tolerable up to at least 60 minutes - 3 - 6 Gx

Progressive tightness in chest, chest pain; loss of peripheral vision; difficulty in breathing and speaking; blurring of vision, effort required to maintain focus

6 - 9 Gx

Increased chest pain and pressure; breathing difficult, shallow respiration from position of nearly full inspiration; further reduction in peripheral vision, increased blurring, occasional tunneling, great concentration required to maintain focus; occasional lacrimation; body, legs, and arms cannot be lifted at 8 Gx; head cannot be lifted at 9 Gx

9 - 12 Gx

Breathing difficulty severe, increased chest pain, marked fatigue, loss of peripheral vision, diminution of central acuity, lacrimation

15 Gx

Extreme difficulty in breathing and speaking, severe viselike chest pain; loss of tactile sensation, recurrent complete loss of vision

d. Backward Acceleration Effects ( - Gx)

Similar to those of + Gx acceleration with modifications produced by reversal of the force vector. Chest pressure reversed, hence, breathing is easier; pain and discomfort from outward pressure toward restraint harness manifest at 8-Gx; forward head tilt cerebral hemodynamic effects akin to Gz; feeling of insecurity from pressure against restraint

e. Lateral Acceleration (± Gy) Little information available

± 3 Gy

Discomfort after 10 seconds; pressure on restraint system; feeling of supporting entire weight on clavicle; inertial movement of hips and legs; yawing and rotation of head toward shoulder; petechiae and bruising; engorgement of dependent elbow with pain

± 5 Gy

14.5 seconds leads to external hemorrhage; severe headache after exposure

5.3.2.2.3 Specific Effects of Linear Accelerations

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There is a large amount of data that describes the effects of accelerations on specific body systems. Refer to the references cited in Figure 5.3.2.2.3-1 for detailed discussions.

Figure 5.3.2.2.3-1 Sources of Data for Specific Physiological Effects of Linear Acceleration

Specific physiological effects	Reference number	Page number
a. Posture changes (also refer to Paragraph 3.3.1)	92	49-154
b. Mobility changes (also refer to Paragraph 3.3.2)	10	7-48
c. Vision changes (also refer to Paragraph 4.2)	36	525-527
	92	155-160
d. Grayout and blackout	36	525
	92	152
e. Reaction time (also refer to Paragraph 4.7)	36	525
f. Cardiovascular work	92	154, 373-379
	7	18
g. Vestibular effects (also refer to Paragraph 4.5)	10	7-64 to 7-95
	86	35-45
	92	387-389
h. Gas exchange	7	154, 373-389
	92	35-45, 387-389
I. Fluid pooling	7	17
j. Motion sickness	86	36-37
	92	553

NASA-STD-3000-48

5.3.2.3 Human Responses to Rotational Accelerations

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Tolerance to rotational accelerations depends on the interaction of at least three factors: 1) center of rotation with respect to the body, 2) axis of rotation, and 3) the rotation rate.

Most subjects, without prior experience, can tolerate rotation rates up to 6 rpm in any axis or combination of axes.

Most subjects cannot initially tolerate rotation rates in the region of 12 to 30 rpm and rapidly become sick and disoriented above 6 rpm unless carefully prepared by a graduated program of exposure.

Rotation rates of 60 rpm for up to 3 or 4 minutes around the pitch axis (y-axis) and around the yaw axis (z-axis) have been described by subjects as being not only tolerable but pleasant.

Rotational rates at about 80 rpm in the pitch mode and at about 90-100 rpm in the yaw mode are intolerable.

In the pitch axis, with the center of rotation at the heart level, symptoms of backward acceleration ( - Gx) are demonstrated at about 80 rpm and are tolerable for only a few seconds. Some effects of forward acceleration (+ Gx), namely numbness and pressure in the legs, are also observed but develop slowly, with pain being evident at about 90 rpm. No confusion or loss of consciousness is found, but in some subjects disorientation, headache, nausea, or mental depression are noted for several minutes after a few minutes of exposure.

With rotation in the yaw mode, when the head and trunk are inclined forward out of the z-axis, rotation becomes close to limiting at 60 rpm for 4 minutes, although, some motivated subjects have endured 90 rpm in the same mode. Except for unduly susceptible subjects, tolerance tends to improve with exposure.

Long-duration runs in the pitch mode have been endured up to about 60 minutes at 6 rpm in selected subjects.

Unconsciousness from circulatory effects alone occur after 3 to 10 seconds in the pitch mode at 160 rpm with the center of rotation at the heart and at 180 rpm with the center of rotation at the iliac crest.

Severe disorientation and performance degradation have been experienced by air and space crewmembers during random tumbles. Serious problems persist through the period of tumbling causing disorientation, reach and manipulative performance degradation ultimately interfering with the ability to make corrective actions.

It has been recommended that if rotation is used to create artificial gravity, the following general principles should be observed to minimize the effects of rotational acceleration on the human:

a. Radial traffic should be kept to a minimum.

b. The crewmembers should not traverse through the spin axis unless the hub is nonrotating.

c. The living and working areas should be located as far as possible from the axis of rotation.

d. The compartments should be oriented so that the primary traffic paths are parallel to the vehicle spin axis.

e. Workstation positions should be oriented so that, during normal activity, the lateral axis through the crewmember's ears is parallel to the spin axis. In conjunction with this, the controls and displays should be designed so that left/right heat rotations and up/down arm motions are minimized.

5.3.2.4 Human Responses to Impact Accelerations

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Tolerance to impact and shock is usually based upon skeletal fracture levels. Damage to the vertebrae is most common. At higher levels of impact, injury to the head is the most frequent and severe manifestation.

There are two main factors which, when combined with the amplitude of acceleration, determine tolerance. These are 1) the time function, i.e., the total time of acceleration exposure and 2) the orientation of the subjects with respect to the direction of acceleration, primarily the relationship between the longitudinal (spinal) axis and the acceleration vector. For linear impact accelerations, those applied at right angles to the spinal axis are better tolerated than those applied parallel to this axis.

Figure 5.3.2.4-1 shows impact survival experience. It should be noted that numerous biophysical factors influence survival, so the approximate survival limit shown is only an estimate.

Figure 5.3.2.4-1 Impact Survival Experience

Reference: 92, Figure 6-3, p. 228; 407; 408; 409; 417; NASA-STD-3000 49

5.3.3 Acceleration Design Requirements

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5.3.3.1 Linear Acceleration Design Requirements

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The following linear acceleration design requirements shall be observed:

a. Linear Acceleration Limits for Unconditioned and Suitably Restrained Crewmembers - The accelerations in any vector shall not exceed those magnitudes and durations specified in Figure 5.3.3.1-1.

b. Linear Acceleration Limits for Preconditioned and Suitably Restrained Crewmembers - Accelerations shall not exceed those magnitudes and durations specified in Figure 5.3.3.1-2.

(Refer to Paragraph 11.7.2.3.3.2, Body Restraint Loads, for seat belt and shoulder harness design loads