"Fire! ...We have a fire in the cockpit!...We've got a bad fire...get us out. We're burning up!" Those were the horrifying words heard in the headsets of flight controllers on January 27th, 1967, as the United States lost it's first astronauts in the race to land a man on the moon. The three astronauts, Vigil I. (Gus) Grissom, Edward H. White and Roger B. Chaffee perished when a fire broke out in the cockpit of their Apollo I command module. In an irony that compounded the tragedy, the fire occurred not in space but during pre launch tests in preparation for the February launch of the first manned Apollo mission.

One of the numerous papers that I made note of when I had a quest in understanding activated sterols and ionization radiation. No scanner, so the figures and graphs are missing.

What factor would be used to show the number of problems in maintaining the super feat of Space Flight ?

Some words that might be used:
ionization radiation, defecation, urination, moisture, digestive micro flora, micro flora of the skin, microbial contamination, enzymes human and microbial, fungi, bacterium, symbiotic, nonsymbiotic, electronic circuitry, electrical and electronic sheilding, compostion of structural materials, structural design, structural stress,physiological stress, psychological stress, synthetic material gassing, adhesives, vision, hearing, physiology, cabin pressure, cabin atmosphere,.............
try to think of some points that rocket scientists have missed.

RADIATION RESEARCH SUPPLEMENT 7, 413 - 422 (1967).



PHYSIOLOGICAL EFFECTS OF SPACE CABIN ATMOSPHERES
E. M. ROTH

The Lovelace Foundation for Medical Education and Research, Albuquerque, New Mexico.

In the past, the presence of engineering commitments involved in the development of spacecraft required that decisions on space cabin atmosphere be made early, often before the physiological tolerance to unnatural gaseous environments could be determinded. Selection has been based primarily on engineering grounds, with the burden of proof on the physiologist that the proposed environments could not be tolerated. While this approach has been adequate for previous flights, it has serious drawbacks in the longer and more hazardous missions projected for the future.

The variables for space cabin environment which must be considered are shown in TABLE I. There are also many physiological and pathological considerations with which these environmental variables may interact (TABLE II).

The interactions indicated above are quite complex and quite beyond the scope of the present conference. I shall limit my discussion to those interactions that may influence the response of an astronaut to the radiation environment within a space vehicle.

To satisfy the engineers, one would like to keep the total pressure in the cabin as low as possible. Analysis of the weight penalty for different total pressures in the cabin suggests that, at pressures below 5 to 7 psi. , the weight of a cabin wall is independent of the pressure. Factors other than hoop tension determine the weight. Above 7 psi., the weight penalty rises steeply with increasing pressure.

The selection of oxygen percentage is strongly determinded by the total pressure. Fig 1. indicates the constraints of the pressure-oxygen system.

The line marked "sea level equivalent" represents those pressures and oxygen percentages equivalent to the partial pressure of 20% O2 at sea level. Pressure must be reduced to 33,000 feet or to less than 4 psi. to maintain the same partial pressure with 100% O2. The heavy lines enclose the zone of unimpaired function. Hypoxia is limiting on the lower border, and oxygen toxicity on the upper boarder. Acclimatization to hypoxia allows lowering of the curve still further. (The horizontal dashed line represents the altitude above which decompression sickness may become the limiting factor.) There is no evidence at present for the development of tolerance to high pressures of oxygen. There is great individual variability in tolerance to oxygen toxicity. The highest oxygen level tolerable in all subjects for long periods of time seems to follow the upper fine-line of Fig 1. However, in recent years evidence has accumulated that the zone of 80 to 100% O2 (hatched area) there may well be more severe time limits than previously expected. (2)

TABLE I
Cabin Environment Variables
----------------------------------------
1.Total pressure
2.Oxygen pressure
3.Carbon Dioxide pressure
4.Inert gas pressure
5.Gaseous trace contaminants
6.Water vapour pressure
7.Thermal properties of a gas
8.Circulation of gas
9.Temperature of gas
10.Leakage rate of gas
11.Duration of exposure
12.Gravitaion level
----------------------------------------

TABLE II
Physiological and Pathological Considerations
----------------------------------------
1.Alertness and performance
2.Communication
3.Time of useful function
4.Decompression syndromes
a.Aeroembolism and bends
b.Barootitis and barosinusitis
c.Cardiovascular collapse
5.Respiratory physiology
a.Atelectasis
b.Hypoxia
c.Hypo- and hypercapnia
d.Hemoglobin control
6.Oxygen toxicity syndrome
7.Radiation sensitivity
8.Fire and blast hazards
a.Meteoroid penetration effects
b.Cabin fire control
9.Bacteria flora changes and infections
10.Water phsiology
11.Thermal control problems
----------------------------------------
Fig. 2 indicates the time and pressure dependence of oxygen toxicity. The ordinate is partial pressure of O2. The abscissa is time to first symptoms. At high pressures of several thousand torr, symptoms occur within 10 minutes; these are mostly of the central nervous system. In the zone of 1 atmosphere to 1/2 atmosphere, the first symptoms are usually of the respiratory tract such as bronchitis and pulmenary edema.

Fig 1. Oxygen pressure effects. Adapted from Luft (1)

Fig 2. Time of first symptoms in oxygen toxicity. Adapted from Welch (3) White - McDivitt flights; Cooper - Conrad flight.

TABLE III
Rates of Collapse of One Lung - with Airway Obstructed - When N2 is Present (Breathing Air) and When N2 is Absent (Breathing O2) a.
---------------------------------------
Breathing Rate of collapse
observed(cc/min/kg)relative
----------------------------------------
Air at 1 atm 0.046 1
O2 at 1 atm 2.87 62
O2 at 0.26 atm17.20 b. 370

Time to collapse lung (minutes)
370 b.
6
1 b.
----------------------------------------

a. After Dale and Rahn (4)
b. Calculated.

Below 1/2 atmosphere, the symptoms are variable. Atelectasis or alveolar collapse is seen in susceptable subjects, especially in the presence of high g<b/> loads. This is manifested by pains in the chest exaggerated by deep inspiration. In one 14-day experiment at 5 psi. 100% O2, subjects experienced an oxidative hemolytic anemia and excessive protein and casts in the urine suggestive of renal damage. Under similar circumstances, other subjects have gone as long as 30 days without symptoms. Symptoms may well be due to trace contaminants in the atmosphere. At present, the interactions between 100% O2 at low pressure and trace contaminants of known toxic materials are being studied at the Aeromedical Laboratory at Wright - Patterson AFB with technical aid from the Aerojet General Corporation.

In operational vehicles, we have reached the 100 hours of the White - McDivitt flight and the 200 hours of the Cooper - Conrad flight with no apparent symptoms, but with possible changes in the red blood cells. The asymptote at the abscissa of Fig 2. is therefore uncertain. The oxygen pressure of 5 psi. (about 100 torr above normal sea level oxygen pressure) used in the Mercury and Gemini may represent borderline O2 toxicity. Antioxidant defenses have been selected in biological systems (through evolution) for less than sea level equivalents of oxygen pressure. Over long periods of time, 100 torr excess pressure may lead to oxidation injury of the body.

The 5 psi. pressure was used in Mercury and Gemini for two reasons. It reduced the danger of decompression sickness on going from sea level to cabin pressure, and it decreased the actlectatic tendency at lower pressures. TABLE III demonstrates the difference in collapse tendency between lung filled with O2 at 0.26 atmospheres. There is a 370-fold difference between the first and the third. Nitrogen acts to brake the collapse, as would any of the inert gases. The lower the water or blood solubility of a gas, the less the collapse tendency (see TABLE III).

It can be seen in TABLE IV that helium and neon would be even more effective than nitrogen in preventing atelectasis. Fortunately, forced deep breathing, espscially during take-off and re-entry and during other high maneuvers, is also effective in reducing the tendency toward collapse of the alveoli.

TABLE IV a
----------------------------------------
Biochemical Properties of Inert Gases
----------------------------------------
a After Roth (5)

Oxygen toxicity is of great concern to the radiobiologist. It raises the question of whether the excess of about 100 torr above normal pO2 can increase the radiation sensitivity of astronauts. Hyperbaric oxygen has been used in radiotherapy to increase the sensitivity of tumors to radition. However, several atmospheres of oxygen pressure are required to produce a significant effect (2).

Several years ago, studies at the Republic Aviation Corporation suggested that mice subjected to 250-kVp x-rays were more sensitive at 5 psi. of 100% oxygen than air. Recent studies at the Douglas Aircraft Coporation with larger number of animals suggest that there is no operationally significant difference in sensitivity produced by the 5 psi. of 100% oxygen environment (6)

Fig 3. is a review of these results. The slightly greater resistance of animals at 5 psi. of 100% oxygen may be a result of the atelectasis seen in some of the animals in the pure oxygen environment. Atelectasis could lead to systemic hypoxia and a possible paradoxical increase resistance to radiation.

The problem of CO2 toxicity is also present in a space vehicle. The data on toxic levels are much more clear than with oxygen.

Fig 4. represents the time dependence of toxic levels of CO2 in man. Below 1 1/2% there are no changes in gross physiological function. However, above 1/2% CO2 there are biochemical adaptive changes which may alter the response of animals to radiation. There are no studies of this relationship in the literature. Current design limits for CO2 are set at 1/2%. Emergency modes may take the CO2 higher. It may be well to get data on changes in radiation sensitivity which may be produced by higher pressures of CO2.

Fig 3. Effect of 5.0 psi, 100% O2 on radiation sensitivity. After Kelton and Kirby (6)

Fig 4. Time dependence of carbon dioxide toxicity. After Roth (7)

Another question of interest is the fire and blast hazard of pure oxygen environments. In a recent publication, the physical chemistry of fire and blast in numerous gaseous environments was reviewed (8). There are many variables involved. Let us look at just a few.

Fig 5. represents the minimum quenching distance and minimum ignition energy for different propane mixtures at different total pressures and with different oxygen-nitrogen compositions. Effects of several orders of magnitude are seen on going from 20% to 100% O2. The data would be similar for hydrocarbon sprays and mists from disrupted hydraulic lines and coolant systems.

Fig 5. Ignition of propane in varied oxygen environments. After Lewis and von Elbe (9)

Fig 6. shows the effect of different gas mixtures on the rate of burning of cloth. The ordinate compares the burning rate with that in a standard aircraft cabin at 8000 feet. The solid lines are lines of constant pO2. The dotted lines are lines of constant O2 %. It can be seen that 5 psi of 100% O2 environment of Mercury and Gemini would allow cloth to burn 3.5 times as fast as in a standard aircraft cabin. Atmosheres of 7 psi. of 50% O2 - 50% N2 recently suggested for Apollo and subsequent missions would allow cloth to burn twicw as fast as in jet cabins. Fireproofing material is also much less effective in 100% O2 (8). For present-day missions of high risk, 100% O2 must be used with much caution.

An alternative atmosphere is 7 psi. of 50% O2 - 50% inert gas. This approach decreases the fire hazard but brings into play two other factors: the difficulty of controlling a two-gas system and the increased hazard of decompression sickness. Control of two-gas systems is much less reliable than a simple anaeroid control of 100% O2 atmospheres. Only recently have reliable, flyable O2 sensors become available.

Decompression sickness arises from the release of inert gas bubbles in the joint tissues and bloodstream produced by the decrease in ambient pressure. In decompression to altitude, the pressure must be decreased by a factor of more than 2 before symptoms arise (3). If a cabin at 7 psi. of 50% O2 - 50% N2 were to decompress and the crew were to inflate their suits to the standard 3.5 psi., there would be a small but definite chance of getting into trouble. In a recent study, it was calculated that less than 7% of a physically conditioned astronaut population should have mild symptoms of bends after severe exercise following such a decompression. At rest, less than 1% should get into trouble (5).

TABLE V
Radioprotective Effect of "Metabolically Inert" Gases a
----------------------------------------
Gas Pressure for 50%effect a
Helium 55
Hydrogen 55
Nitrogen 12.5
Argon 2
Xenon 1
Krypton 2
----------------------------------------
a Adapted from Ebert and Howard (2).
bPressure of metabolically inert gases added to 1 atmosphere of air to reduce the oxygen-dependent radiosensitivity of Vicia faba roots by 50%.

How effective are the other inert gases in decreasing the symptom frequency of severity ? For bubbles forming in fat, helium should be about one-fourth as dangerous, and neon about one-sixth as dangerous. For the more frequent bubbles forming in joint tissue or in the bloodstream, the calculations are less reliable, but neon again appears to be best, and helium equal to nitrogen in degree of danger. The gases argon, xenon, and krypton are much more dangerous than nitrogen in decompression (5).

Neon has been used by the Royal Navy with preliminary favorable results in diving (5). Neon also has favorable characteristics for decreasing the lung damage in explosive decompression and decreasing the hazard of ebullism, the formation of water vapor and gas bubbles at altitudes above 70,000 feet (5). The cryogenic storage penalties for neon are also lower than those for helium. Neon, therefore, looks like a favorable substitute for nitrogen in mixed gas systems. There are few direct data to support this conclusion. There is a dearth of knowledge about long duration inhalation of NE - O2 mixtures. There is also little known about the biochemical role of even N2 gas in body and the effect of chronic exposure to atmospheres with no N2. One would expect little trouble from the chronic lack of nitrogen or from the presence of helium or neon, but no clear-cut data are available on this subject.

The role of inert gases in modifying radiation sensitivity has also been poorly studied (2). The work of Ebert and Howard (TABLE V) suggests that very high pressures of helium,neon,and argon are required to alter the radiation response of plants (11). One can predict that the absence of inert gas or other alterations of our normal inert gas environment (60S torr nitrogen pressure) will probably have little effect on radiation sensitivity of mammals. Supporting data are certainly required on this point.

Thus, we see that selection of an ideal space cabin atmosphere for present operations represents a complex decision. It must be tailored to the mission and to our state of knowledge regarding many physical and biochemical variables. The more complex space stations and interplanetary missions of the future further complicate the story. However, as in most scientific areas, early confusion usually leads to more complete understanding, simplification, and utilization.

SUMMARY

Physiological criteria for the selection of space cabin atmospheres are reviewed. Emphasis is placed on the implication of these factors on engineering constraints and radiobiological considerations. The current operational atmospheres with 5 psi. of 100% oxygen do not appear to significantly effect the radiation hazard within the cabin. Analysis of the complex man machine interactions and engineering trade-offs suggest that selection of the ideal space cabin atmosphere must be specific for each space mission in question.

REFERENCES

1. U. C. Luft, Data on oxygen pressure effects compiled for the Garrett Corporation. The Lovelace Foundation, Albuquerque, New Mexico, (1962).

2. E. M. Roth, Space-cabin atmospheres, Part I, Oxygen toxicity, NASA-SP-47 (1964).

3. B. E. Welch, T. E. Morgan, Jr., and H. G. Clamann, Time concentration effects in relation to oxygen toxicity in man, Federation Proc. 22, 1053 - 1056 (1963).

4. W. A. Dale and H Rahn, Rate of gas absorption during atelectasis, Am. J. Physiol. 170, 606 - 615 (1952).

5. E. M. Roth, Space-cabin atmospheres, Part III, Physiological factors of inert gases., NASA-SP-117 (1965).

6. A. A. Kelton and J. K. Kirby, Total oxygen pressure and radiation mortality in mice. Douglas paper 2030, May (1964), Douglas Aircraft Company, Santa Monica, California.

7. E. M. Roth, Carbon dioxide effects, p. 8. NASA-SP-3006, (1964).

8. E. M. Roth, Space-cabin atmospheres, Part II, Fire and blast hazards. NASA-SP-48 (1964).

9. B. Lewis and G. von Elbe, Combustion, Flames and Explosions of Gases, Academic Press, New York, (1951).

10. H. A. Klein, The effects of cabin atmospheres on combustion of some flammable aircraft materials. Wright Air Development Center Technical Report 59 - 456, (1960) (ASTIA No. AD - 238367).

11. M. Ebert and A. Howard, Effect of nitrogen and hydrogen gas under pressure on the radiosensitivity of the broad bean root. Radiation Res. 1, 331 - 341 (1957).

Regards,