Toxicological Aspects of the Skylab Program

WAYLAND J. RIPPSTEIN, JR. AND HOWARD J. SCHNEIDER

A TOXICOLOGICAL SUPPORT CAPABILITY was established during the early developmental phases of the Skylab Program. From past experiences with closed-loop environmental operations, such as in submarines and manned chamber tests, it had been found that the buildup of trace contaminant gases could result in conditions which could cause mission termination. It was also recognized from the experience gained in the Apollo Program that the use of newly developed nonmetallic material, especially the fluoronated polymers, required toxicological considerations, and that special consideration be given to the testing for outgassing products.

It was known early in the program that the possibility of carbon monoxide buildup in the spacecraft cabin would also require special attention. None of the environmental control life support systems in previous spacecraft nor in Skylab were designed to provide carbon monoxide removal. It was therefore imperative that the selection of materials for use in the Skylab interior include consideration for the outgassing of carbon monoxide. It should be noted at this point that toxicological support provided for the Skylab Program included considerations not only for inhalation toxicity, but also ingestion, eye contact, and skin contact toxicity. Since the latter three areas of toxicology required attention so infrequently, they are not discussed in this paper.

Procedures

To provide a safe breathing, habitable environment for the Skylab crew, several measures were adopted early in the program. The most important of these was a nonmetallic materials screening program which was designed to eliminate those materials that would cause problems from their outgassed products. The screening program was based upon measuring the amounts of carbon monoxide and total organics outgassed per unit weight of each candidate material. Levels of acceptance were established for both carbon monoxide and total organics based upon the spacecraft habitable volume, the trace gas removal rate by the environmental control life support systems, and the cabin leak rate.

Where newly developed polymers were considered for use as electrical component potting compounds or electrical wire insulators, pyrolysis products of these materials were used to determine toxicological limits. The amount of material required to kill 50 percent of the exposed animals identified as lethal dose 50 (LD50) was determined. In these cases, material selection included both outgassing data and LD50 information. To prevent inhalation exposures to toxic effects from chemical compound(s) contained in the pyrolysis products, chemical analyses using mass spectral and gas chromatographic procedures were performed. These analytical procedures were also performed when a waiver was requested on any candidate spacecraft material that failed the carbon monoxide and total organics screening tests.

Problems

Following the loss of the Skylab 1 micrometeoroid shield, a significant toxicity problem developed as a direct result of the overheating of the Orbital Workshop interior wall insulation material. The sensors for wall temperature indicated that the interior walls of the Orbital Workshop had attained a projected temperature of 177 ° C (350 ° F) on the skin side of the insulation and 71 ° C (160 ° F) on the interior volume side of the spacecraft insulation. Since the insulation was known to be a rigid polyurethane foam, a potential hazard could develop as a result of the decomposition of the polymer to produce an isocyanate derivative. Of secondary concern was the accelerated offgassing rate of the entire nonmetallic materials contained in the Skylab habitable volume.

Solutions

Using a piece of foam identical with that in Skylab 1 (same chemical lot and age), a solids probe mass spectral analysis was conducted. Polymer decomposition begins at about 200 degrees C (392 ° F), but toluene diisocyanate was detected in trace quantities from 50 ° C (122 ° F) to about 200 °C (392 ° F). The manufacturer reported that an excess of toluene diisocyanate is used in the processing of a rigid foam and the excess toluene diisocyanate was apparently diffusing from the foam during the lower temperatures prior to thermal decomposition. Also, the blowing agent contained in the foam, trichlorofluoromethane, reached a maximum release rate at about 150 ° C (302 ° F). No accurate quantitative results were available from these analyses due to the unavailbility of toluene diisocyanate standards. Furthermore, at the time of the overheating of the polyurethane foam, there existed no spacecraft requirements for acceptable atmospheric concentrations of toluene diisocyanate. The maximum allowable exposure (8-hour weighted average) limits established by the Occupational Safety and Health Administration (ref. 1) for toluene diisocyanate is 0.14 mg/m³ [0.02 ppm standard temperature and pressure (STP)]. Reports in the literature (refs. 2,3,4,5,6,7) all substantially support this exposure limit.

Prior to the launch of the Skylab 2 crew two types of gas analysis detector tubes and two activated charcoal and hopcalite masks were put aboard the Command and Service Module to protect the unsuited crewmen upon their initial entry into the Orbital Workshop to sample its atmosphere. The tubes were of the colorimetric design and included one type for carbon monoxide and another for toluene diisocyanate detection. The lower sensitivity of the carbon monoxide tubes was 11 mg/m³, and for the toluene diisocyanate tubes, 0.14 mg/m³. Atmospheric samples were taken by using a syringe-type pump to flow air through the analyzer tubes (fig. 10-1).

Prior to the entry of the crew into the space station cluster, two precautionary measures were undertaken to ensure that the habitable areas were safe for manned operations. The first was a series of pressurization-depressurization cycles of the Skylab 1 atmosphere designed to discharge and dilute any contaminating gases of potentially toxic levels. In the second measure the crew sampled the air for carbon monoxide and toluene diisocyanate first in the Multiple Docking Adapter and then in the Orbital Workshop, using the supplied analyzer tubes. The results of their analyses indicated no detectable toluene diisocyanate and an extrapolated 5 mg/m³ level of carbon monoxide.

The crew energized the Skylab 1 Environmental Control Life Support System which contained 9.02 kg (20 lbs) of activated carbon, specifically designed to remove trace levels of contaminating compounds. From prior tests it was known that the spacecraft-type activated carbon would very efficiently remove toluene diisocyanate. After a 30-minute atmospheric circulation period, the crew was given instructions to enter the space station for manned operations. This mission and Skylab missions 3 and 4 were accomplished without any other atmospheric trace gas problems.

In addition to potential offgassing problems from excessive internal temperatures in the Orbital Workshop, a leak was suspected in the coolant system of the spacecraft. To determine the composition and concentration of any atmospheric trace contaminants a unique device was used (Appendix. A.II.c.-1, fig. A.II.c-2, A.II.c-3). The device consisted of two small glass tubes, mounted in parallel in an aluminium cartridge, such that an atmospheric gas flow could pass equally through both tubes at the same time. Each of these tubes was partially filled (4.5 ml/tube) with a gas chromatographic absorbent material. Aproximately 60 liters (STP) of cabin atmosphere were passed through the device during a time span of 15 hours. Three such samples were taken by the Skylab 3 crew on mission days 11, 46, and 77.

The analyses of the absorbed contents of the three samples (three pairs of tubes) indicated the presence of more than 300 compounds in the Skylab atmosphere during the occupancy of the Skylab 3 crew. Of this number, 107 (ref. 8) were identified by mass spectral methods. The molecular weights for the identified compounds ranged from 60 to 584. These data revealed that there was no coolant fluid leaking into the interior of the Orbital Workshop.

When the three atmospheric samples taken on mission days 11, 46, and 77 were compared, the results indicated only minor differences in the levels of contamination. This indicated that a state of equilibrium had been attained earlier between the gas generation rates of the contaminant sources and the removal rate by the Environment Control Life Support System.

Conclusion

The experiences and data gained in the Skylab program have demonstrated that the crew was provided with as safe an environment as could be attained using the current state-of-the-art trace gas removal technology. The knowledge gained in solving the trace contaminant problems encountered in the Skylab Program will greatly aid in providing safe, habitable spacecraft environments for the future missions of man in space.

Acknowledgment

The authors of this paper wish to acknowledge the important contribution of E.S. Harris of the National Institute for Occupational Safety and Health (formerly Head of the NASA-JSC Toxicology Laboratory) in directing the toxicology program in support of Skylab.

References

1. Federal Register. August 13, 1971. OSHA Rules and Regulations, Table G-1, 36(157):15101.

2. ZAPP, J.A. Hazards of isocyanates in polyurethane foam plastic production. AMA Arch., Ind. Health, 15:324-330, April 1957.

3. NIEWENHUIS, R., L. SCHEEL, K. STEMMER, and R. KILLENS. Toxicity of chronic level exposures to toluene diisocyanate in animals. AIHA J., 26:143-149,1965.

4. DUNCAN, B., L. SCHEEL, E.J. FAIRCHILD, R. KILLENS, and S. GRAHAM. Toluenediisocyanate inhalation toxicity: pathology and mortality. AIHA J., 23:447-456.

5. BRUGSCH, H.G., and H.B. ELKINS. Toluene diisocyanate (TDI) toxicity. New Eng. J. Med., 268:353-357,February 14, 1963.

6. Hygienic Guide Series. Toluene diisocyanate (tolylene diisocyanate, TDI). AIHA J., 28:90-94, 1967.

7. DERNEHL, C.U. Health hazards associated with polyurethane foams. J. Occup. Med., 8:59, 1966.

8. The Proceedings of the Skylab Life Sciences Symposium, I:163-166. NASA TM X-58154, November 1974.

 

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