Spacecraft Maximum Allowable Research Articles

Quantitative Measurements of Hazardous Gas Effluents from the Combustion of Crew Waste Simulant in Microgravity

Abstract In August 2021, Blue Origin launched their un-crewed NS-17 mission aboard their New Shepard launch vehicle. Among the scientific payloads was NASA’s Orbital Syngas Commodity Augmentation Reactor (OSCAR), a flight-capable test rig allowing the combustion of ~10 g of simulated astronaut trash. Developed at NASA’s Kennedy Space Center, OSCAR measured differences in the combustion of complex mixed waste materials between terrestrial gravity and microgravity conditions. OSCAR is self-contained and collects its own effluent gases, which were subsequently analyzed for trace volatile organic compounds (VOCs) with a modified EPA Method TO-15. It was found that combustion in microgravity produced higher levels of VOCs (2,883 mg measured VOCs per kg trash) than for analogous triplicate (terrestrial) laboratory experiments (1,237±286 mg measured VOCs per kg trash with 95% confidence interval), indicating significant differences that were consistent with previously reported combustion efficiencies. Also, the concentrations of the measured VOCs were compared to NASA’s Spacecraft Maximum Allowable Concentrations (SMAC) values. These results provide a basis for understanding important design considerations for spacecraft waste disposal systems as NASA and their commercial partners develop crewed vehicles for missions to the Moon and Mars.

Spacecraft Maximum Allowable Concentrations for Hydrogen Fluoride.

BACKGROUND: Spacecraft maximum allowable concentrations (SMACs) provide guidance on allowable chemical exposures for nominal and emergency situations aboard spacecraft. SMACs are set to mitigate or preclude potential crew health effects and performance degradation. Hydrogen fluoride (HF) gas is highly irritating. Inhaled HF produces irritation primarily in the upper respiratory tract. HF is not routinely present in spacecraft atmospheres. However, it can be produced in spacecraft due to overheating or combustion events involving materials containing fluorinated organics.METHODS: Toxicological data relevant to SMAC development were collected from electronic databases using principles of systematic review, and from previous assessments and reviews of HF.RESULTS: The human inhalation data of Lund (short-term) and Largent (subchronic) showed that HF at approximately 3 ppm caused very mild respiratory irritation. NASA HF SMACs are based on these findings.DISCUSSION: The 1-h and 24-h SMACs are set at 3 ppm, a value consistent with NASA short-term SMAC criteria where crew may experience mild irritation. The 7-d, 30-d, 180-d, and 1000-d SMACs are set at 0.3 ppm to protect against any long-term crew health or performance effects that could be produced from HF exposures.Lam C-W, Ryder VE. Spacecraft maximum allowable concentrations for hydrogen fluoride. Aerosp Med Hum Perform. 2022; 93(10):746-748.

Acceptable Limits for n-Hexane in Spacecraft Atmospheres.

INTRODUCTION:The Spacecraft Maximum Allowable Concentrations (SMACs) for C2-C9 alkanes set by NASA in 2008 under the guidance and approval of the National Research Council specifically excluded SMACs for n-hexane. Unlike other C2-C9 alkanes, n-hexane can cause polyneuropathy after metabolism in humans or rodents and so requires more stringent SMACs than the other members of this group do. This document reviews the relevant published studies of n-hexane toxicity to develop exposure duration-specific SMACs for n-hexane of 200 ppm for 1 hour, 30 ppm for 24 hours, and 2.4 ppm for 7 days, 30 days, 180 days, and 1000 days.Garcia HD. Acceptable limits for n-hexane in spacecraft atmospheres. Aerosp Med Hum Perform. 2021; 92(12):956-961.

Spacecraft Maximum Allowable Concentrations for Manganese Compounds in Mars Dust.

INTRODUCTION: Exposure to excess manganese (Mn) can cause multiple toxicological outcomes in humans, most notably neurotoxicity. Ample epidemiological evidence suggests that chronic, low-level exposure causes subclinical cognitive effects. Because NASA astronauts will be exposed to Mars regolith, Spacecraft Maximum Allowable Concentrations (SMACs) were developed following an extensive literature review.METHODS: Multiple databases were searched for information relevant to derivation of Mn SMAC values. An additional search for Mars dust data was performed. Risk assessment approaches were applied, including adjustments for space-relevant susceptibility to Mn effects, to develop limits for 1-h to 1000-d exposures. Rover data informed the assessment and enabled calculation of allowable total dust exposure based on Mn content.RESULTS: Over 400 relevant sources were identified. Applicability of exposure characteristics and data collection methods influenced key study choice. SMACs ranging from 3 mg · m-3 (1 h) - 0.0079 mg · m-3 (1000 d) were set to protect primarily against neurocognitive and respiratory effects. Considering 0.38 wt% total Mn presence in the dust, maximum recommended total dust exposure should not exceed 790 mg · m-3 (1 h) - 2 mg · m-3 (1000 d).DISCUSSION: This literature review allowed for identification of relevant studies to inform SMAC development. Manganese is one of several components to consider when developing an appropriate total dust limit for Martian dust; other dust elements may alter Mn bioavailability. Mission-specific activities may require alteration of assumptions regarding Mn dust concentration and exposure duration. However, based on expected toxicity of particulate matter itself, the acute SMACs are protective, even with transient exposure during activities that could produce higher concentrations.Romoser AA, Ryder VE, McCoy JT. Spacecraft maximum allowable concentrations for manganese compounds in Mars dust. Aerosp Med Hum Perform. 2019; 90(8):709-719.

Predicting Air Quality at First Ingress into Vehicles Visiting the International Space Station.

NASA regularly performs ground-based offgas tests (OGTs), which allow prediction of accumulated volatile pollutant concentrations at first entry on orbit, on whole modules and vehicles scheduled to connect to the International Space Station (ISS). These data guide crew safety operations and allow for estimation of ISS air revitalization systems impact from additional pollutant load. Since volatiles released from vehicle, module, and payload materials can affect crew health and performance, prediction of first ingress air quality is important. To assess whether toxicological risk is typically over or underpredicted, OGT and first ingress samples from 10 vehicles and modules were compared. Samples were analyzed by gas chromatography and gas chromatography-mass spectrometry. The rate of pollutant accumulation was extrapolated over time. Ratios of analytical values and Spacecraft Maximum Allowable Concentrations were used to predict total toxicity values (T-values) at first entry. Results were also compared by compound. Frequently overpredicted was 2-butanone (9/10), whereas propanal (6/10) and ethanol (8/10) were typically underpredicted, but T-values were not substantially affected. Ingress sample collection delay (estimated by octafluoropropane introduced from ISS atmosphere) and T-value prediction accuracy correlated well (R2 = 0.9008), highlighting the importance of immediate air sample collection and accounting for ISS air dilution. Importantly, T-value predictions were conservative 70% of the time. Results also suggest that T-values can be normalized to octafluoropropane levels to adjust for ISS air dilution at first ingress. Finally, OGT and ingress sampling has allowed small leaks in vehicle fluid systems to be recognized and addressed.Romoser AA, Scully RR, Limero TF, De Vera V, Cheng PF, Hand JJ, James JT, Ryder VE. Predicting air quality at first ingress into vehicles visiting the International Space Station. Aerosp Med Hum Perform. 2017; 88(2):104-113.

Quantification and characterization of volatiles evolved during extrusion of rice and soy flours.

NASA-Johnson Space Center is designing and building a habitat (Bioregenerative Planetary Life Support Systems Test Complex, BIO-Plex) intended for evaluating advanced life support systems developed for long-duration missions to the Moon or Mars where all consumables will be recycled and reused. A food system based on raw products obtained from higher plants (such as soybeans, rice, and wheat) may be a central feature of a biologically based Advanced Life Support System. To convert raw crops to edible ingredients or food items, multipurpose processing equipment such as an extruder is ideal. Volatile compounds evolved during the manufacturing of these food products may accumulate and reach toxic levels. Additionally, off-odors often dissipated in open-air environments without consequence may cause significant discomfort in the BIO-Plex. Rice and defatted soy flours were adjusted to 16% moisture, and triplicate samples were extruded using a tabletop single-screw extruder. The extrudate was collected in specially designed Tedlar bags from which air samples could be extracted. The samples were analyzed by GC-MS with special emphasis on compounds with Spacecraft Maximum Allowable Concentrations (SMACs). Results showed a combination of alcohols, aldehydes, ketones, and carbonyl compounds in the different flours. Each compound and its SMAC value, as well as its impact on the air revitalization system, was discussed.