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NASA’s Office of the Chief Health and Medical Officer (OCHMO) convened a specialized panel to assess the Spacecraft Maximum Allowable Concentration (SMAC) values for hydrogen sulfide (H2S). This working group conducted three virtual meetings between February and March 2023, culminating in individual expert opinions submitted in April 2023.
Chosen for their expertise in toxicology and their understanding of H2S impacts, the panel included Dr. Terry Gordon, Dr. Tee Guidotti, and Dr. Joyce Tsuji. The OCHMO team facilitated the discussions with members Dr. Doug Ebert, Dave Francisco, Sarah Childress, Kristin Coffey, and Kim Lowe.
The primary objectives of the working group were to:
- Review and endorse proposed changes to the H2S SMAC values suggested by the Johnson Space Center (JSC) Toxicology Laboratory.
- Examine a draft manuscript addressing H2S SMAC prepared by the JSC Toxicology Laboratory.
- Offer insights regarding H2S toxicity applicable to spaceflight operations.
Background
The NASA Spaceflight Human-System Standard (NASA-STD-3001) mandates that all vehicle systems must control atmospheric contamination within established limits, specifically outlined in the document that outlines contaminant thresholds.
The JSC Toxicology Laboratory maintains the “JSC 20584 Spacecraft Maximum Allowable Concentrations for Airborne Contaminants” document. This comprehensive source provides SMAC values for various harmful substances, including carbon monoxide and a range of volatile organic compounds, addressing exposure limits over several time frames from one hour to 1,000 days.
The established SMAC values specify not only the maximum safe concentrations of each chemical but also the respective health effects based on exposure levels. For further details, refer to the guidelines on SMACs and the NASA Exposure Guidelines as well as the OCHMO Spaceflight Toxicology technical brief.
Hydrogen sulfide SMAC values have not been a primary focus in the past due to low concern in earlier spacecraft operations. However, with upcoming Artemis missions aimed at lunar sample returns, there is an increased risk of H2S exposure due to its presence in lunar polar ice.
This compound is characterized by its distinct rotten egg odor, which may pose a psychological distraction for astronauts. At lower concentrations, H2S acts as an irritant, while higher concentrations could potentially lead to more severe neurological effects or even unconsciousness.
Setting H2S SMAC values is crucial for establishing safe exposure limits for future missions and could necessitate new safety protocols for monitoring this gas throughout space flight activities.
Key conclusions of the panel’s review included:
- Panel members endorsed the recommended SMAC values for 1-hour, 24-hour, 7-day, 30-day, and 180-day exposures.
- The suggested 1,000-day exposure limit may not be feasible, as the panel concluded these levels may not present a significant toxicological threat, prompting a recommendation to either discard this limit or classify it as a guideline.
- The overall methodology for calculating SMAC values and the use of safety factors were acknowledged as reasonable, though the panel suggested further explanation may be beneficial.
- Interactions between H2S and other substances at low concentrations were deemed negligible, and microgravity effects were not expected to heighten exposure risks.
- Recommendations emphasized that these SMACs should apply to astronauts who have been thoroughly screened while noting that private spaceflight participants may require additional health assessments given potential sensitivities to H2S.
- The panel also suggested the implementation of passive dosimetry for ongoing monitoring of low-level exposures.
Following these recommendations, the NASA/TM-20240000101 “Exposure Limits for Hydrogen Sulfide in Spaceflight” was updated and released by the JSC toxicology group in January 2024 and can be accessed here.
Citation: NASA sets new hydrogen sulfide exposure limits for space missions (2025, January 20) retrieved from https://phys.org/news/2025-01-nasa-hydrogen-sulfide-exposure-limits.html
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