Hydrogen Sulfide (H2S) Independent Assessment

Jan 17, 2025

NASA’s Office of the Chief Health and Medical Officer (OCHMO) assembled a small working group to review hydrogen sulfide Spacecraft Maximum Allowable Concentration (SMAC) values. The group met virtually three times during February and March 2023, and panel members submitted individual opinion statements in April 2023. Toxicology subject matter experts were chosen to result in a balanced panel with respect to expertise in SMAC determinations and specific hydrogen sulfide considerations, from both clinical and epidemiological perspectives. Panel members were Dr. Terry Gordon, Dr. Tee Guidotti, and Dr. Joyce Tsuji; OCHMO team members who facilitated the discussions were Dr. Doug Ebert, Dave Francisco, Sarah Childress, Kristin Coffey, and Kim Lowe.

The goals of the working group were to:

1. Endorse or recommend changes to H₂S SMAC levels that had been proposed by the JSC Toxicology Laboratory
2. Review a draft H₂S SMAC manuscript prepared by the JSC Toxicology Laboratory
3. Provide any additional insight and consideration regarding H₂S toxicity that should be considered for spaceflight programs

Background

The NASA Spaceflight Human-System Standard (NASA-STD-3001) establishes that vehicle systems shall limit atmospheric contamination below established limits [V2 6050] Atmosphere Contamination Limit. The JSC Toxicology Laboratory maintains the JSC 20584 Spacecraft Maximum Allowable Concentrations for Airborne Contaminants document, which contains a table of SMAC values for a variety of chemicals including carbon monoxide, ammonia, heavy metals, and a wide range of volatile organic compounds. SMACs are documented for 1-hr, 24-hr, 7-day, 30-day, 180-day, and 1000-day time spans for each chemical, and express the maximum concentration to which spaceflight crew can be exposed for that duration.

The organ system that is affected as well as the effect (symptoms) are also documented for each SMAC. For more information on SMACs, see this article Exposure Guidelines (SMACs and SWEGs) – NASA and the OCHMO Spaceflight Toxicology technical brief technical brief.

A SMAC value for hydrogen sulfide has not previously been established since it has not been of concern in spacecraft. However, with Artemis missions returning to the moon there is a possibility that H₂S could be released within spacecraft during lunar sample return, given that this compound may be a component of lunar polar ice. H₂S has an intense smell of rotten eggs and therefore has a distracting psychological element. Physiologically it has been shown to be an irritant at low concentrations and in high concentrations can potentially lead to neurological effects and unconsciousness.

Hydrogen sulfide SMAC values will define safe limits for spaceflight crews on future missions and could drive new requirements for monitoring and mitigation of this chemical during spaceflight.

Conclusions

Key points of the review were:

1. The proposed 1-hour, 24-hour, 7-day, 30-day, and 180-day SMAC values were deemed appropriate and were endorsed by each of the panel members.
2. The proposed 1000-day SMAC value is so low that the panel’s opinion is that this SMAC may not be attainable due to human-generated sources, and that these concentrations do not represent a true toxicological risk. The recommendation is to eliminate the 1000-day SMAC, or to call it a guideline.
3. The general SMAC calculation approach and inclusion of safety factors is logical, although some additional rationale would be justified.
4. Interactive and additive effects with other substances are considered negligible, particularly at these low concentrations.
5. Microgravity-induced physiological changes are unlikely to exacerbate hydrogen sulfide exposure at these low concentrations.
6. Recommendations were made with the understanding that these SMACs apply to pre-screened, healthy astronauts. For private spaceflight participants who may not be as well screened, the panel recommended individual physician attention and a review of all SMACs (including hydrogen sulfide), to identify sensitivities in certain populations (existing disease states, etc.).
7. Passive dosimetry technology is available and should be considered for long-term monitoring at these low concentrations.

Following consideration of the panel’s recommendation, the NASA/TM-20240000101 Exposure Limits for Hydrogen Sulfide in Spaceflight was revised and released by the JSC toxicology group in January of 2024 and is available below.

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