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[NASA] Hydrogen Sulfide (H2S) Independent Assessment

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Preparations for Next Moonwalk Simulations Underway (and Underwater)

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AS16-116-18653 (23 April 1972) — Astronaut Charles M. Duke Jr., Apollo 16 lunar module pilot, stands at a big rock adjacent (south) to the huge “House Rock” (barely out of view at right edge). Note shadow at extreme right center where the two moon-exploring crew members of the mission sampled what they referred to as the “east-by-west split of House Rock” or the open space between this rock and “House Rock”. At their post-mission press conference, the crewmen expressed the opinion that this rock was once a part of “House Rock” which had broken away. The two sampled the big boulder seen here also. Duke has a sample bag in his hand, and a lunar surface rake leans against the large boulder. Astronaut John W. Young, commander, exposed this view with a color magazine in his 70mm Hasselblad camera. While astronauts Young and Duke descended in the Apollo 16 Lunar Module (LM) “Orion” to explore the Descartes highlands landing site on the moon, astronaut Thomas K. Mattingly II, command module pilot, remained with the Command and Service Modules (CSM) “Casper” in lunar orbit.
NASA

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

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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

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and the
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.

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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Astronaut Woody Hoburg replaces life support system components inside the International Space Station’s Destiny laboratory module.
NASA

About the Author

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Kim Lowe

Human Systems Standards Integrator

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Last Updated
Jan 17, 2025

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