Provide any additional insight and consideration regarding H2S toxicity that should be considered for spaceflight programs

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 HS could be released within spacecraft during lunar sample return, given that this compound may be a component of lunar polar ice. HS 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.

Key points of the review were:

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.