NASA sets new hydrogen sulfide exposure limits for space missions
NASA’s Office of the Chief Health and Medical Officer (OCHMO) assembled a small working group to review hydrogen sulfide (H2S) 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 H2S 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:
- Endorse or recommend changes to H2S SMAC levels that had been proposed by the JSC Toxicology Laboratory
- Review a draft H2S SMAC manuscript prepared by the JSC Toxicology Laboratory
- 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 1,000-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 the article “Exposure Guidelines (SMACs and SWEGs)—NASA” and the “OCHMO Spaceflight Toxicology 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.