Hydrogen Sulfide (H2S) Impartial Evaluation

The objectives of the working group had been to:

1. Endorse or suggest modifications to H2S SMAC ranges that had been proposed by the JSC Toxicology Laboratory
2. Assessment a draft H2S SMAC manuscript ready by the JSC Toxicology Laboratory
3. Present any extra perception and consideration relating to H2S toxicity that needs to be thought of for spaceflight packages

The NASA Spaceflight Human-System Normal (NASA-STD-3001) establishes that car techniques shall restrict atmospheric contamination beneath established limits [V2 6050] Ambiance Contamination Restrict. The JSC Toxicology Laboratory maintains the JSC 20584 Spacecraft Most Allowable Concentrations for Airborne Contaminants doc, which incorporates a desk of SMAC values for quite a lot of chemical substances together with carbon monoxide, ammonia, heavy metals, and a variety of unstable natural compounds. SMACs are documented for 1-hr, 24-hr, 7-day, 30-day, 180-day, and 1000-day time spans for every chemical, and specific the utmost focus to which spaceflight crew will be uncovered for that length.

The organ system that’s affected in addition to the impact (signs) are additionally documented for every SMAC. For extra info on SMACs, see this text Publicity Pointers (SMACs and SWEGs) – NASA and the OCHMO Spaceflight Toxicology technical transient technical transient.

A SMAC worth for hydrogen sulfide has not beforehand been established because it has not been of concern in spacecraft. Nonetheless, with Artemis missions returning to the moon there’s a risk that H₂S could possibly be launched inside spacecraft throughout lunar pattern return, on condition that this compound could also be a element of lunar polar ice. H₂S has an intense odor of rotten eggs and subsequently has a distracting psychological factor. Physiologically it has been proven to be an irritant at low concentrations and in excessive concentrations can probably result in neurological results and unconsciousness.

Hydrogen sulfide SMAC values will outline secure limits for spaceflight crews on future missions and will drive new necessities for monitoring and mitigation of this chemical throughout spaceflight.

Key factors of the evaluation had been:

1. The proposed 1-hour, 24-hour, 7-day, 30-day, and 180-day SMAC values had been deemed applicable and had been endorsed by every of the panel members.
2. The proposed 1000-day SMAC worth is so low that the panel’s opinion is that this SMAC is probably not attainable attributable to human-generated sources, and that these concentrations don’t signify a real toxicological danger. The advice is to remove the 1000-day SMAC, or to name it a suggestion.
3. The final SMAC calculation strategy and inclusion of security components is logical, though some extra rationale can be justified.
4. Interactive and additive results with different substances are thought of negligible, notably at these low concentrations.
5. Microgravity-induced physiological modifications are unlikely to exacerbate hydrogen sulfide publicity at these low concentrations.
6. Suggestions had been made with the understanding that these SMACs apply to pre-screened, wholesome astronauts. For personal spaceflight contributors who is probably not as properly screened, the panel really helpful particular person doctor consideration and a evaluation of all SMACs (together with hydrogen sulfide), to determine sensitivities in sure populations (current illness states, and many others.).
7. Passive dosimetry know-how is offered and needs to be thought of for long-term monitoring at these low concentrations.

Following consideration of the panel’s advice, the NASA/TM-20240000101 Publicity Limits for Hydrogen Sulfide in Spaceflight was revised and launched by the JSC toxicology group in January of 2024 and is offered beneath.