Hydrogen Sulfide (H2S) Independent Assessment

NASA's Office of the Chief Health and Medical Officer (OCHMO) has formed a small working group to review spacecraft hydrogen sulfide (H2S) maximum allowable concentration (SMAC) values. The group met virtually three times during February and March 2023, and group members submitted individual statements of opinion in April 2023. Toxicology subject matter experts were selected to form a balanced group with expertise in SMAC determination and specific H2S considerations, including clinical and clinical studies. and epidemiological perspectives. Panelists included Dr. Terry Gordon, Dr. Tee Guidotti, and Dr. Joyce Tsuji; OCHMO team members leading the discussion included Dr. Doug Ebert, Dave Francisco, Sarah Childress, Kristin Coffey, and Kim Lowe.

The goals of this working group are:

  1. Endorse or recommend changes to the JSC Toxicology Laboratory’s proposed H2S SMAC levels
  2. Review of the draft H2S SMAC manuscript prepared by the JSC Toxicology Laboratory
  3. Provide any additional insights and considerations regarding H2S toxicity that should be considered by space programs

NASA's Standard for Spaceflight Human Systems (NASA-STD-3001) stipulates that aircraft systems should limit air pollution to air pollution limits below established limits (V2 6050). The JSC Toxicology Laboratory maintains the JSC 20584 Maximum Allowable Concentrations for Spacecraft Air Pollutants document, which contains tables of SMAC values ​​for various chemicals, including carbon monoxide, ammonia, heavy metals, and various volatile organic compounds. SMAC records time spans of 1 hour, 24 hours, 7 days, 30 days, 180 days and 1000 days for each chemical and represents the maximum concentration to which space crews can be exposed during that time.

Each SMAC also records the organ system affected and the effects (symptoms). For more information about SMAC, see this article Exposure Guidelines (SMAC and SWEG) - NASA and OCHMO Space Toxicology Technical Brief.

The SMAC value for hydrogen sulfide has not been previously determined because it has not received attention in spacecraft. However, with the Artemis mission returning to the Moon, it is possible that H2Given that this compound may be a component of the lunar polar ice, S may have been released within the spacecraft during lunar sample return. H2S has a strong smell of rotten eggs and therefore has a distracting psychological component. From a physiological perspective, it has been shown to be irritating at low concentrations, while at high concentrations it may cause neurological effects and unconsciousness.

Hydrogen sulfide SMAC values ​​will define safety limits for space crews on future missions and may drive new requirements for monitoring and mitigating this chemical during spaceflight.

The key points of the review are:

  1. The proposed 1-hour, 24-hour, 7-day, 30-day, and 180-day SMAC values ​​were considered appropriate and endorsed by each panelist.
  2. The proposed 1000-day SMAC value is so low that the expert panel's opinion is that this SMAC may not be achievable due to anthropogenic sources and that these concentrations do not represent a true toxicological risk. It is recommended to cancel the 1000-day SMAC, otherwise known as the guideline.
  3. The general SMAC calculation method and the inclusion of safety factors are logical, although some additional rationales are justified.
  4. Interactions and additive effects with other substances are considered negligible, especially at these low concentrations.
  5. Physiological changes induced by microgravity are unlikely to exacerbate these low concentrations of hydrogen sulfide exposure.
  6. Recommendations are made on the assumption that these SMACs are suitable for prescreened healthy astronauts. For private spaceflight participants who may not be well screened, the panel recommends individual physician attention and review of all SMACs (including hydrogen sulfide) to determine susceptibility in certain populations (existing disease states, etc.).
  7. Passive dosimetry techniques are available and should be considered for long-term monitoring at these low concentrations.

After considering the expert panel's recommendations, the JSC Toxicology Panel revised and published NASA/TM-20240000101 Hydrogen Sulfide Exposure Limits in Spaceflight in January 2024 as follows.