Safe human exposure limits for airborne linear siloxanes during spaceflight.

Area of Science:

• Aerospace Medicine
• Toxicology
• Environmental Health

Background:

• Low molecular weight siloxanes are utilized in various industries and consumer goods.
• Siloxane vapors have been detected in spacecraft environments, necessitating health protection measures for astronauts.
• Previous spacecraft maximum allowable concentrations (SMACs) for siloxane vapors require reevaluation due to new scientific data and risk assessment methodologies.

Purpose of the Study:

• To reevaluate existing SMACs for octamethyltrisiloxane (L3) across exposure durations from 1 hour to 180 days.
• To establish a new 1000-day SMAC for L3 to support extended space missions.
• To broaden the applicability of these safety values to the broader family of linear siloxanes.

Main Methods:

• Conducted a comprehensive literature review to identify relevant studies published since the initial SMACs for L3 were established in 1994.
• Analyzed updated toxicological data to identify sensitive toxicity endpoints.
• Evaluated current risk assessment approaches and dosimetric adjustment methods.

Main Results:

• Updated SMACs for L3 were determined for 1-hour, 24-hour, 30-day, and 180-day exposure periods.
• A new 1000-day SMAC was developed for L3, crucial for long-duration space exploration.
• Group SMACs were established for the linear siloxane family (L2, L3, L4, L5) for exposures spanning 1 hour to 1000 days.

Conclusions:

• New SMACs were established based on acute pulmonary and neurotoxicity from high-dose exposure to hexamethyldisiloxane (L2).
• Potential liver effects from longer-term exposure to L2 and octamethyltrisiloxane (L3) were considered in the updated limits.
• These revised SMACs are designed to safeguard astronauts from adverse health effects associated with linear siloxane exposure during space missions.