Updates to Spacecraft Maximum Allowable Concentrations for 2-Butanone

Introduction
The original Spacecraft Maximal Allowable Concentrations (SMACs) for 2-butanone were first established by the National Aeronautics and Space Administration (NASA) in 1996. 2-butanone is an irritant that may also cause central nervous system (CNS) effects at high concentrations. EPA limits are set to protect against developmental effects that are not currently considered for NASA purposes. Limits for 1 hour and 24 hours, which are used for off-nominal scenarios that may allow for minor, reversible health effects, were both set at 50 ppm based on limited human exposure data from the 1940s. Limits for 7, 30, and 180 days were set at 10 ppm using the same human data but further accounting for extrapolation from mild effects to no effects by the addition of a safety factor of 3 and the small number of human volunteers by applying the square root of the number of subjects (10 in this case) to ensure no adverse effects for nominal operations. Limits were not set at that time for long-duration missions of 1000 days.
Methods
A literature search was conducted using keywords ‘methyl ethyl ketone’ and further narrowed with keywords ‘acute toxicity’ or ‘irritation’ for short-term SMAC development and ‘sub-chronic toxicity’, ‘chronic toxicity’, ‘CNS effects’, or ‘reproductive effects’ for SMAC durations of 7 days or more. Additionally, studies cited in the development of existing limits, including occupational limits, acute exposure guidelines, and state permitting and monitoring limits, were assessed.
Results
Several toxicity studies were published after publication of the original SMACs in 1996 and are summarized here. Acute (1 h and 24 h) SMACs were increased from 50 ppm to 200 ppm. The SMAC for 7-d was increased from 10 ppm to 67 ppm, and the SMACs for 30- and 180-d were increased from 10 ppm to 22 ppm. A SMAC for 1000-d was not previously set but has now been set at 22 ppm.
Discussion
As part of a periodic review of historical limits we have revised SMACs for all durations for 2-butanone. Increased limits based on the most recent evidence and risk assessment methodologies will ensure the appropriate degree of conservatism in future spacecraft design.