Revisions to Limits for Propylene Glycol in Spacecraft Air

INTRODUCTION: The previous Spacecraft Maximal Allowable Concentrations (SMACs) for propylene glycol were established based on a study of rodents exposed to propylene glycol (PG) aerosol for 6 h/d, 5 d/wk for 90 d. This study has been used as the basis for the few existing limits, but all exposure concentrations were well above the saturated vapor concentration of ∼100 ppm for pure propylene glycol at room temperature. For this reason, the Environmental Protection Agency and the Agency for Toxic Substances and Disease Registry noted that the method used to generate the aerosols for the two published studies of animal exposures are not relevant to exposure conditions for the general public, and most regulatory agencies have not established inhalation limits for propylene glycol, citing lack of data. Since publication of the PG SMACs in 2008, an acute inhalation study was conducted in healthy human subjects which allows us to revise our assessment. This manuscript provides the rationale for increasing the prior limits for PG in spacecraft air from 32 and 17 ppm to 64 and 32 ppm for off-nominal scenarios/releases (1-h and 24-h limits) and from 9, 3, and 1.5 ppm to 32 ppm for all nominal timeframes (7, 30, and 180 d). Due to a lack of longer-term exposure data, NASA has elected to eliminate the 1000-d SMAC limit at this time.Ryder VE, Williams ES.