Revisions to Acute/Off-Nominal Limits for Benzene in Spacecraft Air

BACKGROUND: The previous short-term (1-h and 24-h) Spacecraft Maximal Allowable Concentrations (SMACs) for benzene were established at 10 and 3 ppm by NASA in 1996, based on a study of mice in which no hematological effects were noted following two 6-h exposures to benzene. When the benzene SMACs were updated in 2008, there was no revision to the short-term SMAC limits. Rather, that effort developed a long-term SMAC (1000-d) for Exploration mission scenarios.Acute benzene exposures can cause numerous neurological effects, and long-term exposure to low levels is well-known to cause acute myeloid leukemia. Since publication of the original benzene SMACs, the National Academy of Sciences developed interim Acute Exposure Guideline Limits (AEGLs) for unintentional releases of benzene into the air. Based on the data used to establish the AEGLs, we have increased our short-term, off-nominal limits for benzene in crewed spacecraft to 40 ppm and 6.7 ppm for 1-h and 24-h, respectively.Ryder VE, Williams ES.