title: Safe human exposure limits for airborne linear siloxanes during spaceflight.
abstract: BACKGROUND
  Low molecular weight siloxanes are used in industrial processes and consumer products, and their vapors have been detected in the atmospheres of the Space Shuttle and International Space Station. Therefore, the National Aeronautics and Space Administration (NASA) developed spacecraft maximum allowable concentrations (SMACs) for siloxane vapors to protect astronaut health. Since publication of these original SMACs, new studies and new risk assessment approaches have been published that warrant re-examination of the SMACs.
  
  
  OBJECTIVE
  To reevaluate SMACs published for octamethyltrisiloxane (L3) for exposures ranging from 1 hour to 180 days, to develop a 1000-day SMAC, and to expand the applicability of those values to the family of linear siloxanes.
  
  
  METHODS
  A literature review was conducted to identify studies conducted since the SMACs for L3 were set in 1994. The updated data were reviewed to determine the sensitive toxicity endpoints, and current risk assessment approaches and methods for dosimetric adjustments were evaluated.
  
  
  RESULTS
  Recent data were used to update the original 1-hour, 24-hour, 30-day, and 180-day SMACs for L3, and a 1000-day SMAC was developed to protect crewmembers during future exploration beyond Earth orbit. Group SMACs for the linear siloxane family, including hexamethyldisiloxane (L2), L3, decamethyltetrasiloxane (L4), and dodecamethylpentasiloxane (L5), were set for exposures of 1-hour to 1000 days.
  
  
  CONCLUSIONS
  New SMACs, based on acute pulmonary and neurotoxicity at high doses only achievable with L2 and potential liver effects following longer-term exposures to L2 and L3, were established to protect crewmembers from the adverse effects of exposure to linear siloxanes.
authors.affiliation: National Aeronautics and Space Administration Johnson Space Center , Houston, TX , USA .
authors.forename: Valerie E
authors.initials: VE
authors.lastname: Meyers
authors.forename: Hector D
authors.initials: HD
authors.lastname: García
authors.forename: Tami S
authors.initials: TS
authors.lastname: McMullin
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Tobin
authors.forename: John T
authors.initials: JT
authors.lastname: James
mesh_terms: D000395:Air Pollutants, Occupational / Q000592:standards* / Q000633:toxicity; D000818:Animals; D006801:Humans; D019570:Inhalation Exposure / Q000592:standards*; D016273:Occupational Exposure / Q000592:standards*; D018570:Risk Assessment; D012833:Siloxanes / Q000592:standards* / Q000633:toxicity; D013026:Space Flight / Q000592:standards*
chemical_list: D000395:Air Pollutants, Occupational; D012833:Siloxanes