Safe human exposure limits for airborne linear siloxanes during spaceflight

Abstract

BackgroundLow 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.ObjectiveTo 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.MethodsA 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.ResultsRecent 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.ConclusionNew 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.

The inhibitory effect of Mesembryanthemum edule (L.) bolus essential oil on some pathogenic fungal isolates

“…The use of some of these chemical compounds has been well studied. Compounds such as Tetra-cosamethylcyclo-dodecasiloxanes and Tetra-decamethylcyclo-heptasiloxane that ends with ‘siloxanes’ belongs to the wider class of organosilicon [ 21 ]. These compounds are made up of both organic and inorganic chemical compounds comprised of silicon, oxygen, carbon and hydrogen [ 22 ].…”

“…These compounds are made up of both organic and inorganic chemical compounds comprised of silicon, oxygen, carbon and hydrogen [ 22 ]. Siloxanes are commonly used in the cosmetic industries to produce deodorants, sunblocks, hairsprays and skincare [ 21 ]. In addition, siloxanes are an important product in the cook ware industry and kitchen utensils [ 21 ].…”

“…Siloxanes are commonly used in the cosmetic industries to produce deodorants, sunblocks, hairsprays and skincare [ 21 ]. In addition, siloxanes are an important product in the cook ware industry and kitchen utensils [ 21 ]. They are also used as effective industrial cleaning agents and in dry cleaning industries.…”

Environmental Chemistry of Organosiloxanes

“…For several dimethylsiloxanes such as Me 2 Si­(OMe) 2 , viSi­(OMe) 3 (vi = vinyl), MM, M vi M vi , L 3 , MD H M, M 4 Q, D 3 , D 6 , initial assessment profiles are available in the OECD Existing Chemicals Database, and for some of these detailed dossiers, for L 3 and M 4 Q, see also refs , . Several unpublished studies on the inhalation toxicity of L 2 –L 4 were reviewed and used to derive 1-h to 1000-day spacecraft maximum allowable air concentrations for the group of linear dimethylsiloxanes …”

High-Magnification SEM Micrograph of Siloxanes

“…HMDS, the initiator molecule, is converted to siloxanes. As is visible, regular geometric structures vary from HMDS[18] molecules. It is possible that the foliar structures present the linear form of siloxanes molecule[14] and the cyclic form of siloxane molecule, as was shown inFigure 5.…”