applications, formaldehyde is a common combustion byproduct and can be measured at low concentrations in almost all ambient air samples. Although it is often thought of as strictly an environmental pollutant, formaldehyde is a normal metabolic product formed endogenously from the breakdown of serine (Restani and Galli 1991) and serves as an intermediary cellular metabolite in the biosynthesis of purines, thymidine, and several amino acids (ATSDR 1999). Formaldehyde is also a normal byproduct from the metabolism of many N-methyl substituted drugs and other xenobiotics (Dahl and Hadley 1983). Endogenous concentrations of formaldehyde in blood are estimated at 2.6 micrograms per gram (µg/g) for humans (Heck et al. 1985). Dietary exposure is also relevant because formaldehyde is found naturally (and even as a food additive) in some animal products, fruits, vegetables, cheeses, seafood, and other commodities (Restani and Galli 1991).

Formaldehyde is a common contaminant that may be discharged to the spacecraft environment from both direct and indirect sources. Its occurrence in water is also closely tied to releases to air, thus providing a good example of the interdependency of these media onboard the space shuttle and International Space Station (ISS). Formaldehyde is occasionally detected, although at relatively low concentrations (1-3 µg per liter [L]), in drinking water provided to the ISS from ground-based sources (Schultz 2004). However, much higher concentrations of formaldehyde (up to 9,000 µg/L) have been measured in the humidity condensate onboard the ISS (Schultz 2004).

The relatively high concentrations of formaldehyde in this condensate are largely attributable to the number of sources that may release formaldehyde, as well as to its chemical and physical properties. Formaldehyde is one of the most common indoor air pollutants. Formaldehyde can be off-gassed from textiles, foam insulation, resins, epoxies, and a myriad of other substances commonly encountered in the indoor environment (both ground based and in orbit). Furthermore, formaldehyde can be formed through secondary reactions of other indoor air pollutants (for example, methane, pinene), especially in the presence of higher temperatures and/or chemical oxidizers. Studies by the National Aeronautics and Space Administration (NASA) have frequently observed formaldehyde releases from Delrin and other commonly used industrial materials (James 2004). Once present in the air, the high water solubility of formaldehyde relative to its vapor pressure (a relationship expressed by its Henry’s law constant) results in a significant removal of formaldehyde from the air by condensing moisture. Given this interdependency, exposure to formaldehyde through drinking water ingestion is a relevant exposure pathway on-orbit, despite ground-based experiences that suggest



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