these end points were reviewed and are incorporated in this reassessment. Any other relevant end points for which data emerged since publication of Volume 3 were addressed as they were identified. Another important aspect of this review is the need to set 1,000-d SMACs for longer-term crew exposures. Longer exposures would be of interest in assessing possible lengthy missions on the International Space Station (ISS) or in future lunar or Martian exploration efforts.
The intent of this reassessment is not to revisit all the studies that were considered in the original Volume 3 chapter. Much of the foundation for this review (e.g., discussion on metabolism, rationale for not addressing fetal risks) was provided in Volume 3; thus, these two write-ups should be viewed as complementary.
Volume 3 was written before NASA’s full involvement with ISS and thus does not contain information on the relevance of ethanol in that environment or on monitored concentrations of ethanol in the ISS atmosphere. As a compound potentially used in a variety of spacecraft applications, ethanol can be introduced to the ISS atmosphere through many sources, including cleaning products such as alcohol wipes, payloads, substances in medical kits, and crew hygiene products. Other possible contributors to consider are the small amounts of ethanol that can be formed and released endogenously by humans. Ethanol is generally monitored (near-instantaneous readings) around 5-8 milligrams per cubic meter (mg/m3) in the ISS atmosphere, although measurements have approached 20 mg/m3 on occasion.
Because of its extreme solubility in water, a main concern with ethanol in the ISS atmosphere is its potential to affect the processing of humidity condensate by the Russian water processing system. Recycled humidity condensate provides a significant percentage of potable water on ISS (50%+). Ethanol is a common organic component of ISS condensate, being measured at concentrations as high as 156 mg/liter (L), with average ethanol concentrations around 50-55 mg/L. For reasons that are not fully understood, U.S. Laboratory condensate frequently contains higher ethanol concentrations than condensate from the Russian Service Module (Figure 11-1).
As these ethanol concentrations can double the system design limit for the Russian processing equipment (80 mg/L), significant efforts have been made to identify and limit releases of ethanol and other volatiles to the ISS atmosphere. The presence of excess volatiles in humidity condensate can affect the processing system in several ways. As the Russian system includes an oxidizing reactor and downstream multifiltration beds, system resources may be spent in oxidizing and removing relatively low-toxicity liquid components (e.g., ethanol). This has operational impacts for these limited life items and can impair performance when the system is also challenged with more toxic organic compounds.