The monitoring of trace pollutants in spacecraft air is essential to protect the crew from harmful exposures. Monitoring requirements are focused on those sources of pollutants that pose the highest risk to crew health. Deciding which sources pose the greatest risk is done based on years of experience with the Space Shuttle, and more recently with the Russian Mir space station. Combustion of nonmetallic materials associated with electric circuits or heat-generating devices poses the greatest risk to crew health. Major leaks of fluids from systems or payloads also pose a significant risk. Other potential risks include accumulation of metabolites, entry of propellants, and excess offgassing, especially in modules that have been sealed for long periods.