In normal conditions, there are trace contaminant gases such as ammonia from normal off-gassing at ambient temperatures and elevated temperatures. Other gases arise from the breathing gas supply reservoirs and crew members themselves. In emergencies, gases can arise from overheating, spills, a rupture in the coolant loop (ethylene glycol) and from the pyrolysis of non-metallic components. Carbon monoxide is a major concern for space crews; this was evident during the Apollo missions. The emitted trace gases can be controlled using lithium hydroxide filters to trap carbon dioxide and activated carbon filters to trap other gases.