are positioned in the avionic bays, and three portable extinguishers are located in the crew module; the fixed and portable extinguisher tanks each contain 1.73 and 1.25 kg of Halon 1301, respectively (M. Hoy, NASA Fire Detection and Suppression Branch, Crew and Thermal Systems Division, personal commun., 1993). Halon 1301 has been detected, probably from small leaks in the tanks, in the in-flight air samples in about two-thirds of the shuttle missions flown to date. Detected concentrations ranged from 2.3 to 77 mg/m³ for the first 24 missions (Coleman, 1988); in more recent missions (STS-26 to 52, missions after the Challenger accident), concentrations were generally below 10 mg/m³, and half of those were below 1 mg/m³. None of the extinguisher tanks has ever been discharged during a mission. However, in the event of a fire or a false alarm in the avionics bay, all three tanks could be discharged, resulting in a cabin concentration of Halon 1301 at 1% (10,000 ppm, 61,000 mg/m³). This low concentration of halogenated methane cannot be removed effectively by activated charcoal and would remain in the confined environment of the spacecraft. The crew could be exposed to Halon 1301 for up to 24 h before the shuttle could return safely to earth.

Current environmental policy in the United States calls for phasing out chlorofluorocarbons (CFCs). Although the National Aeronautics and Space Administration (NASA) is phasing out noncritical uses of CFCs, Halon 1301 will continue to be used in the shuttle fire-suppression system. It will not be used in the space station, because the air-revitalization system planned for the station could not remove this compound efficiently (M. Hoy, NASA Fire Detection and Suppression Branch, Crew and Thermal Systems Division, personal commun., 1993). However, if Halon 1301 is discharged in the shuttle while it is docked with the station, the gas could diffuse into the station, and crews could be exposed to low concentrations of Halon 1301 for extended periods during their tours of duty.

An inhalation study sponsored by NASA was conducted with eight pairs of human subjects exposed to 1% (10,000 ppm) Halon 1301 for