The performance of radiant and catalytic heaters fuelled by liquefied petroleum gas (LPG) in a sealed room has been evaluated. Vitiation of the atmosphere by radiant heaters produced toxic conditions by the generation of carbon monoxide. Catalytic heaters did not produce enough carbon monoxide for a toxic level to exist before the fonnation of carbon dioxide and depletion of oxygen produced hazards. Although the heaters did not create explosive atmospheres during the reported tests, and are unlikely to do so when operated in a room of a volume commensurate with the output of the heaters, the situation could be changed for catalytic heaters if their efficiency of combustion was seriously impaired, for example, by damage or deterioration after prolonged use. Heaters when operated in badly ventilated spaces can produce a risk to life. This risk can be avoided if the heater is equipped with a device which will shut down the heater when the carbon dioxide in the atmosphere reaches a specific but low concentration. Such devices should also reduce the possibility of explosive atmospheres being formed.