Discusses toxic and flammable gases and vapours that lead to hazards in buildings. Examines trends in accidental deaths in the home in England and Wales from gas poisoning. Discusses influence of buoyancy on the dilution by ventilation air of accidental leaks of toxic and flammable gases and shows where buoyancy dominates layers can readily form. Presents theoretical results for controlling gas hazards in buildings by ventilation for a wide range of practical situations. Suggests new technique for measuring ventilation, mainly for mechanically ventilated buildings with appreciable adventitious ventilation. This uses a steady release of tracer gas and enables both the mechanical ventilation output rate and the adventitious input rate to be determined from a single set of measurements.