Abstract: The explosibility of an atmosphere which may contain a combustible gas or vapor or mist or aerosol is sensed by drawing a known volume of the atmosphere through a filter (14) and into a container (16); in a safe environment drawing the sample through the filter which is heated; completely oxidizing any combustible material present; and sensing the quantity of oxygen required for complete combustion. The concentration of combustible material as a percentage of the lower explosive limit can be approximately determined even when the combustible material is unknown.