An air bag inflator for inflating an air bag includes a housing for generating gas having only two walled chambers, including a centrally disposed ignition chamber and an outer chamber in coaxial relation therewith. The outer chamber provides a dual function of containing a quantity of gas generating material in an inner portion thereof and holding a filter for cooling and collecting contaminants from the generated gas in an outer portion thereof. The filter is mounted with an inlet face of substantial area in direct contact with and surrounding a body of gas generating material and no wall is provided between the filter and the generating material.