Combustion of fossil fuels produces avariety of contaminants, many of which can be hazardous to health. Mostcombustion equipment in buildings vents these contaminants directly tooutdoors, but products exist that release the contaminants directly to theindoor air. This article reviews the relevant combustion process, thecontaminants and their acceptable levels in the context of ASHRAE standards andposition documents, with particular emphasis on unvented residential spaceheaters.