During operation of cooker hoods or extraction fans smoke or exhaust from open fireplaces closed fireplaces and gas heaters can be drawn back into the room this can lead to potentially fatal poisoning caused by the odourless and invisible gases carbon monoxide and carbon dioxide for this reason according to the furnace regulations m feuvo para 4 simultaneous operation of an air extractor and a flame source is only permitted if there is provision of an adequate supply of fresh air eg