Carbon monoxide is produced by the incomplete combustion of materials (in this case gas). Idealy no appliance should produce carbon monoxide only carbondioxide (and water) but if the air flow is resticted then carbonmonoxide is produced. This can happen with both gas fires and gas cookers (or indeed anywhere where something is being burnt, so no barbaques indoors!). The air flow about a gas fire is more easily restriced and therefore problems with them are more apparent. Carbon dioxide is also poisonous but much larger amounts are required and in any normal situation this should not pose a problem. Hope this helps, Hamish