It is ___________ to sleep in a closed room with a coal fire burning.
Carbon mono-oxide is a colourless, odourless, non- irritating but very poisonous gas. It is produced by incomplete combustion of fuel such as natural gas, coal or wood. Vehicular exhausts are the largest source of carbon mono- oxide. It is, however, not a persistent pollutant. Natural processes convert CO to other compounds that are not harmful. In congested and closed places, it, however, proves quite harmful. Another source of CO is cigarette smoke.
So, the correct answer is 'Dangerous'.