Carbon monoxide can accidentally form when there is too little air to burn all the fuel into carbon dioxide. Such a situation may happen if the oven shutters are closed too early or if a mobile cooker is used in a small tent with no ventilation (Ventilation is fresh air coming in and smoke going out). Many people have died from carbon monoxide poisoning. Carbon monoxide poisoning can cause feelings of paranoia and hallucinations, and has been determined to be a major cause of "haunted" houses. Small amounts of it are found in coal gas, a fuel produced by heating coal without any air.