You cannot see or smell carbon monoxide, but undetected it can kill you within minutes. The colorless, odorless gas is produced by the burning of gas, oil, kerosene, wood and charcoal. The correct use of properly working appliances does not generally produce harmless levels of carbon monoxide. Since it is common to be exposed to carbon monoxide, it's smart to know the symptoms of carbon monoxide poisoning. Low levels of exposure can cause shortness of breath, nausea and headaches, and fatigue.
Higher levels can produce severe headaches, dizziness, mental confusion, nausea and fainting.
Substantial exposure to high concentrations can even be fatal. Use vented space heaters, proper fuel in kerosene space heaters, and an exhaust fan vented to the outdoors over gas stoves. Don't sleep in rooms with unvented heaters in use. Open fireplace flues. Don't use a charcoal grill indoors, even in a fireplace.