Carbon Monoxide is a toxic gas, but, being colorless, odorless, tasteless, and non-irritating, it is very hard to detect. Carbon Monoxide is a product of incomplete combustion of gasoline, natural gas and propane or improper ventilation of these gases.  Symptoms may include headaches and flu like effects; larger exposures can lead to significant toxicity of the central nervous system. (you may have trouble keeping your balance or walking). Prolonged exposure can even cause death.  There is only one way to detect carbon monoxide and that is with a UL listed detector.  There are several of these available to choose from at your local retail stores.  The electric with battery backup and digital read out is one of the most preferred.