Carbon Monoxide (CO) - Carbon monoxide is a colorless, odorless,
and (at much higher levels) poisonous gas, formed when carbon in
fuels is not burned completely. It is a product of motor vehicle
exhaust, which contributes about 60 percent of all CO emissions
nationwide. High concentrations of CO generally occur in areas
with heavy traffic congestion. In cities, as much as 95 percent
of all CO emissions may emanate from automobile exhaust. Other
sources of CO emissions include industrial processes such as
carbon black manufacturing, non-transportation fuel combustion,
and natural sources such as wildfires. Woodstoves, cooking,
cigarette smoke, and space heating are sources of CO in indoor
environments. Peak CO concentrations typically occur during the
colder months of the year when CO automotive emissions are
greater and nighttime inversion conditions are more frequent.