Carbon Monoxide (CO) is a colorless, odorless gas formed when carbon in fuel is not burned completely (incomplete combustion). Vehicle exhaust contributes about 60 percent of all CO emissions in the United States. Construction equipment and boats contribute about 22 percent of all CO emissions nationwide. Higher levels of CO generally occur in areas with heavy traffic congestion. In cities, 95 percent of all CO emissions may come from motor vehicle exhaust. Other sources of CO emissions include industrial processes (such as metals processing and chemical manufacturing), residential wood burning, and natural sources such as forest fires. Woodstoves, gas stoves, cigarette smoke, and unvented gas and kerosene space heaters are sources of CO indoors. The highest levels of CO in the outside air typically occur during the colder months of the year when CO automotive emissions are greater and nighttime inversion conditions are more frequent. In inversion conditions the air pollution becomes trapped near the ground beneath a layer of warm air. Overexposure to carbon monoxide can cause asphyxiation and death.