Carbon monoxide (CO) is a colorless, odorless, tasteless gas. CO results from incomplete combustion of fuel and is emitted directly from vehicle tailpipes. For more general information about carbon monoxide, please read EPA’s Carbon Monoxide Information page.
CO can cause harmful health effects by reducing oxygen delivery to the body’s organs and vital tissues, affecting the cardiovascular and nervous systems. Symptoms include dizziness, headaches, nausea, fatigue, memory loss, visual impairment, and decreased muscular control. At extremely high levels, CO can cause death.
CO is released when engines burn fossil fuels; this includes automobiles, buses, trucks, lawnmowers, snow blowers, power generators, etc. Other sources include furnaces, gas boilers, some industrial processes and wood stoves.
High concentrations can be found in confined spaces like parking garages, poorly ventilated tunnels, or traffic intersections, especially during peak traffic hours. The highest levels of CO tend to be localized near roadways in urbanized areas, such as Nashua and Manchester in New Hampshire. Please see EPA’s Air Emissions Sources page for more about CO sources.
Attainment Status and Trends
Recent improvements in motor vehicle technology and more rigorous inspection programs have achieved major success in reducing CO emissions nationwide. Modern passenger cars are capable of emitting 90 percent less CO over their lifetimes than their uncontrolled counterparts of the 1960s. As a result, airborne CO levels have dropped, despite large increases in the number of vehicles on the road and the number of miles they travel.