Carbon monoxide is a tastless, odorless, and invisible gas. It is often around us in fairly high concentrations. Carbon monoxide can be formed when gasoline is burned without enough oxygen. 75 percent of CO emissions are from automobiles. Cigarette smoke also releases fairly high concentrations. The U.S. governement has set a danger level for CO at 9 ppm over 8 hr and 35 ppm over 1 hr. If you take a reading in many urban areas of the U.S. you will get a reading between 7 and 8 ppm. If you are close to the street then these levels are exceeded much of the time.

CO binds to hemoglobin and interferes with its normal function of transporting O2. The result is drowsiness. It also forces your heart to work harder as it requires more effort to get an oxygen supply. Chronic exposure to high levels of CO may result in physical and mental impairment.

It might be an instructive science project to measure CO levels in your neighborhood. Or, go to a busy street with your carbon monoxide detector and see if it goes off :-) TSI Inc. offers a Q-Trak device which monitors CO and CO2 levels. The Beijing 2008 Olympics used TSI Inc's services to monitor air quality during the Olympics but I'm not aware of the results.