Carbon monoxide (CO) is an odorless, colorless, and tasteless gas that is lethal. Auto exhaust is one common source, but CO can result from the combustion of almost any material containing carbon. The molecules bond to hemoglobin in the blood, preventing the hemoglobin from transporting oxygen through the body as it normally does. Depriving organs and other tissues of oxygen can result in death within minutes. The early symptoms of carbon monoxide poisoning, however, include drowsiness, disorientation, and headaches.

In well-ventilated areas, carbon monoxide poisoning should not be a problem, but in closed-in areas, such as a garage, it is hazardous. This is why you should never leave your car running inside a garage. But carbon monoxide can also come from clogged chimneys, unvented space heaters, gas appliances, grills, and lawn mowers. Homes should be equipped with carbon monoxide monitors as a precaution.

While carbon monoxide poisoning is more likely inside a home or garage than outdoors, this pollutant can be a problem in large urban areas. In 1995, for example, there was a strong temperature inversion in the city of Chicago that caused carbon monoxide levels to be pushed toward the ground, rather than dissipating. The toxic gas then found its way into some homes.