Carbon Monoxide (CO) is a colorless, odorless gas resulting from the incomplete combustion of fuels. Higher levels of CO occur in more urbanized areas, such as Nashua and Manchester, where there is heavy, congested motor vehicle traffic. CO levels have improved significantly since the 1980s in Manchester and Nashua due to federal tailpipe standards for new automobiles, traffic flow improvements in the two cities, and in Nashua, a required annual emissions inspection program for most motor vehicles between 1986 and 1994.