Gasoline engines are fueled with two chemical compounds, gasoline and air. Basically, gasoline is made of two elements, hydrogen (H2) and carbon (C). In the form of gasoline, these two chemicals combine to make what we call a hydrocarbon (HC). Air is made up of basically two elements, oxygen (02) and nitrogen (N2).

If gasoline engines burned their fuel as efficiently as possible, they would produce three by-products: water vapor (H2O), carbon dioxide (CO2) and nitrogen (N2).

For the most part, none of these chemical compounds are harmful; however, environmentalists argue that excessive amounts of carbon dioxide promote the formation of the greenhouse effect. Nonetheless, H2O, CO2 and N2 are the most desirable by-products of combustion, and automotive engineers strive to create emission control systems that allow a vehicle to produce only these three chemical groups.

Unfortunately, engines do not run perfectly, and as a result they also produce three by-products commonly referred to as the "terrible trio" of automotive pollutants. This trio includes the following:

• Carbon monoxide (CO) - is an odorless, tasteless, poisonous gas that can cause a variety of health problems and even death. Many urban areas experience critically high levels of carbon monoxide, especially during the cold winter months when engines take longer to warm up and run cleanly.
• Unburned hydrocarbons (HC) - cause a variety of respiratory problems, crop damage, and promote the formation of smog.
• Oxides of nitrogen (NOx) - like unburned hydrocarbons, cause respiratory problems, and promote the formation of smog.