The automobile is a primary source of hazardous air pollution.  This is especially true in urban areas where photochemical smog, produced by cars, is a major pollutant.  Emissions of carbon monoxide are also dangerous.

Some automotive engines produce more harmful pollutants than others.  For instance, smoke from diesel engines can easily pollute the air.  Also, soot from such engines has been found to contain carcinogenic material.  Many of these are polycyclic aromatic hydrocarbons, or PAHs, e.g. benzo-a-pyrene, which has been linked to cancer for over one hundred years. 

Diesel emissions are thought to be more dangerous than those from most gasoline engines.  However, gas emissions can be carcinogenic as well.  Benzene is the most prevalent of these emissions.  The compound toluene, from gas engines, causes photochemical smog.  Dioxin, an extremely dangerous compound, may also be present in auto emissions.

Chemicals added to improve fuel performance can also result in dangerous emissions.  One example of this is tetraethyl lead, which sends lead particles into the air.  These can cause a range of human health problems, including lowered IQs.  However, this problem is beginning to be solved with the increasing use of unleaded gasoline.

There are auto emissions from sources other than the engine.  Volatile materials can evaporate from the crank case, carburation system, and fuel tank.  These materials release hydrocarbons.  The wearing down of car parts, such as brake linings, can also release chemical particles, such as asbestos.  Malfunctioning auto parts pose further problems, including that of carbon monoxide, from the exhaust systems; this can cause drowsiness and impair judgment.

Cars also emit carbon dioxide to the air, thus enhancing the greenhouse effect.  In addition, nitrogen oxide emissions can become nitric acid and thereby contribute to rainfall acidity.  Finally, diesel-powered cars can add to sulfur compounds to the atmosphere.

The Health Effects Institute on auto pollution