If people really want further improvements in air quality, their automobiles are going to be affected. Cars are among the last targets from which significant additional reductions in air emissions can be wrung.
Automobiles now account for as much as half of total air-pollution emissions in many U.S. cities.
These aggregate figures obscure a critical detail. It is these so-called gross polluters that states are trying to target with their new smog check programs. The idea is to identify, through vehicle inspections, those cars that are really dirty and then require that they be repaired to reduce their emissions. Gross polluters are not just all those smoking old junkers easily identified by make, model, and year. Untuned or mechanically ailing newer cars can also be gross polluters.