The EPA (Environmental Protection Agency) has many guidelines set in place the control the use of dangerous air contaminants such as asbestos, aerosols, carbon monoxide, chlorofluorocarbons, mercury, etc. It also discusses in great detail ground level ozone which is of particular concern, especially in the summer time when the temperature rises.
Ozone can be either good or bad. When it is low it is considered bad ozone. This bad ozone is caused by poor vehicle exhaust, gasoline vapors, pollutants from industry, etc. The EPA has guidelines that must be lawfully followed to help protect the environment from these dangerous pollutants. For example, some areas require that people take their vehicles in for emissions tests. Industry is also heavily fined for breaking rules set in place by the EPA.