Almost every major city in the world measures air pollution. There are good reasons for doing so, because air pollution can pose a major threat to people's health if levels get out of control.
Measuring air pollution is a highly complicated process. Different types of equipment have to be used to measure each potentially dangerous chemical in the atmosphere.In the United States, every urban area measures air pollution.
They are required to do so by the Clean Air Act of 1970.
Under the Clean Air Act, the Environmental Protection Agency (EPA) must enforce the National Ambient Air Quality Standards (NAAQS). These regulations set permitted levels of certain harmful pollutants. Some examples are carbon monoxide, sulfur dioxide, nitrogen oxides, and ozone.
Collecting so vast an amount of information requires many stations throughout
the country that monitor the quality of the air. This data can then be collected and tabulated for analysis by EPA scientists.
The EPA measures the data collected from around the nation against the NAAQS standards. If a pollution level exceeds the standard, the polluting area may be breaking the law and the EPA will have to take action.
Some people advocate massive monitoring and measurement efforts that would
check air pollution levels throughout the atmosphere. Some international programs are already beginning to work towards this goal.