Air quality is determined with respect to the total air pollution in a given area as it interacts with meteorological conditions such as humidity, temperature and wind to produce an overall atmospheric condition. Poor air quality can manifest itself aesthetically (as a displeasing odor, for example), and can also result in harm to plants, animals, people, and even damage to objects.

As early as 1881, cities such as Chicago, Illinois, and Cincinnati, Ohio, had passed laws to control some types of pollution, but it wasn't until several air pollution catastrophes occurred in the twentieth century that governments began to give more attention to air quality problems. For instance, in 1930, smog trapped in the Meuse River Valley in Belgium caused 60 deaths. Similarly, in 1948, smog was blamed for 20 deaths in Donora, Pennsylvania. Most dramatically, in 1952 a sulfur-laden fog enshrouded London for five days and caused as many as 4,000 deaths...