The U.S. Environmental Protection Agency (EPA) developed the Air Quality Index as a measure to better advise citizens of pollutants in the atmosphere. To calculate the index, the EPA takes into account levels of carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide. Each pollutant is measured in parts per billion and compared to an acceptable standard over a certain period of time (24 hours for most pollutants, but ozone is calculated over an eight-hour period). This number is then multiplied by 100 to give the Air Quality Index. In other words, the formula would be (pollutant concentration)/(pollutant goal concentration) X 100 = Air Quality Index. The table below explains the different categories in the index.