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The air pollution index is a value derived from an air quality scale which uses the measured or predicted concentrations of several criteria pollutants and other air quality indicators, such as coefficient of haze (COH) or visibility. The best known index of air pollution is the pollutant standard index (PSI).
The PSI has a scale that spans from 0 to 500. The index represents the highest value of several subindices; there is a subindex for each pollutant, or in some cases, for a product of pollutant concentrations and a product of pollutant concentrations and COH. If a pollutant is not monitored, its subindex is not used in deriving the PSI. In general, the subindex for each pollutant can be interpreted as follows
| Air Pollution Stages | |
| Index Value | Interpretations |
| 0 | No concentration |
| 100 | National Ambient Air Quality Standard |
| 200 | Alert |
| 300 | Warning |
| 400 | Emergency |
| 500 | Significant harm |
The subindex of each pollutant or pollutant product is derived from a PSI nomogram which matches concentrations with subindex values. The highest subindex value becomes the PSI. The PSI has five health-related categories:
| PSI Range | Category |
| 0 to 50 | Good |
| 50 to 100 | Moderate |
| 100 to 200 | Unhealthful |
| 200 to 300 | Very unhealthful |
| 300 to 500 | Hazardous |
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This section contains 191 words (approx. 1 page at 300 words per page) |
