Air Pollution Index

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Air pollution index

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