pollutant standards index

A scale devised by the U.S. Environmental Protection Agency to provide a way for broadcasts and newspapers to report air quality on a daily basis.

The index is based on measurements of the concentrations of five pollutants: particulate matter, sulfur dioxide, carbon monoxide, nitrogen dioxide, and ozone. Air quality standards have been developed for each of these. For each pollutant, a value of 100 is assigned to the maximum permitted concentration of that pollutant. After determining a value for each of the five pollutants, the highest of the five numbers is reported as the pollutant standards index for the day. Standard verbal descriptions for various ranges have been specified.

Index Value PSI Descriptor General Health Effects Cautionary Statements
Up to 50 Good None for the general population. None required.
50 to 100 Moderate Few or none for the general population. None required.
100 to 200 Unhealthful Mild aggravation of symptoms among susceptible people, with irritation symptoms in the healthy population. Persons with existing heart or respiratory ailments should reduce physical exertion and outdoor activity. General population should reduce vigorous outdoor activity.
200 to 300 Very Unhealthful Significant aggravation of symptoms and decreased exercise tolerance in persons with heart or lung disease; widespread symptoms in the healthy population. Elderly and persons with existing heart or lung disease should stay indoors and reduce physical activity. General population should avoid vigorous outdoor activity.
Over 300 Hazardous Early onset of certain diseases in addition to significant aggravation of symptoms and decreased exercise tolerance in healthy persons. At PSI levels above 400, premature death of ill and elderly persons may result. Healthy people experience adverse symptoms that affect normal activity. Elderly and persons with existing diseases should stay indoors and avoid physical exertion. At PSI levels above 400, general population should avoid outdoor activity. All people should remain indoors, keeping windows and doors closed, and minimize physical exertion.

“A PSI level of 400 or above would constitute an ‘Emergency,’ and would require a cessation of most industrial and commercial activity, plus a prohibition of almost all private use of motor vehicles.”

U.S. Environmental Protection Agency, Office of Air and Radiation.
Measuring Air Quality. The Pollutant Standards Index.
EPA 451/K-94-001

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