Interpreting the Pollutant Standards Index



Robert T. Edgar
Human Ecology Research Foundation of the Southwest, Dallas, Texas

The pollutant standards index (PSI) is a method of reporting ambient air quality across the United States on a uniform basis. The index is based on five pollutants for which National Ambient Air Quality Standards have been established. These pollutants are: Total suspended particulates (TSP), sulfur dioxide (SO2), carbon monoxide (CO), ozone (O3) and nitrogen dioxide (NO2). For each pollutant, a subindex is calculated from a segmented linear function that transforms ambient concentrations onto a scale extending from 0 through 500. One hundred corresponds to the primary National Ambient Air Quality Standards and 500 corresponds to the level which is expected to cause significant harm to the general population. The PSI is chosen as the maximum of the calculated subindexes.

The main purpose of the PSI is to present urban air quality data from across the nation on a consistent basis. However, the PSI for a particular day does not indicate the level of air contaminants to which an individual is exposed. The PSI can be interpreted as an indicator of meteorological conditions with which either high or low levels of air contaminants can occur.

An individual’s exposure to outdoor ambient air pollutants depends primarily upon his proximity to air contaminant sources and the prevailing meteorological conditions. The gasoline motor vehicle is the major source of carbon monoxide and nitrogen oxides in the United States. Coal combustion is the major source of particulates and sulfur dioxide. Ozone is a photochemical oxidant produced in the atmosphere by a mixture of nitrogen oxides, hydrocarbons, and particulates along with sunlight. Therefore, the closer an individual lives or works near a heavily traveled highway, the higher his exposure will be to carbon monoxide and nitrogen oxides. Depending on wind speeds, the highest concentration of ozone can occur many miles downwind from the urban center.

Meteorological conditions such as wind speed and atmospheric stability determine air contaminant concentrations downwind from a source. Inversions, usually radiational and subsidence types, act as a lid on air contaminants released into the air. Wind disperses pollutants with levels decreasing exponentially from the source. Precipitation, such as rain and snow, can remove some types of air contaminants from the atmosphere.

Since the PSI is based upon a small number of air quality monitors in an urban area, a high PSI level indicates that source levels along with meteorological conditions are favorable for the exposure of the population to high levels of air contaminants. However, since PSI levels are announced after the fact, their value as a warning to protect people from high levels of air contaminants can be questioned.