The U.S. Environmental Protection Agency has recommended a daily air pollution index for use by State and local air pollution control agencies--the Pollutant Standards Index (PSI). The new index makes use of segmented linear functions which convert measured concentrations of each air pollutant into a normalized number. The overall index is reported for the pollutant with the maximum subindex value, or the critical pollutant. To evaluate the index during its design phases, a computer analysis was undertaken using air quality data from selected cities. This analysis examined the statistical properties of the new index, the time series of index values, and the frequency distribution of descriptor words and critical pollutants. The index was found to be an effective tool for compactly representing a year of air quality data at any air monitoring station. It showed sufficient day-to-day variation to give useful information to the public on daily air quality levels. This paper provides sufficient technical detail for the engineer who wishes to apply the index on a large-scale computer, minicomputer, or hand calculator.