Pollutant Standards Index

   The Pollutant Standards Index (PSI) provides a nationally uniform method for reporting air quality as related to health impacts. PSI values are calculated on a pollutant-specific basis and are reported daily. The daily PSI value reported is the highest pollutant-specific index noted for that day, at any monitoring site, for any criteria pollutant, within the area identified. A numerical scale for the PSI ranges from 0 to 500 with five different intervals or health categories as identified below:

   A pollutant-specific PSI reading of 100 is equivalent to the federal health standard for that pollutant. The two categories above a PSI value of 199 correspond to the first two air pollution episode

levels as defined in the Washington Episode Avoidance Plan-Alert and Warning.
    Trends. The first bar graph shows the trend of the number of days per year since 1985 with a PSI greater than 100-unhealthful or worse. No exceedances of health standards were recorded anywhere in our four-county area in 1996. In 1994, ozone monitoring recorded two exceedances in Enumclaw and one in LaGrande (Pack Forest).
    The subsequent three bar graphs show trends since 1985 of days with a PSI of 50 or less-good, or the healthiest air quality. The trend shows increasing numbers of good days per year over the past eight to 10 years.
    Since there were no measured exceedances of the health-based standards in 1996, all measured PSI values were either good or moderate. PSI readings in the good range are now so predominant that even moderate levels are becoming infrequent to rare.