Air Quality Standards

Here you will find information about:

• Why do we measure?
• What are the standards?
• What do the numbers mean? The Pollutant Standard Index
• Where do we measure? Map of monitoring sites

Why do we measure?

Our agency, in cooperation with the Washington State Department of Ecology, monitors air quality in our four-county region.  Our primary concern and responsibility is for the health and welfare of humans, animal and plant life. In the process of assuring the health of the air we breathe, we also monitor our performance in terms of complying with federal and state air quality standards.

What are the standards?

There are three terms that describe an area's air quality with relation to the federal standards for our primary pollutants -- attainment, exceedance and violation.  An exceedance of the  federal standard occurs when a monitoring station records a higher level than allowed by the standard during the time frame average for a specific pollutant.  For example, if an ozone monitoring station measures a one-hour concentration higher than allowed, we have an exceedance.  For particulate matter (PM₁₀), the standard is a 24-hour average.  There is a scientific basis to the different averages.  Some pollutants are deemed more hazardous at lower, prolonged levels, while others are more hazardous at short-term peak concentrations.

A violation of a standard occurs when an individual monitoring site measures more exceedances than allowed during a specific time frame.  In the case of ozone, we are allowed three exceedances within a three year period.  For example, if we were to register four exceedances at the same monitoring site in three years, we would be in violation of the standard. 

The Puget Sound area is currently classified as an "attainment area" for carbon monoxide and ozone.  We are currently working on gaining "attainment" status for particulate matter.

The following table shows the national, state and regional standards for our primary pollutants.  In the case of pollutant gases, the standards are measured in parts of pollution per million parts of air.  Particulate matter and lead are measured by weight, in micrograms per cubic meter.

*These new standards were adopted into Federal law in 1997 and therefore apply to the Puget Sound region. They are soon to be formally adopted by PSAPCA's Board of Directors.

Notes:

ppm = parts per million by volume
µg/m3 = micrograms per cubic meter

a attained when the 3-year average of the annual arithmetic mean concentration, as determined in accordance with 40 CFR Part 50, Appendix N, is less than or equal to 50 µg/m3
b never to be exceeded
c attained when the 3-year average of the 99^th percentile 24-hour average concentration, as determined in accordance with 40 CFR Part 50, Appendix N, is less than or equal to 150 µg/m³
d not to be exceeded more than once per year
e attained when the 3-year average of the annual fourth-highest daily maximum 8-hour average concentration is less than or equal to 0.08 ppm, as determined in accordance with 40 CFR Part 50, Appendix I
f not to be exceeded more than twice in 7 consecutive days
g attained when the 3-year average of the annual arithmetic mean concentration, as determined in accordance with 40 CFR Part 50, Appendix N, is less than or equal to 15.0 µg/m3
h attained when the 3-year average of the 98^th percentile 24-hour average concentration, as determined in accordance with 40 CFR Part 50, Appendix N, is less than or equal to 65 µg/m³

What do the numbers mean? The Pollutant Standard Index

Even though pollution is measured in parts per million or micrograms per cubic meter, these numbers can be difficult to grasp. When was the last time you saw a microgram (or millionth of a gram) of anything?  However, these pollutants all have real health effects, even if we can't see them.

To make it easier to understand pollution's health effects, the Pollutant Standard Index (PSI) was developed by the U.S. EPA.  We also began using the index in the late 70s to standarize air pollution reporting and to allow for air quality comparisons between different pollutants and geographical areas. The PSI converts air pollution concentrations to simple values ranging from zero to 500. The PSI values are graded into levels, which are identified by descriptors relevant to human health. The index value of 100 was selected to correspond to the short-term (24-hour or less) standard set by EPA. This standard represents the highest concentration of pollution we consider to be healthy for most people.  Adverse health effects have not been proven below this level.

Our agency converts measurement data (in values shown above) to the PSI index for the three most common daily pollutants. We update this web site weekdays with the latest PSI readings for the Puget Sound area. Weekday readings are available in many newspapers, near the weather forecast.

Updated December 1996

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