Air Quality

Definition of Indicators

  • PM2.5, Percentage of days exceeding US standard is the percentage of days in a year in which the daily PM2.5 average concentrations exceed the U.S. standard (National Ambient Air Quality Standard (NAAQS) of 35 μg/m3.
  • PM2.5, Number of person-days exceeding US standard is an estimate of the number of people exposed to high daily PM2.5 concentrations exceeding the U.S. standard (National Ambient Air Quality Standard (NAAQS) of 35 μg/m3, expressed as person-days per million.
  • PM2.5, Annual Average Concentration is the average daily PM2.5 levels.
  • Ozone, Percentage of days exceeding CA standard is the percentage of days in a year in which the daily maximum 8-hour average concentrations exceed the CA standard of 0.070 ppm.
  • Ozone, Number of person-days exceeding CA standard is an estimate of the number of people exposed to high daily maximum 8-hour average ozone exceeding the CA standard of 0.070 ppm., expressed as person-days per million.
  • California statewide measures are an average of the county measures.

How to Read Tables, Charts, and Maps

  • If a county has a value of N/A the data are not available due to the lack of air monitors in that county.
  • PM2.5 is measured in micrograms per cubic meter (μg/m3).
  • Ozone is measured in parts per million (ppm).
  • In October of 2017, the U.S. Standard changed from 0.075 ppm to 0.070 ppm. As of January 2018, we are only reporting on the CA Standard since the U.S. and CA Standards are now the same. It should be noted that while the standards are the same, the methods for calculating the state 8-hour ozone averages and national 8-hour (70 ppb) ozone averages are different.

Data Sources

  • California Air Resources Board
  • Denominators for person-days are based on estimates from the California Department of Finance.

Methods

  • Person-days of exposure, days exceeding national or state standards are multiplied by population values for each county
  • Percentage of days, the number of days for each county exceeding the national  or state standard are divided by 365.2.
  • Annual Average concentration, the average levels from each monitor (weighted by the number of samples) in a county is calculated for each year. If there is more than one monitor per county, then the means are averaged.

Limitations of the data

  • The majority of PM monitors do not take measurements every day and operate on different schedules. Thus the number of short term events (e.g. days exceeding the standard) is uncertain, and estimating measures that are representative of short term exposures over a year is complex.
  • There can be considerable variability in the number of high PM and ozone days per year so tracking trends over time needs to be done carefully.
  • For short-term measures, the monitor with the highest reading on any day is used in the measure. Larger areas will have a broader range of pollution values and perhaps more monitors that may measure a high value on a given day. Thus day and person day estimates for larger areas may be biased higher than estimates for smaller areas.

Suggested Citation

Tracking California, Public Health Institute. Air Quality data. Accessed [Month/Day/Year] from www.trackingcalifornia.org/air/query.


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