Air Quality

PM2.5, Percentage of days exceeding US standard is the percentage of days in the 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 days exceeding US standard is the number of days in the 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, Annual Average Concentration is the average daily PM2.5 levels for the year.
Ozone, Percentage of days exceeding US standard is the percentage of days in the year in which the daily maximum 8-hour average ozone concentrations exceed the U.S. standard of 0.070 ppm.
Ozone, Number of days exceeding US standard is the number of days in the year in which the daily maximum 8-hour average ozone concentrations exceed the U.S. standard of 0.070 ppm.

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).
The map legend displays the percentage/ number of days exceeding the US standard with four class breaks, where the data is divided into four equal bins known as quartiles.

Percentage of days, the number of days for each county exceeding the national 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.

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 estimates for larger areas may be biased higher than estimates for smaller areas.

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