Air Quality Indicators
The Tracking Network includes data about ozone and particulate matter (PM2.5).
- Ozone - Days Above Regulatory Standard: The number of days in which the daily maximum 8-hour average ozone concentration exceeds a standard provides an indication of short-term spikes in ozone concentrations. This may give you an idea of how many days per year you may be exposed to unhealthy levels of ozone.
- PM2.5 - Days Above Regulatory Standard: These data help summarize short-term trends in particle pollution concentrations. This may give you an idea of how many days per year you may be exposed to unhealthy levels of particulate matter.
- Annual PM2.5 Level: These data help summarize long-term trends in particle pollution concentrations. This will give you an idea of what the yearly level of PM2.5 is in an area.
Monitor + Model Air Data
Air monitoring in the United States is conducted by many federal, state, local, and tribal air agencies. The Environmental Protection Agency (EPA) provides air pollution data about ozone and particulate matter (PM2.5) to CDC for the Tracking Network. The EPA maintains a database called the Air Quality System (AQS) which contains data from approximately 4,000 monitoring stations around the country, mainly in urban areas. Data from the AQS is considered the "gold standard" for determining outdoor air pollution. However, AQS data are limited because the monitoring stations are usually in urban areas or cities and because they only take air samples for some air pollutants every three days or during times of the year when air pollution is very high.
CDC and EPA have worked together to develop a statistical model (Hierarchical Bayesian) to make modeled predictions available for environmental public health tracking purposes in areas of the country that do not have monitors and to fill in the time gaps when monitors may not be recording data.
There are two primary benefits to creating modeled air pollution data:
- Approximately 20% of counties in the United States have actual air monitors. With modeled data, the Tracking Network is able to create indicators for counties that do not have monitors (excluding Alaska and Hawaii);
- Most PM2.5 air monitors take samples every three days and many ozone monitors sample only during the ozone season. Modeled data helps to fill in these time gaps.
After careful study, EPA and CDC found that air pollution modeled predictions are very similar to actual monitor data in areas where the two can be compared. In some areas, the modeled data underestimates or overestimates the air pollutant concentration levels when compared to AQS monitoring data. Therefore, the best way to use modeled air data is in conjunction with actual monitoring data. On the Tracking Network, both AQS and modeled datasets are available to track possible exposures to ozone and PM2.5, evaluate health impact, conduct analytical studies linking health effects and the environment, and guide public health actions.