Air Quality Metadata: Particulate Matter (PM2.5)


Air quality data are from the US Centers for Disease Control and Prevention (CDC) and U.S. Environmental Protection Agency (EPA); population data are from the US Census Bureau.

Dates Available:

1999 - 2014

Geographic Resolution:



This dataset contains the annual average concentration of particulate matter (PM2.5) and the percent of days with daily average particulate matter (PM2.5) concentration over the U.S. standard (National Ambient Air Quality Standard, NAAQS) of 35.0 micrograms per cubic meter (ug/m3), by year and county. The dataset also contains the annual number of person-days over the standard, per year and by county, based on the total county population multiplied by the annual number of days over the standard. The CDC and EPA worked together to develop a statistical model, called Downscaler, that combines predictive air pollutant models with measurements from continuous air monitors.

The dataset contains the following measures:

  1. Annual average concentration of particulate matter (PM2.5) in micrograms per cubic meter. Monitor only measures, 1999-2014; Downscaler measures, Monitor + Modeled, 2001-2011.
  2. Annual percent (%) of days with daily average PM2.5 concentration over 35.0 micrograms per cubic meter. Monitor only measures, 1999-2014; and Downscaler measures, Monitor + Modeled, 2001-2011.
  3. Annual number of person-days with daily average PM2.5 concentration over 35.0 micrograms per cubic meter. Monitor only measures, 1999-2014; and Downscaler measures, Monitor + Modeled, 2001-2011.


This data set supports efforts to improve public health in Maine and contributes to the U.S. CDC’s National Environmental Public Health Tracking (EPHT) Network. A key activity of participants in this network is to track and make available environmental health measures on state and national data portals. Measures derived from the data set described here can be used to compare levels of ambient particulate matter across the state, between groups of people, over time, and in relation to other risk factors, exposures, and health outcomes.

The Maine Tracking Network, a member of the National EPHT Network, connects communities, public health professionals, policy makers, state agencies, and others to the data they need to monitor public health, respond to health concerns, prioritize resources for public health action, and evaluate prevention activities. Maine tracks certain health effects, exposures, and environmental hazards that have known relationships, as well as some health effects and environmental hazards that have suspected relationships. By making health and environmental data available through the Maine Tracking Network, more people have access to data they need to think critically and hypothesize about health outcomes and their relationships to conditions in the environment.

Supplemental Information:

Particle pollution, or particulate matter, consists of particles that are in the air, including dust, dirt, soot and smoke, and little drops of liquid. Some particles, such as soot or smoke, are large or dark enough to be seen. Other particles are so small that you cannot see them.

Particle pollution can come from primary or secondary sources. A primary source, such as wood stoves or forest fires, lets off particle pollution directly. A secondary source lets off gases that react and form particles. Examples of secondary sources are coal fires and power plants. Particle pollution also comes from motor vehicles, factories, and construction sites. These can be primary or secondary sources. Particle pollution can be a problem at different times of the year, depending on where you live.

Particle pollution includes:

  • coarse particles that are between 2.5 and 10 micrometers,
  • fine particles that are between 0.1 micrometers and 2.5 micrometers; also known as PM2.5, and
  • ultrafine particles that are smaller than 0.1 micrometers.

Particles bigger than 10 micrometers can irritate your eyes, nose, and throat but do not usually reach your lungs. Ten micrometers is about seven times thinner than one human hair.

Fine and ultrafine particles are the most concerning because they are most likely to cause health problems. Their small size allows them to get into the deep part of your lungs and even into your blood.

Entity and Attribute Overview:

This dataset includes: county, year, state,average annual concentration of particulate matter (PM2.5), percent of days wth daily average above 35.0 micrograms per cubic meter (ug/m3), percent of days above same threshold, and person-days above same threshold (population in county x days above). 

Data Limitations:

  • The measures provide a general indication of the overall trend in annual particulate matter (PM2.5) concentrations; they do not directly reflect personal exposure. The relationship between ambient concentrations and personal exposure is subject to many factors, including individual activity patterns and microenvironments.
  • Air monitors provide concentration data around the specific location of each monitor. Intra-county variation in concentrations will likely exist but cannot be captured in this measure.
  • The Downscaler model (“Monitor + Modeled”) is used to fill in air quality estimates in areas and time periods without monitoring data, but for counties without any monitors, temporal (seasonal) and spatial (regional) factors can lead to systematic under- or over-prediction. 

Access Constraints:


More Information:

Suggested Citation for Data Displays:

Maine Center for Disease Control and Prevention, Maine Tracking Network. Air Quality: Particulate Matter (PM2.5). Available online: Accessed on [Date].