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Massachusetts Department of Public Health seal Massachusetts Environmental Public Health Tracking

Fine Particles (PM2.5)

Picture of three-lane highway congested with cars and trucks, which pertains to outdoor air quality indicators for fine particulate matter.

"Particulate matter," also known as particle pollution or PM, is a complex mixture of extremely small particles and liquid droplets made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, soil or dust particles, and allergens (such as fragments of pollen or mold spores).The size of particles is directly linked to their potential for causing health problems. Particulate matter less than 10 micrometers in diameter pose the greatest problems because these particles can deposit deep into the lungs.

The National Ambient Air Quality Standards (NAAQS) listed below for particulate matter address two categories of particle pollution: "inhalable coarse particles" (PM10) and "fine particles" (PM2.5). Inhalable coarse particles (PM10) are those that are larger than 2.5 micrometers but are 10 micrometers in diameter or less (PM10) and is associated primarily with road dust. Fine particles are those that are 2.5 micrometers in diameter or less (PM2.5) and are generated from residential wood burning, road dust, and fossil fuel combustion. Even though local sources (such as wood smoke emissions) contribute to PM2.5 concentrations, statewide levels are frequently influenced by the transport of these fine particulates from out of state sources, such as ongoing forest fires many miles upwind.

Pollutant Measure Standards
Pollutant Averaging Time NAAQS
Pollutant PM10 Averaging Time 24-hour NAAQS 150 µg/m3
Pollutant PM2.5 Averaging Time Annual Arithmetic Mean NAAQS 12 µg/m3
Pollutant PM2.5 Averaging Time 24-hour NAAQS 35 µg/m3

There are a large number of health studies that link exposure to fine particles (i.e., PM2.5) at levels measured in the ambient air with a variety of heart and respiratory diseases. Long-term exposures, such as those experienced by people living in areas with high particle levels for many years, have been associated with problems such as reduced lung function and the development of chronic bronchitis and even premature death. Short-term exposures to particles (hours or days) can aggravate lung disease, causing asthma attacks and acute bronchitis, and may also increase susceptibility to respiratory infections. In people with heart disease, short-term exposures have been linked to heart attack and arrhythmias. Healthy children and adults have not been reported to suffer serious effects from short-term exposures, although they may experience temporary minor irritation when particle levels are elevated. However, people with heart or lung disease, older adults, and children are considered at greater risk from particles than other people, especially when they are physically active. Exercise and physical activity cause people to breathe faster and more deeply and therefore take more particles into their lungs.

Did You Know?
Fine particles (PM2.5) are the main cause of reduced visibility (haze) in parts of the United States.

PM2.5 outdoor air quality measures are available in a number of formats, including monitored (only) in counties where monitors are located, as well as monitored and modeled (combined) and modeled (only) for all counties, communities, and census tracts. In addition to annual average PM2.5 concentrations, daily outdoor air quality measures are also available. The daily measures include percent of days exceeding the daily PM2.5 NAAQS as well as information on the number of people exposed to levels of PM2.5 pollution above the NAAQS. When comparing across geographies, a higher number of person-days indicates an area with a larger exposed population and a larger number of high pollution days.

Data Considerations

There are important limitations to consider when evaluating the outdoor air quality measures for PM2.5 in your community:

  • Monitoring data are available for certain counties where monitors are located
  • Variations in pollutant concentrations within a county will likely exist but will not necessarily be captured by the monitoring network.
  • Modeled data are used in instances when monitoring data are not available. Modeled data are available by county, community, and census tract.
  • Personal exposure and related health risks depend on such factors as the amount of time and level of activity outdoors and cannot be determined solely from outdoor air quality measures.

For additional information, please read the FAQ

Available Data on Outdoor Air Quality Measures for Fine Particles (PM2.5)

Use the Explore Maps & Tables link on this page to access the following measures for Fine Particles (PM2.5) in your community. The most current available data will be shown. Be sure to check the site periodically as new data are added each year. To protect privacy, no information is shown that could identify an individual.

  • Annual average PM2.5 concentration by county
  • Percent of monitored days that PM2.5 24-hour concentration exceeds daily NAAQS by county
  • Number of person-days that PM2.5 24-hour concentration exceeds daily NAAQS by county

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