Particle pollution (also known as "particulate matter" or PM) includes a mixture of solids and liquid droplets. Some particles are emitted directly; others are formed in the atmosphere when other pollutants react. Particles come in a wide range of sizes.

  • Fine Particles (PM2.5). Particles up to 2.5 microns in diameter are called "fine" particles. These particles are so small they can be detected only with an electron microscope. All sources of fuel combustion, including power plants, cars, buses, trucks and wood burning, as well as some industrial processes, generate fine particles. Because of their miniscule size, these particles can penetrate deeply into the lungs and accumulate in the respiratory system. Large concentrations of fine particles can be seen as haze.
  • Coarse Particles (PM10). Particles between 2.5 and 10 microns in diameterPM2.5 Emissions are referred to as "coarse." These come from a variety of sources, including natural wind erosion of soil and airborne residue from commercial and industrial operations. Individual particles cannot be seen with the naked eye, but collectively can appear as haze, dust or soot


PM 2.5 2005 Emissions (50,139 TPY)


Particle pollution can be bad at any time or location, but especially:

  • When the weather is calm, allowing pollution to build up.
  • Around factories, during rush hour and near busy roads.
  • When there is smoke in the air from wood stoves, fireplaces, forest fires or burning vegetation.


Health Effects of Particle Pollution & Who is Most at Risk

Exposure to particle pollution can affect both your lungs and your heart. Health research suggests that short-term exposure to coarse particles can lead to coughing, minor throat irritation and reduced lung function, while long-term exposure may increase the rate of respiratory and cardiovascular illness. A large number of scientific studies have definitively linked fine particles with a number of significant health problems, including:

  • Asthma and chronic bronchitis
  • Acute respiratory symptoms, such as coughing and chest tightness
  • Decreased lung function, experienced as shortness of breath
  • Heart attacks
  • Premature death in people with heart or lung disease


These conditions contribute to work and school absences, emergency room visits, and hospital admissions. Long-term exposure can make existing conditions worse and even reduce life expectancy.

The following "sensitive groups" are particularly susceptible to health problems when fine particle levels are high:

  • Children, because their lungs are still developing
  • Elderly, particularly those with or prone to cardiovascular disease
  • People with asthma or other respiratory ailments

Otherwise healthy adults who exert themselves during periods of elevated fine particle concentrations also may be affected because they tend to breathe more while working or exercising.

Learn more on EPA’s website.

Environmental Effects of Particle Pollution

Vast quantities of coarse and fine particles can produce haze that can impair outdoor visibility, reducing visual range by as much as 70 percent from natural conditions. Airborne particles and droplets also tend to remain suspended in the air for extended periods of time and can travel long distances. When they eventually settle to the surface, they can damage property, acidify lakes and streams, and harm plants and animals.

Particle Pollution Standards

The U.S. Environmental Protection Agency (EPA) established National Ambient Air Quality Standards (NAAQS) for two sizes of particle pollution: coarse particles (PM10) and fine particles (PM2.5).

The health-based primary standard for PM10 is 150 micrograms per cubic meter (ug/m3) averaged over a 24-hour period, and was established in 1987.  The welfare-based secondary standard is the same as the primary standard.

The primary standards for PM2.5 are 12 ug/m3 averaged over an entire year and 35 ug/m3 averaged over a 24-hour period.  These were established in 2012 and 2006, respectively.  The welfare-based secondary standards are 15 ug/m3 averaged over an entire year and 35 ug/m3 averaged over 24 hours.  

Monitored levels of PM10 and PM2.5 in Massachusetts meet the applicable standards, although there are a few days each year when the PM2.5 24-hour standard is exceeded, often due to winter weather “inversions” that trap cold air near the ground surface combined with wood smoke from home heating.   

For more information on particle pollution monitoring, standards, and trends see MassDEP’s Annual Air Quality Reports.