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Particulate Matter

Particulate Matter (PM) Research

Graphic of PM2.5

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What is Particulate Matter?

Particulate matter (PM) is an air pollution term for a mixture of solid particles and liquid droplets found in the air. The pollutant comes in a variety of sizes and can be composed of many types of materials and chemicals. Particles that are small enough to be inhaled have the potential to cause health effects. Of particular concern is a class of particles known as fine particulate matter or PM2.5 that gets deep into the lung.

There are many sources of PM. The air pollutant can originate from natural processes, like forest fires and wind erosion, and from human activities, like agricultural practices, smokestacks, car emissions, and construction. Examples include dust, dirt, soot, soil, and smoke.

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EPA Research on PM

EPA accelerated its investigations of fine particulate matter (PM2.5) in 1998 to improve understanding of the potential health effects of the small particles in the outside air and to find ways to reduce risks from the air pollutant. Studies at that time had provided compelling evidence that air pollution particles were responsible for thousands of deaths and hospitalizations, as well as substantial loss of work and school days.

Research has since confirmed the links between exposure to PM2.5 and increases in respiratory health problems, hospitalizations and premature death. EPA's PM research also has affirmed the need for air quality standards to reduce PM in the air to protect human health. However, many questions remain about particles and why they are associated with such significant health effects.

Research is being conducted to better understand:

  • Which attributes of particles may be causing these health effects?
  • Who may be most susceptible to their effects?
  • How are people exposed to PM air pollution?
  • How are particles formed in the atmosphere?
  • How should particles be measured so data is relevant to health protection?
  • What are the contributions from various sources in the different regions of the country?
Areas of research include:
Atmospheric Science

To learn how PM forms, interacts and where it comes from and travels

Exposure Science

To learn how and where people become exposed to PM

Health Effects Research

To understanding the impacts PM has on human health, particularly with those more susceptible

Sources Research

To describe and profile the many different sources of PM

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Types of PM

Although it can be categorized in a number of ways, PM has traditionally been classified by size. In general, the smaller the particle, the stronger its potential impact on human health because it can be more easily inhaled. For this reason, EPA monitors and regulates particles in two size categories depending on their predicted penetration into the lung. These categories are:

Coarse particles (PM10)

Inhalable particles less than 10 micrometers (μm) in diameter used as a nominal surrogate for particles between 2.5 and 10μm in diameter; found near roadways and dusty industries

Fine particles (PM2.5)

Inhalable particles less than 2.5μm in diameter; generally found in smoke and haze, emitted from natural sources like forest fires and industrial combustion sources, or formed when gases react in the air. Ultrafine particles (PM0.1) are a subset of inhalable PM2.5 particles less than 0.1μm in diameter. They are not specifically regulated but have a strong link to combustion and therefore are garnering special attention.

PM can also be classified by its source:

  • Primary particles: directly emitted from a natural or human source
  • Secondary particles: produced when chemicals from natural and human sources react in the atmosphere often energized by sunlight

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Human Health Effects

Inhalable particles, particularly fine particles, have the greatest demonstrated impact on human health. Their small size allows them to get deep into the lungs and from there they can reach or trigger inflammation in the lung, blood vessels or the heart, and perhaps other organs. Studies have linked PM exposure to health problems such as:

  • Irritation of the airways, coughing, and difficulty breathing
  • Reduced lung function
  • Aggravated asthma
  • Chronic bronchitis
  • Irregular heartbeat
  • Nonfatal heart attacks
  • Some cancers

Research has found that certain populations are more vulnerable to these health effects, such as people with pre-existing heart or lung diseases, children, and older adults.

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Environmental Effects

Effects of PM on the environment include:

  • Reduced visibility (haze)
  • Increased acidity of lakes and streams
  • Nutrient balance changes in coastal waters and river basins
  • Reduced levels of nutrients in soil
  • Damage to forests and crops
  • Reduced diversity in ecosystems
  • Damage to stone and other materials

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History of PM

When fossil fuels empowered new industries in the late 1800s, most people saw smoky skies and haze as signs of a city's prosperity, and there was little concern about any health effects they might have. That perception started to change in the mid-20th century as severe episodes of air pollution (e.g., Meuse Valley, Belgium; Donora, PA; London, UK) were linked to spikes in death rates and hospitalization.

Soon groups began measuring soot levels and noticing actual correlations between urban fogs and death rates. In response, local areas began monitoring and regulating emissions of these particles, but regulatory efforts remained uneven and localized until national awareness about the environment rose in the 1960s.

In 1970, the Clean Air Act established the first set of National Ambient Air Quality Standards (NAAQS) and created EPA as a federal Agency to protect the public health and welfare.

At that time, Congress designated seven (now six) criteria air pollutants to be of particular concern. What little research existed had shown these to be ubiquitous urban pollutants with potential negative health effects. The concern was at a nationwide level, not just localized in a particular region, city or industrial area. EPA currently regulates six criteria pollutants (PM, ozone, sulfur oxides, nitrogen oxides, carbon monoxide, and lead) to protect public health and the environment.

In the 1980s, as many smoke stacks cleared their emissions, the air looked clean and standard health assessments generally concurred with these improvements. The irritating haze of urban smog and ozone became the next focus of interest as towns and cities with associated vehicular traffic began to grow and expand. However, new studies (some supported by EPA) in the 1990s, using novel methods, began to reveal the insidious nature of forgotten PM, particularly a smaller class of inhalable particles, PM2.5, that was associated with respiratory health problems, heart and lung disease, and premature death at levels once thought to be safe.

In 1997, in response to these findings, EPA established a new standard for allowable levels of PM2.5 in the ambient environment to augment the existing regulation for larger PM particles or PM10.

In 1998, Congress augmented EPA's budget and mandate for enhanced emphasis on PM research and specifically called for EPA to fund up to five PM Research Centers to address the PM issue over the next five years. In 2005, these grants were re-awarded for a second five-year cycle.

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Controlling PM Emissions

Efforts by EPA, other organizations, and the general public have successfully reduced ambient levels of PM in the United States. Between 2000, when monitoring began, and 2007, average national levels of PM2.5 declined by 11%. Similarly, national levels of PM10 declined by 28% between 1990 and 2007. However, these declines were regionally uneven; some areas experienced bigger declines while others actually experienced increases in PM levels during this period.

Revisions to the Clean Air Act in 1990 required each state to develop a State Implementation Plan (SIP) describing how it will reach and maintain the national standards. These SIPs vary by state, but generally include local monitoring of PM levels, strategies to reduce PM emissions, and steps to evaluate these strategies. Individual actions that can also make a difference include recycling, using energy-efficient products and appliances, planting deciduous trees, and driving less.

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Related Links:

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Selected References:

1. United States Environmental Protection Agency. (2006 June). The PM Centers Program 2005-2010 Overviews and Abstracts. Available online at http://www.epa.gov/ncer/publications/workshop/11-30-2005/pmcentersabstract.pdf.

2. Bachmann, John. (2007 June). Will the circle be unbroken: A history of the U.S. National Ambient Air Quality Standards. Journal of the Air & Waste Management Association. 57: 650-697.

3. United States Environmental Protection Agency. (2008 May). Particulate Matter. Retrieved on June 9, 2008 from http://www.epa.gov/oar/particlepollution/.

4. United States Environmental Protection Agency. (2008 April). Air Trends: Particulate Matter. Retrieved on June 9, 2008 from http://www.epa.gov/airtrends/pm.html.

5. United States Environmental Protection Agency. (2007 March). Air and Radiation: What You Can Do to Clean the Air. Retrieved on June 9, 2008 from http://www.epa.gov/air/actions/.

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