What is PM?

PM stands for particulate matter, which refers to solid particles or liquid droplets found in the air. PM is also called particle pollution. PM is a mixture of many substances with different properties. Some particles, such as dust, pollen, dirt, soot, or smoke, are large or dark enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope.

Size categories of PM:

Size categories of PM are :

• PM_2.5 : Particles suspended in the air with diameters that are generally 2.5 micrometers and smaller. How small is 2.5 micrometers? The average human hair is about 70 micrometers in diameter – making it 30 times larger than the largest particle of PM_2.5.

• PM₁₀ : Particles suspended in the air with diameters that are generally 10 micrometers and smaller. PM₁₀ includes all PM_2.5 particles as well as particles between 2.5 and 10 micrometers sometimes generated from grinding or crushing. Windblown dust is a common example that is mostly PM₁₀.

What are the sources of PM into the air?

Particles in the air can be made up of hundreds of different chemicals and come in many sizes and shapes. Some are emitted directly from a source, such as industrial smokestacks, power plants, industrial boilers, cement manufacturing, construction sites, unpaved roads, fields, or fires. However, most particles form in the atmosphere as a result of complex reactions of chemicals such as sulfur dioxide and oxides of nitrogen.

What are the harmful human health effects of PM?

Particulate matter contains microscopic solids or liquid droplets that are so small that they can be inhaled and cause serious health problems, especially over long time periods of exposure. The size of particles is directly linked to their potential for causing health problems. Particles less than 2.5 micrometers in diameter, also known as PM_2.5 or “fine” particles, pose the greatest risk to health because they can be inhaled deeply into your lungs and some may even get into your bloodstream and have effects on many systems in the body. Particles less than 10 micrometers in diameter can also be inhaled and cause serious health problems, but generally aren't inhaled as deeply into the lungs.

Exposure to such particles can affect both your lungs and your heart. Numerous scientific studies have linked particle pollution exposure to a variety of problems, including:

• Premature death in people with heart or lung disease
• Nonfatal heart attacks
• Irregular heartbeat
• Aggravated asthma
• Decreased lung function and lung function growth
• Increased respiratory symptoms, such as irritation of the airways, coughing or difficulty breathing

People with heart or lung diseases, children, and older adults are most sensitive to and affected by exposure to PM_2.5 and PM₁₀.

Environmental effects: Visibility impairment and environmental damage

PM_2.5 is the main cause of reduced visibility due to haze in many parts of the United States, including many national parks and wilderness areas.

Particles, especially PM_2.5, can be carried over long distances by wind and then settle on ground or water. Depending on the chemical composition of the particles, the effects of this settling may include:

• Contributing to acid rain, making lakes and streams more acidic
• Changing the nutrient balance in coastal waters and large river basins
• Depleting nutrients in soil
• Damaging sensitive forests and farm crops
• Affecting the diversity of ecosystems

The US Environmental Protection Agency regulates emissions of substances that form PM in the air.