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Particles or particulate matter (PM) is an air pollution term for a mixture of solid particles and liquid droplets found in the air, 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, domestic heating, car emissions, and construction. Examples include dust, dirt, soot, soil, and smoke.

Particles can be classified on the basis of their size, referred to as their 'aerodynamic diameter':-

  • Coarse particles (PM10) - inhalable particles less than 10 micrometers (µm) in diameter found near roadways and dusty industries such as quarrying, and from natural sources such as dust storms and pollens
  • 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, industrial combustion sources, and woodheating, or formed when gases react in the air.
  • Ultrafine particles (PM0.1) - a subset of inhalable PM2.5 particles less than 0.1µm in diameter.

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, the brain, 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
  • Cardiorespiratory illness and death
  • Infant respiratory mortality
  • 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.

Electron Microscope Image of ambient particulate matter, consisting of aggregates of very small carbon spherules. The scale bar at bottom right is 100 nano-metres, so the whole image is around 1 micron across.
(Image source: Particulate Matter exposure in Children; J. Grigg, The Proceedings of the American Thoracic Society 6:564-569 (2009)
 (A) Image of an airway macrophage obtained by induced sputum from a healthy child and imaged under light microscopy. The black material is inhaled carbon.

(B) A section of an airway macrophage obtained from a healthy infant undergoing elective surgery using bronchoalveolar lavage and imaged using electron microscopy. The inhaled material is agglomerations of carbon spherules. The scale bar at bottom is 1 micron.
(Image source: as above)

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