PM 2.5 is the mixture of extremely small particles and liquid droplets that tend to stay suspended in the air as smoke and haze. At just 2.5 microns in diameter and smaller, 20 to 100 or more fine PM 2.5 particles could fit across the diameter of an average human hair. The average human hair measures 70 micrometers. Read the Environmental Protection Agency’s explanation on particle size.
A micron (also called a “micrometer” or “µm”) is a millionth of a meter. To give you another idea of how small PM 2.5 is, the dot above the letter “i” in a typical newspaper measures about 400 microns and could hold 160 or more PM 2.5 particles.
PM 10 are slightly bigger particles and more easily settle out of the air as dust. PM 10 particles measure 10 microns in diameter or less. That same typical newspaper printed dot above the “i” could hold 40 or more PM 10 particles. One PM 10 particle could hold 4 or more PM 2.5 particles.
Both PM 2.5 and PM 10 particles are inhalable, although PM 2.5 are also called respirable. See the Wikipedia on Particulates. Also, see the Minnesota Pollution Control Agency site on health effects of wood smoke.
Learn more at the Environmental Protection Agency’s sites on Particulate Matter and Particulate Matter Research. There are many sources of PM 10 and PM 2.5 pollution.
[PM 10] are “(i)nhalable coarse particles,” such as those found near roadways and dusty industries, are larger than 2.5 micrometers and smaller than 10 micrometers in diameter. [PM 2.5] particles can be directly emitted from sources such as forest fires, or they can form when gases emitted from power plants, industries and automobiles react in the air.
Now, in addition to monitoring PM 10, scientists and technicians monitor fine particles called PM 2.5, these particles measure 2.5 microns in diameter or smaller, or about 1/10,000 of an inch. These tiny particles are about 30 times smaller than the width of a hair on your head! These tiny particles are small enough to get inhaled past our defensive nose hairs and into our lungs…. and be carried throughout our bodies.
After inhalation, wood smoke particles may grow inside the lung. Wood smoke particles are hydroscopic, attracting and holding water. At relative high humidity, about 99%, “particles from wood embers showed a significant growth”. Hygroscopic Growth of Assorted Indoor Aerosols, S. K. Dua, P. K. Hopke, Aerosol Science and Technology Vol. 24, Issue 3, 1996, p 159.
What is Particulate Matter? from the Department of Environmental Quality, Pima County (Tucson), Arizona provides helpful background to better understand particulate pollution including what types of activities create PM 10 and PM 2.5 particles and factors leading to higher health consequences. PM 2.5 particles can stay in the air for days or weeks and travel many hundreds of miles.
For a wealth of specific information on the range of air pollutants, view EPA’s page on Air Pollutants.