It's a safe bet that most of the people reading this never spent much time thinking about particles. Or wondered how fast they drop to earth. Or whether they can become aerosolized and float for a long time.
But, to cite a familiar refrain, COVID-19 changed all that. We are all now paying attention to the behaviors of airborne virus particles, which are among the smallest of the small—between 0.05 microns and 0.3 microns. COVID-19 particles have been measured at approximately 0.165 microns and larger. With virus particles piquing our curiosity, now is a great time to learn about pollution particles of all sizes and how they can affect our health, indoors and out.
Fine, ultrafine? How small is small?
Making distinctions between fine and ultrafine may seem like splitting hairs, and it sort of is. Air pollution from particulate matter (PM) is measured in micrometers (or microns um) and nanometers (1000x smaller than a micrometer). Because they are too small to be seen without magnification (the smallest size visible to the human eye is above 25-30 microns), it's easier to visualize micro-and nanoparticles by comparing them to something small but familiar, like a grain of sand or a human hair. As an example: 2.5 microns in diameter is 30 times smaller than a typical human hair.
Why does size matter?
It's simple: air pollution all around us, in the form of particulate matter too small to see, can penetrate the upper respiratory system and from there move into the lungs, blood stream, and internal organs. The smaller the particles, the further they can penetrate and the more lasting the health damage may be.
According to the World Health Organization, "An estimated 4.2 million premature deaths globally are linked to ambient air pollution, mainly from heart disease, stroke, chronic obstructive pulmonary disease, lung cancer, and acute respiratory infections in children…." See the complete report
Recent studies have also found markers for and symptoms of Alzheimer's and Parkinson's diseases in children living in heavily polluted environments. And in 2020, in a landmark ruling, air pollution was formally cited as the cause of death of a nine-year-old child in in England.
PM10? PM2.5? Why are these the sizes we hear about?
Particles are classified by size ranges. Medium-sized particles, as large as 100 microns, are considered inhalable and can cause throat irritation and make allergies worse. Particles up to 10 microns (PM10) in diameter are categorized as coarse. Despite the name, these particles are inhalable and can make their way into your lungs to cause respiratory discomfort and short-term breathing difficulty. Fine particles up to 2.5 microns (PM2.5) are considered the most dangerous to your health, as they can penetrate even deeper into your lungs. Ultrafine particles less than 0.1 microns can travel into your blood stream and can lodge in internal organs.
To protect public health and the environment, the EPA establishes safe standards and monitors PM 2.5 levels outdoors, providing warnings when levels are unsafe. There is no comparable standard for indoor air quality, which has been reported to be two to five times worse than the air outside.
Learn more in this EPA report on the health effects of particulate pollution.
A closer look at PM10 and PM2.5
PM10 Coarse particles, 10 microns in diameter and less, settle out of the air slowly, and can pass through the nose and throat into the lungs creating respiratory irritation. Examples are fine silt, mold spores, asbestos, and fine pollen.
PM2.5 Fine particles, 2.5 microns or less, are 30 times smaller than an average human hair, and can penetrate the lungs and circulate in the bloodstream. Examples are fine dust and wood smoke. Outdoor sources can make their way into your home, but indoor sources are a concern on their own. PM2.5 indoors can come from, among other sources, wood-burning stoves, kerosene heaters, cooking, especially with gas, burning candles, tobacco smoke, and pets.
PM0.1 Ultrafine particles, 0.1 microns in diameter or less, are included in the PM 2.5 classification, but deserve a little more attention. PM0.1 penetrates deeply through the lungs to reach the bloodstream and internal organs. PM0.1 remains in the lungs for longer periods of time than larger particles. Ultrafine particles are the byproduct of forest fires, combustion, including gas automobiles, power plant emissions, household cooking, aerosol sprays, printer toner, and e-cigarette vapor, among other things. Virus particles are also ultrafine.
For a deep dive into the health effects of ultrafine particles, read this article published the journal Experimental and Molecular Medicine.
With invisible, damaging particles in the air outside and inside your home, how do you protect yourself? How do you pick the right air purifier?