1. What Does PM2.5 Actually Mean?
“PM” stands for particulate matter, which is also called particle pollution. It is a mixture of solid specks and liquid droplets floating in the air. The number is a size cutoff, measured as the particle’s diameter. PM10 means particles that are generally 10 micrometers wide or smaller, and PM2.5 means particles that are generally 2.5 micrometers wide or smaller. A micrometer is a thousandth of a millimeter [1].
The U.S. Environmental Protection Agency (EPA) gives a way to feel how small that is: an average human hair is about 70 micrometers across, about 30 times wider than the biggest PM2.5 particle [1]. Some particles, such as soot or smoke, are large or dark enough to see. Others can be detected only with an electron microscope [1]. You cannot pick out a single PM2.5 particle with your eyes. What you notice is the haze that appears when millions of them scatter sunlight [3].
Where do they come from? Some come straight from construction sites, unpaved roads, fields, smokestacks and fires. The EPA says most form in the air itself, when gases from power plants, factories and vehicles, such as sulfur dioxide and nitrogen oxides, react with one another [1]. The WHO adds that burning fuel for transport, energy, homes, industry and farming is the main source [2].
2. How Does Your Body Normally Keep Dust Out of Your Lungs?
Your airways are not an open tube. The nose warms and filters the air, and the lining of the airways is covered with sticky mucus that traps particles. Tiny hairlike cilia then sweep the mucus along so you can cough, sneeze or swallow it. The American Lung Association says these natural defenses help get some coarse particles (the 2.5 to 10 micrometer size) out of the body [3].
A simple way to picture it is a net with holes: big objects are caught, while the smallest ones pass through. This is a picture to help you remember, not how the airways are literally built.
3. Why Do the Smallest Particles Get Past Those Defenses?
The same association says that these defenses do not keep out the smaller fine and ultrafine particles. Those particles get trapped in the lungs instead. The very smallest, called ultrafine particles and defined as under 0.1 micrometer, are described as small enough to pass through lung tissue into the blood, much as oxygen does [3].
The EPA puts the idea more carefully: particles under 10 micrometers can get deep into the lungs, and some may even reach the bloodstream. Of these, the fine particles under 2.5 micrometers pose the greatest risk to health [1]. The WHO says that both PM2.5 and PM10 can penetrate deep into the lungs, while PM2.5 can even enter the bloodstream [2].
Notice the wording “some” and “may.” The agencies describe where particles of different sizes can go. The pages read for this article do not give exact percentages of how many particles reach the blood, so this article does not either. That is a good question for a curious reader to look up in the research papers.
4. What Can Fine Particles Do to the Body Once They Get In?
The EPA lists several effects linked to particle pollution: aggravated asthma, decreased lung function, and irritation of the airways, coughing or difficulty breathing. In people with heart or lung disease it has also been linked to heart attacks, irregular heartbeat and early death [1]. The WHO says that in children, exposure to air pollution can include reduced lung growth and function, respiratory infections and worse asthma [2].
The heart is on that list because the harm does not stay in the lungs. Particles can cause cardiovascular as well as respiratory problems, and the WHO says other organs may be affected too [2]. In 2013 the WHO’s cancer agency, IARC, classified outdoor air pollution and particulate matter as carcinogenic [2].
The scale is large. The WHO estimates that outdoor air pollution caused 4.2 million premature deaths worldwide in 2019, with the cause being fine particles, and that nearly 9 in 10 of those deaths happened in low- and middle-income countries [4]. These are estimates of effects across whole populations, not a prediction for any one person on any one day.
5. How Can You Find Out How Clean the Air Is Today?
Since you cannot see single particles, people rely on measurements. In the United States, the AirNow website turns air-quality data into the Air Quality Index, with numbers and colors that show when it may be wise to take action to protect health [1]. Other countries have their own versions of such an index, and many weather apps show one.
Some schools use an air quality flag program, in which a colored flag tells students whether to adjust outdoor activity [1]. On a day when the index is high, a teacher or parent may suggest less strenuous outdoor activity or staying indoors for a while. Children, older adults and people with heart or lung disease are the groups the EPA says are most likely to be affected [1].
If you want to dig deeper, try these questions: why are particles formed in the air from gases counted as pollution just like soot? How do particle size and other factors affect whether particles reach the lungs? How do scientists measure particles in the blood? The sources below are a good place to start.