Fine particle pollution, or PM2.5, has been linked with increased rates of serious illnesses and premature deaths in numerous studies, even at levels that fall below current regulatory standards. It is the air pollutant that kills the most people worldwide.

What is particle pollution?

Particle pollution, also called particulate pollution or particulate matter (PM), is a mixture of solid particles and liquid droplets in the air. Particles come in many sizes and shapes and can be made up of hundreds of different chemicals.  

A graphic showing the comparative sizes of a grain of sand, a human hair, and  fine and coarse particles.

Particulate matter is classified by size. “Fine particles,” or PM2.5, are those smaller than 2.5μm (micrometers, or microns) in diameter, which is about 30 times smaller than the diameter of an average human hair. Coarse particles, or PM10, are between 2.5 and 10μm in diameter. Smaller than PM2.5 are ultrafine particles, PM0.1.

In the lungs and bloodstream

Particles larger than PM10 do not go into the deepest parts of the lungs, and they can be coughed out. Particles smaller than PM10, however, are inhaled into the deepest parts of the lungs, where they become embedded and can cause disease.

The smallest fine and ultrafine particles, which are abundant in wood smoke, are so minute they behave like gases, passing through the lungs and into the bloodstream.

Once in the bloodstream, these toxin-laden particles can be carried to every organ in the body, causing inflammation and harm wherever they go.

Ultrafine particle pollution

Most particle pollution research and air quality regulations have been focused on PM2.5. However, there is growing evidence that ultrafine particles, which are sometimes also referred to as nanoparticles, are an even greater health concern due to their size and characteristics.

For the same amount of PM2.5 mass, ultrafines have a much higher particle number and surface area. For example, a billion 10nm particles has the same mass as just one PM10 particle, but a combined surface area a million times larger.

More reactive than larger particles

This higher surface to volume ratio allows ultrafines to more easily absorb organic compounds, which makes them more reactive than larger-sized particles.

Bypassing the blood/brain barrier

Once inhaled, ultrafine particles can become embedded in virtually any cell or organ of the body.

They can also travel up through the nose and, rather than passing down into the lungs, be delivered directly into the brain and central nervous system via the olfactory nerve, bypassing the body’s protective blood/brain barrier.

Ultrafine particle counts are not considered with regulatory air monitoring

Regulatory air quality monitoring is based on the mass of particles that are collected on the monitors’ filters. Since ultrafines contribute little to the pollution mass, but occur in greater numbers, PM2.5 readings usually will not capture the full magnitude of the associated health hazard from air pollution sources that, like wood smoke, contain a large number of ultrafines.

Particle pollution is bad for the heart

Particle pollution is linked to a variety of cardiovascular diseases and outcomes, including heart attacks.

A large study that was published in the New England Journal of Medicine, for example, found that long-term exposure to fine particle pollution is linked with cardiovascular disease and death in postmenopausal women.

Irregular heart rhythms in teenagers

It has long been known that particle pollution is linked to irregular heart rhythms in adults, but a 2022 study in the Journal of the American Heart Association also found an association between short-term particle pollution exposure and ventricular arrhythmias in teenagers.

The effects were seen within 30 minutes to two hours of exposure, and at levels “well below the US Environmental Protection Agency–mandated, health‐based air quality standards.”

Dr. Fan He, the lead author of the study and an instructor at the Penn State College of Medicine, noted:

While relatively rare, irregular heart rhythms can lead to sudden cardiac death in otherwise healthy adolescents and young adults. Our findings linking air pollution to irregular heart rhythms suggest that particulate matter may contribute to the risk of sudden cardiac death among youth.

More heart failure deaths and cardiac arrests

A review in the Lancet found a close and consistent association between short-term exposure to ambient particle pollution levels and heart failure hospitalizations and deaths.

Small increases in annual average exposure to particle pollution have similarly been linked to increased risk of death for patients with heart failure.

Small increases in PM2.5 levels have also been linked to sudden cardiac arrest.

A coffin at a funeral.
The death rate rises when there are more fine particles in the air, even at levels that are below current regulatory thresholds.

Low PM2.5 levels are linked to deaths

A significant and growing number of studies have linked fine particle pollution with an increase in the death rate, even at low levels that meet current standards for “good” air quality.

A study of New England Medicare recipients over age 65 determined that the death rate rises with each 10μg/m3 increase in PM2.5, both from short-term and long-term exposure, and even when pollution levels do not exceed US EPA or World Health Organization thresholds.

These findings were consistent with those of several other studies that have also found an association between particle pollution levels and an increase in the mortality rate, even at levels that are considered low by regulatory standards.

Like lead pollution, there’s no safe level

According to Joel Schwartz, senior author of the New England study and professor of environmental epidemiology at Harvard T.H. Chan School of Public Health, this was the first study to examine the effect of airborne soot particles over an entire region, including rural areas. Dr. Schwartz noted:

The harmful effects from the particles were observed even in areas where concentrations were less than a third of the current standard set by the EPA.

Particulate pollution is like lead pollution; there is no evidence of a safe threshold even at levels far below current standards, including in the rural areas we investigated.

In a later study, he and other researchers at Harvard looked at data covering 60 million US seniors. This much larger study, which covered approximately 97% of US residents aged 65 and older, similarly linked long-term exposure to PM2.5 with an increased risk of premature death, even at levels below current regulatory standards.

PM2.5 is carcinogenic

In 2013, the WHO’s specialized cancer agency, the International Agency for Research on Cancer (IARC), officially classified outdoor air pollution and particulate matter as carcinogenic.

Particle pollution and breast cancer

A study in 2023 found that PM2.5 was associated with an increased risk of estrogen receptor–positive breast cancer.

Particle pollution and vision risk

Smoking and secondhand tobacco smoke exposure are known risk factors for cataracts and eye diseases such as age-related macular degeneration. Similarly, fine particle pollution exposure, even at low levels, has also been linked to higher risk of age-related macular degeneration. Laboratory evidence has also suggested that wood smoke exposure can promote the development of cataracts.

Links to brain shrinkage and strokes

Research has shown that long-term exposure to even slightly elevated PM2.5 levels is associated with a .32% smaller total brain volume and a 46% higher risk of silent strokes, even in people who haven’t yet developed dementia or had obvious strokes.

Older women living in areas with higher levels of particulate pollution have been found to have smaller white matter volumes.

Stroke survival rates reduced

Other research has also shown an association between strokes and particulate pollution, especially from “air pollutants related to combustion.” Risk of death from an ischemic stroke is higher when levels of PM2.5 are higher. Even five years after an initial stroke, survival rates are reduced significantly with increased exposure to PM2.5.

Particle pollution and Parkinson’s and Alzheimer’s diseases

Exposure to particulate pollution has been linked to the development and progression of neurodegenerative diseases such as Alzheimer’s and Parkinson’s.

For example, a study in Ontario, Canada, found that PM2.5 was associated with increases in cases of Parkinson’s disease.

Another study linked clusters of Parkinson’s disease cases to localized areas of air pollution.

A man with a caregiver washing his face
Particulate pollution plays a role in the development of diseases such as Parkinson’s and Alzheimer’s.

Short-term increases in PM2.5 are associated with an increased risk of hospitalization and an increased risk of death for people with diabetes and, even more so, for people with Parkinson’s disease.

Long-term exposure to PM2.5 “at levels well below current EU air pollution limit values,” have been associated with an increase in Parkinson’s disease-related deaths.

Researchers who study how air pollution affects the brains of children and young adults have concluded that exposure to air pollution early in life should be considered a risk factor for later Alzheimer’s and Parkinson’s diseases.

More likely to develop dementia

There is growing evidence linking particle pollution exposure to increased risk of cognitive decline and dementia.

For example, one study found that older women residing in places where fine particle pollution exceeded EPA standards had an 81% increased risk of global cognitive decline, and were 92% more likely to develop dementia from any cause, including Alzheimer’s. The risk was even stronger in women who had the APOE4 gene, a genetic variation that increases the risk for Alzheimer’s.

More links with PM2.5 and cognitive impairment

Another study has also linked fine particle pollution with worsened cognitive function. It was found that episodic memory is most impaired by exposure to PM2.5. “Episodic memory impairment is one of the core diagnostic criteria used to determine mild cognitive impairment and dementia in older adults, and it has been suggested that impairments in this aspect of memory represent some of the earliest signs of dementia.”

An older woman sits in front of a fire holding a mug.
Exposure to higher levels of fine particle pollution is linked with increased risk of cognitive decline and dementia.

The researchers noted that particulates can affect the respiratory and cardiovascular systems, which can in turn affect the vascular pathology in the brain. Particles may even directly damage the brain itself. “Studies of both humans and animals have confirmed that exposure to PM is linked to harmful changes in brain health and functioning.”

Other studies have also confirmed an association between cognitive decline and long-term exposure to PM2.5.

For example, it was found that exposure to particle pollution at levels “typically experienced by many individuals in the United States is associated with significantly worse cognitive decline in older women.”

A study in Sweden found that  those who lived in an area with a 1 µg/m3 increase in PM2.5 due to residential wood burning had a 55% higher likelihood of developing dementia compared to those who were not living in an area polluted by wood smoke. Those who had a wood stove and who lived in an area where wood burning was common were 74% more likely to develop dementia over the course of the study.

Conversely, improved air quality in later life has been linked with reduced dementia risk.

A video from the World Health Organization explains how particle pollution is an invisible killer.