Over time, increased exposure to air pollution is linked to faster "hardening" of the arteries, or atherosclerosis, a leading cause of heart attacks and strokes. Conversely, exposure to reduced levels of air pollution is linked to slowed progression of atherosclerosis. These are the findings of a new study from the US published this week in PLOS Medicine.

Lead author Sara Adar, the John Searle Assistant Professor of Epidemiology at the University of Michigan (U-M) School of Public Health in Ann Arbor, says in a statement:

"Our findings help us to understand how it is that exposures to air pollution may cause the increases in heart attacks and strokes observed by other studies."

Cardiovascular Disease (CVD)

Cardiovascular disease (CVD) is a general term for disease of the heart or blood vessels. It is caused by a thrombosis (blood clot), or atherosclerosis, where arteries get narrow and harden because of build-up of fatty deposits or plaques on their inside walls.

CVD is a major cause of illness and death worldwide. The leading cause of adult deaths in the US is coronary artery disease, a CVD caused by atherosclerosis of the heart's blood vessels. The fourth leading cause of death among adult Americans is stroke, a CVD caused by atherosclerotic plaques interrupting the blood supply to the brain.

Smoking, high blood pressure, high levels of cholesterol, diabetes, lack of physical activity and being overweight all raise the risk for developing CVD.

Treatments for CVD include helping patients to change their lifestyle or giving them drugs to lower their blood pressure or cholesterol.

Air Pollution

One way to assess the amount of pollution in the air is to measure the concentration of fine particles with a diameter of less than 2.5 micrometers (µm) which is about 1/30th the width of a human hair. These fine particles travel deeper into the lungs, contain more toxic compounds, and are thought to have more serious effects on health, than larger particles.

Particles of this size (known as fine particulate matter, PM2.5) come mainly from motor vehicles, power plants and other sources that burn fuel.

Studies have shown that PM2.5 also raises the risk of CVD, but it is not clear how it does it. One suggestion is that it triggers or speeds up atherosclerosis.

Another recent study, where researchers looked at data on more than three million births spanning four continents, suggests that air pollution is also linked to low birthweight.

The Study

This prospective cohort study is part of the Multi-Ethnic Study of Atherosclerosis and Air Pollution (MESA Air). It followed individuals over time and assesses whether exposure to certain risk factors affects risk of developing a particular disease.

Sadar and colleagues wanted to find out if there is a link between exposure to PM2.5 and atherolsclerosis: if there is, it might explain the association between PM2.5 exposure and raised risk of CVD.

To assess atherosclerosis they measured the thickness (or IMT) of the inner layers of the carotid artery, one of the major blood vessels that supplies the head and neck with blood. IMT stands for intima-medial thickness, where intima and media are the innermost layers of the artery wall.

The IMT of the carotid artery, which is measured using ultrasound, is often used as a surrogate measure of overall atherosclerosis, which can be present throughout the body even without other obvious symptoms heart disease.

The researchers followed 5,362 participants aged between 45 and 84 from six US metropolitan areas.

They then looked for links between air pollution measures estimated at participants' home location and IMT measures of their carotid artery, taken about three years apart.

The Findings: PM2.5 Air Pollution Linked to Faster Thickening of the Carotid Artery Lining

After adjusting for possible influencers such as smoking, sex, age, and socioeconomic status, Sadar and colleagues found that on average across all participants, the IMT of the carotid artery increased by 14 µm each year.

But the IMT went up faster in those participants exposed to higher levels of PM2.5. Even among participants living in the same metropolitan area, in the main it was those living in the more polluted parts whose carotid arteries thickened more quickly over time.

And they also found that "greater reductions in PM2.5 over follow-up ... were also associated with slowed IMT progression".

Adar says:

"Linking these findings with other results from the same population suggests that persons living in a more polluted part of town may have a 2% higher risk of stroke as compared to people in a less polluted part of the same metropolitan area."

If these findings are confirmed with a longer, 10-year follow up in this group of participants, they may explain why long-term exposure to PM2.5 is linked to cardiovascular events like heart attacks and strokes, conclude the researchers.

Written by Catharine Paddock PhD