Is Air Pollution Contributing To Hardened Arteries?

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Taxis going through steam

Smog and car exhaust can take a toll on the heart, and the latest research explores how.

Previous studies have shown an association between badly polluted air and a heightened risk of heart attack stroke, and researchers have started to investigate how pollutants could exert such harm. Some have documented the increased inflammation that pollution can trigger, as well as changes in blood pressure and the activity of clotting factors in the blood that could promote heart heart disease. The latest research, published in the journal PLOS Medicine, found that exposure to air pollution may increase heart attacks and strokes by accelerating the process of atherosclerosis.

Researchers from the University of Michigan School of Public Health and the University of Washington followed 5,362 people between the ages of 45 and 84 from six regions in the U.S.: Baltimore, Maryland, Forsyth County, North Carolina, Los Angeles County, California, Northern Manhattan and Southern Bronx, New York and St. Paul, Minnesota. The participants were all part of the Multi-Ethnic Study of Atherosclerosis and Air Pollution and none of them had heart disease at the start of the trial.

(MORE: Car Pollution Linked To Childhood Cancers)

To determine the amount of air pollution to which the participants were exposed, the researchers created models to estimate the particulates in the air in and around the volunteers’ homes, using information from the Environmental Protection Agency’s air quality readings, as well as data that took into account density of car traffic, roadways and other sources of pollution near the people’s homes. The scientists also used ultrasound to measure blood vessel characteristics both at the beginning of the study and again three years later.

After accounting for behaviors like smoking, which can independently affect heart disease risk, they found that the thickness of the carotid arteries that supply blood to the head and neck increased by 14 µm each year. Participants who were exposed to higher levels of air pollution in their home had blood vessels that thickened faster compared to others living in their area with lower exposure levels.

Thickening of blood vessels is a sign of hardening of the arteries, as inflammation lures in clotting factors and other immune agents to patch up worn areas; as these compounds build, the vessels stiffen and thicken, losing their ability to flex and adjust to the varying levels of pressure created by the blood flow.

High levels of a fine air pollutant called PM2.5 was specifically linked with faster thickening of the inner two layers of the carotid artery, a vessel that serves as a sentinel for the state of other arteries throughout the body. However, the study also showed that if levels of PM2.5 were reduced in the participants’ homes, the pace of thickening slowed.

(MORE: Exposure to Air Pollution in Pregnancy May Boost Chances of Obesity in Kids)

“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,” said study author Sara Adar, a John Searle assistant professor of epidemiology at the University of Michigan School of Public Health in a statement.

The researchers say that understanding the pathway of PM2.5 and how it influences hardening of the arteries will help future studies investigating how air pollution affects heart disease. Previous studies have linked car pollution to a higher risk of childhood cancers and even autism.

And a study published last year even found an association between an expectant mother’s exposure to pollution during pregnancy and obesity in children. The researchers found that higher levels of polycyclic aromatic hydrocarbons (PAHs) which are present in cigarette smoke and car exhaust, in the mother’s blood during the third trimester can disrupt hormones that regulate growth and development.

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While these studies all highlight associations, and not causal relationships between pollution and health risks, they point to potentially harmful exposures that could trigger damaging physiological changes. They are particularly concerning since many people can’t avoid exposure to pollutants, particularly from cars, so public health experts recommend focusing on heart disease risk factors that are in people’s control — such as not smoking. We can’t chose how many pollutants we breathe in from car and industrial exhaust, but we can avoid exposing our lungs to the pollutants in cigarette smoke.