by Alejandro Davila Fragoso CLIMATE PROGRESS

CREDIT: AP Photo/Amel Emric

A Bosnian man covers his mouth as he walks on the street during heavy smog in the northern Bosnian town of Tuzla, 140 kms. north of Sarajevo.

Air pollution has been linked to heart disease for years, prompting concern as well as some skepticism, as the physiological steps showing a cause-and-effect have gone less understood. But now, a multi-year study has for the first time documented that air pollution thickens blood and hardens arteries, a condition that causes cardiovascular problems like heart attacks and strokes.

“What’s new here is the linkage between air pollution and actual evidence of progression of atherosclerosis, the underlying disease process that leads to most [heart] attacks and strokes,” Joel Kaufman, lead author and University of Washington professor, told ThinkProgress. “The study provides important new information on how pollution affects the main biological process that leads to heart disease.”

The decade-long study, published Tuesday in The Lancet, found that Americans living in areas with more polluted air suffer from accelerated rates of atherosclerosis — at times rates about 20 percent higher than those living in less polluted areas, Kaufman said. This harm to arteries may stem from air pollution triggering cell inflammation, he said, and affect the white blood cells that are involved in protecting the body against both infectious disease and foreign invaders.

Atherosclerosis refers to plaque building up inside arteries that carry oxygen-rich blood to the heart and other parts of the body. The disease disrupts the flow of blood, posing serious cardiovascular complications. Plaque is made up of fat, cholesterol, calcium, and other substances found in the blood, according to the National Heart, Lung and Blood Institute. Atherosclerosis is also common as it occurs in 80 to 90 percent of Americans over the age of 30, and leads to cardiovascular disease, the leading cause of death in people over 45.

For years, scientists have been successful in associating air pollution and vehicle-related emissions with overall mortality, cardiovascular mortality, and cardiovascular disease, to name a few. They have even been able to calculate mortality figures linked to air pollution. Just in February, the Global Burden of Diseases, Injuries, and Risk Factors Study, the most comprehensive international effort to measure epidemiological trends worldwide, calculated that about 5.5 million people prematurely died in 2013 because of indoor and outdoor air pollution. And before that, in September of 2015, another study found that outdoor air pollution kills 3.3 million people worldwide, a number set to double in the next 35 years if emissions continue unabated.

However, past epidemiological studies, though illuminating in quantifying a problem, often depended on data sets collected for other purposes and couldn’t delve further into some of the factors at play. In turn, Tuesday’s paper is considered the most in-depth study of air pollution exposures ever applied to a large study group to examine pollution and cardiovascular health. “By following adults over 10 years, we were able to ask if people who live in places with more pollution have a faster thickening of their blood vessels over time,” Sara Adar, author and assistant professor of epidemiology at the University of Michigan, told ThinkProgress via email.

Funded by the Environmental Protection Agency and the National Institutes of Health, the study was based on more than 6,000 people with no history of clinical cardiovascular disease in six states. To make sure that it was all encompassing, researchers recruited middle-aged and senior men and women from four ethnic groups: white, Hispanic, African American, and Chinese living in different corners of the country. They then conducted body scans to assess participants’ arteries as they calculated, via air monitors and other methods, each participant’s exposure to PM 2.5 — a pollutant that comes primarily from the burning of fuels — as well as exposure to nitrogen oxide, nitrogen dioxide, and black carbon.

By the end of the study, researchers were able to track more than 3,600 people and realized that those living in more polluted places like Los Angeles suffered from worse calcium plaque in their arteries. Calcified plaques in the coronary arteries have been consistently associated with cardiovascular disease.

“These effects seem to come across people regardless of their race or ethnicity, regardless of their socioeconomic status,” said Kaufman, who now aims to use the data set to quantify the influence that air pollution has on heart attack rates.

Another major finding of the study is that air pollution at levels below regulatory standards also accelerates the progression of atherosclerosis. With that, the study raises questions about the quality of the air that people need to avoid environmental harms — particularly because it found that air quality has been in general improving in the United States. “Exposures are quite a lot lower than what they used to be, but we still are seeing these effects,” said Kaufman. “So the question is, how low do we need to go to see the bottom of these effects?”