Bad air apparently isn’t great for good cholesterol.

Researchers from the American Heart Association (AHA) have concluded that air pollution can actually lower levels of good cholesterol in the body, increasing the risk of cardiovascular disease. Their study was published this month in the AHA journal Arteriosclerosis, Thrombosis, and Vascular Biology.

The researchers studied the effects of air pollution on more than 6,000 middle-aged and older adults from a variety of backgrounds in the United States.

They found that people in areas with higher levels of traffic-related air pollution tended to have lower levels of high-density lipoprotein (HDL). HDL, commonly known as “good” cholesterol, can boost heart health.

It’s significant news because while the connection between air pollution and heart health has long been known, the reasons for that connection have been murky.

“Our study helps strengthen the biological plausibility of the link between traffic-related air pollution and cardiovascular disease,” said lead author Griffith Bell, PhD, MPH, and former fellow of the University of Washington School of Public Health, in a release. “We’re slowly beginning to understand some of the biology of how that link works.”

Read more: Recommended cholesterol levels by age »

Reaction to the study

The researchers said components in air pollution seem to reduce the number of small, cholesterol-depleted HDL particles, leaving the average amount of cholesterol in HDL particles higher on a per-particle basis.

The researchers found that both men and women were affected, but the effect of air pollution on women tended to be greater.

Experts in the field found the results of the study intriguing.

“We’ve known for a long time that air pollution seems to be related to a higher incidence of cardiovascular events,” said Dr. Andrew Freeman, director of cardiovascular prevention and wellness at National Jewish Health and co-chair of the Nutrition and Lifestyle Work Group at the American College of Cardiology.

“This study was interesting in that it tried to posit a mechanism, in that maybe HDL is getting lowered by some reason, or its function is getting lowered by some reason, and that may be what’s causing it,” Freeman added. “We find, in general, that in parts of the world where there’s more air pollution, there’s also more cardiovascular disease. I think this is certainly not surprising to see that there’s some effect on some intermediary markers of risk, which would be lower HDL.”

Dr. Ragavendra Baliga, professor of internal medicine at The Ohio State University Wexner Medical Center and editor-in-chief of Heart Failure Clinics of North America, told Healthline that HDL has long been known to be good for heart health — but it can be difficult to raise HDL levels in the body.

“With HDL — or as I tell my patients, the healthy cholesterol — we know that the association between higher levels of HDL is beneficial, but unfortunately we don’t have any medications that are shown to improve HDL,” he said. “HDL is good, but we have no interventions apart from weight loss and exercise that have been shown to be markers of longevity.”

Read more: What are we breathing and how bad is it for us? »

Smoke does damage

Whatever the form the smoke comes in, it’s likely to cause damage to the body’s cardiovascular system.

“I think a lot of this also relates to some of what we’re learning about tobacco, in that the particulate matter seems to really create a lot of inflammation, and that may be the common pathway between air pollution and cigarettes and other things,” said Freeman. “I think, at least as it pertains to the topic here, avoiding smoking of any sort seems to be a key thing, especially now that marijuana’s becoming legal in lots of parts of the world.”

Freeman also shared a story from his training that highlights another way that smoke can harm the body.

“I spent a month in my training in Africa, and a lot of the folks there had developed COPD (chronic obstructive pulmonary disease) or emphysema — not from tobacco, but from cooking smoke, because their homes weren’t ventilated properly,” he said.

And it isn’t just the cardiovascular system that is harmed by smoke.

“Where there’s pollution, where there’s particulate matter in the air, there’s no question that some of what you inhale gets into your bloodstream and is deposited along the walls of the blood vessels,” said Baliga. “It can affect the neural control of the heart, have a direct impact on the way the heart beats, and there’s emerging data that it may cause diabetes as well. So all of these mechanisms are works in progress and are still being teased out by the experts.”

Read more: Children’s lungs benefit from less air pollution »

Steps you can take

While cigarette smoke can be avoided relatively easily, air pollution is more pervasive.

Freeman said there are protective steps that people who live in areas with significant air pollution can take.

He recommends using personal protective gear, if available, keeping windows closed, and investing in a good air filtration system and home ventilation.

Avoiding bad air is important, but it’s just one component of good heart health.

“I would say that the ways to keep one’s heart healthy are not just avoiding air pollution and making sure that you have good quality air and good quality water, but of course to exercise regularly, to eat predominantly plant-based and unprocessed foods, and to work on stress relief and mindfulness, which seem to affect cardiovascular outcomes,” said Freeman. “It’s also important to make sure that one has enough support connections in their life. The people that have the best support systems always seem to do better. So I think combining those things is critical, and obviously avoiding smoking and air pollution is important as well.”