Ingestion of fine particulate matter in polluted air raises the risk of cardiovascular disease (CVD) and mortality just as much as, if not more than, common risk factors such as hypertension, diabetes, dyslipidemia and obesity, according to a large-scale study of 136,094 Seoul, South Korea, residents.

Ambient exposure to fine particulate matter (PM2.5) was ranked by the 2015 Global Burden of Disease Study as the fifth most important risk factor for all-cause mortality across the world, first author Hyeanji Kim, MD, and colleagues wrote in their paper, published this week in the Journal of the American Heart Association. In 2012, the World Health Organization estimated seven million deaths were attributable to air pollution globally.

Despite the known cardiovascular health consequences of air pollution, which exceed other pollution-related repercussions like pulmonary diseases, most studies on the subject have been limited to the U.S. or Europe, Kim et al. wrote, bypassing regions of the world where air quality is a widespread issue. The researchers followed a large cohort of Seoul citizens between 2007 and 2013, all of whom were free from cardiovascular disease at the study’s baseline, though 9.8 percent had hypertension, 7.5 percent suffered from dyslipidemia and 24.2 percent were active smokers. A small percentage were also diabetic.

Kim and co-authors measured air pollutants like PM2.5 particles, carbon monoxide, sulfur dioxide, nitrogen dioxide and ozone in each patient’s zip code by linking those residential areas to outdoor air monitors.

The Financial Times reported earlier this year that Seoul was among the world’s most polluted countries, ranking among Beijing and Delhi for its concentrations of hazardous smog. By March 2017 alone, South Korean authorities had already issued 85 ultrafine dust warnings.

Kim and colleagues followed up with patients for an average of seven years and recorded 1,658 all-cause deaths and 1,856 composite cardiovascular events, according to the research. Ischemic stroke was the most common cardiovascular event, while congestive heart failure, acute myocardial infarction, hemorrhagic stroke and cardiovascular mortality followed suite.

The burden of air pollution weighed heavily on these results, the authors reported, with long-term exposure to Seoul’s average PM2.5 concentration resulting in 30.8 percent of the population-attributable risks for cardiovascular events. The risk of heart events increased with higher mean concentrations of PM2.5 in a linear relationship, with a hazard ratio of 1.36. Every 1-µm/mᶾ increase of long-term exposure to PM2.5 was associated with a 36 percent increase in cardiovascular risks.

The authors wrote this strong correlation could be a result of several biological mechanisms reacting to air pollution, including systemic inflammation caused by oxidative stress, autonomic imbalance and the possibility of pollutants reaching a patient’s systemic circulation, though more research is needed to expand scientific knowledge on the topic.