Exposure to air pollutants — even at levels below World Health Organization (WHO) air quality guidelines — may trigger a heart attack within an hour, claims a new study.

The study found exposure to any level of four common air pollutants — fine particulate matter, nitrogen dioxide, sulfur dioxide, and carbon monoxide — could quickly trigger the onset of the acute coronary syndrome (ACS).

ACS is an umbrella term describing any situation in which blood supplied to the heart muscle is blocked, such as in a heart attack or unstable angina, or chest pain caused by blood clots that temporarily block an artery.

Exposure to nitrogen dioxide was most strongly associated, followed by fine particulate matter, and was most dangerous during the first hour following exposure. The link was strongest among adults age 65 and older with no history of smoking or other respiratory illnesses and for people exposed during the colder months.

“The adverse cardiovascular effects of air pollution have been well documented. But we were still surprised at the very prompt effects,” said Haidong Kan, Professor in the School of Public Health at Fudan University in Shanghai.

“Another surprise was the non-threshold effects of air pollution. Any concentration of air pollutants may have the potential to trigger the onset of a heart attack,” Kan added.

The study was published in the American Heart Association’s journal Circulation.

Exposure to fine particulate matter — microscopic solids or liquid droplets that come from automobile emissions, power plants, construction sites, and other sources of pollution — has been unequivocally linked to heart disease, stroke, and other health issues, as well as 4.2 million premature deaths worldwide. These particles can be so small that when inhaled, they may go deep into the lungs or even the bloodstream.

In the new study, researchers analyzed medical data for nearly 1.3 million people treated for heart attacks and unstable angina at 2,239 hospitals in 318 Chinese cities between 2015 and 2020.

They compared hourly onset times of heart events with concentrations of fine particulate matter, coarse particulate matter, nitrogen dioxide, sulfur dioxide, carbon monoxide, and ozone.

While short-term exposure to any level of the pollutants was associated with the onset of all types of ACS, as levels of the studied pollutants rose, so did the risk for heart attacks.

“The cardiovascular effects of air pollution should be a serious concern for all, including policymakers, clinicians, and individuals,” Kan said.