Acute Exposure to Air Pollution Can Trigger MI-Related Death

New data from China show that greater exposure to some forms of air pollution, even in the short term, is linked to an increased risk of dying from myocardial infarction.

The relationship was seen for particulate matter with an aerodynamic diameter of ≤ 2.5 µg (PM_2.5) and ≤ 10 µg (PM₁₀), as well as for nitrogen dioxide (NO₂), Yuewei Liu, MD, PhD (Sun Yat-sen University, Guangzhou, China), and colleagues report in their paper published online today in the Journal of the American College of Cardiology.

Earlier studies have shown a connection between air pollution and more-chronic forms of heart disease, as well as for MI itself. This study takes a different angle, the researchers explain. “Because MI is a leading cause of death that accounts for over 30% of all deaths from ischemic heart diseases, it is of great importance to understand whether air pollutant exposures also trigger deaths from MI.”

Physicians who commented on the results for TCTMD said this is indeed a relevant question. Its answer, they said, confirms the long-held suspicion that air pollution—whether short- or long-term—is bad for cardiovascular health.

Jonathan D. Newman, MD, MPH (NYU Langone Health, New York, NY), drew attention to the fact that the biggest upswing in risk with particulate matter was actually seen among people with lower exposures. “That is relevant worldwide and to other populations that are less highly exposed to anthropogenic or fossil fuel combustion-related air pollution,” he said, adding, “There’s a significant risk even in the lower groups of exposure, which argues for the importance of control measures even for populations that are less highly exposed.”

The arrows all point to one direction, which is: air pollution is bad for your heart. Robert Kloner

Here, mean exposures of PM_2.5 and PM₁₀ were 63.4 and 100.4 µg/m³, respectively, on days when the MI deaths occurred (on control days, mean levels were slightly lower at 62.1 and 99.5 µg/m³). By comparison, Newman noted, the Environmental Protection Agency’s “gold standard is less than 12 µg/m³ in the United States” for PM_2.5.

Robert Kloner, MD, PhD (Huntington Medical Research Institutes, Pasadena, CA), agreed that while the findings are confirmatory, this doesn’t mean they are uninteresting. What’s unique here is not only the primary endpoint but also the precision by which the time of exposure was captured, he said. Their design “is quite a sophisticated approach” that provides hints on mechanism, specifically oxidative stress, inflammation, thrombosis, and vascular dysfunction. His own research has shown, in animal models, that ultrafine particles delivered directly to the heart can impair function, irrespective of age and whether there’s preexisting cardiovascular disease.

“You don’t get atherosclerosis in 1 day,” stressed Kloner. “This really does point to the concept that air pollution can act as an acute trigger, as an acute risk factor, for death from heart attack.”

Pollution Close to Home

For their study, Liu and colleagues used a time-stratified, case-crossover design to look at 151,608 MI-related deaths that took place in China’s Hubei province from 2013 to 2018. Exposure to air pollution—particulate matter, sulfur dioxide, NO₂, carbon monoxide, and ozone—was assessed based on each person’s home address, with data drawn from local air-quality monitoring stations.

They compared mean exposure on the day of death and the day prior with other weekdays in the same month, finding significant relationships between MI death risk for PM_2.5, PM₁₀, and NO₂. With PM_2.5 and PM₁₀, the risk associated with exposure increased up until the breakpoints of 33.3 µg/m³ and 57.3 µg/m³, respectively, then flattened out, whereas relationship was nearly linear for NO₂.

Increase in Odds of MI Mortality

“These findings add to the understanding of acute adverse effects of air pollution on cardiovascular mortality and highlight the needs for either general population or policy practitioners to take effective measures in reducing air pollution exposures, especially for older adults and those with higher risk of MI occurrence,” the investigators conclude.

From Policy to Personal

Next steps may indeed come at the policy level, and now the world has had a glimpse of what positive changes can produce.

“One silver lining in the dark cloud of COVID-19 has been the singular impact of the pandemic on global air pollution emissions. Early in the pandemic, with complete cessation of road traffic and air travel in many parts of the world, there was a dramatic impact on air pollution levels,” Sanjay Rajagopalan, MD (University Hospitals and Case Western Reserve University, Cleveland, OH), and Jagat Narula, MD (Icahn School of Medicine at Mount Sinai, New York, NY), write in an accompanying editorial. “PM_2.5 plummeted across the globe resulting in ‘blue sky days,’ allowing anyone to ponder a future free from air pollution.”

Newman said that there is a clear need for policies that shift away from fossil fuels, as well as evidence to support these broad interventions. But, as he and others addressed in a recent review on pollution’s cardiopulmonary impact, far less is known about “personal-level interventions,” like face masks or air filters, which could be studied in randomized controlled trials.

As of now, “I would wager that there’s little advice being given in regards to air pollution exposure and risk-mitigation strategies in routine clinical practice,” partly due to the need to address traditional risk factors during the limited time frame of an office visit, Newman said, adding that physicians may be stymied by a lack of knowledge on what to offer, particularly when evidence is absent.

Yet there is growing awareness, he added, and modern tools like smartphones might be leveraged to provide details on air quality for specific regions. Less obvious is how to apply this information. “There are always unintended consequences. For example, I think in general with physical activity, the benefits . . . outweigh for most people any potential added [risk] from air pollution, so you wouldn’t necessarily want to caution people not to exercise or do things outdoors,” he suggested, though this added risk might be more relevant for individuals with comorbidities or in situations, like forest fires, when pollution levels are particularly noxious.

For Kloner, the main advice is for patients to avoid polluted air when possible, especially if they live near a major highway or other potent source. “It will be interesting to see if during the COVID era maybe there are fewer problems with this, because people have been wearing masks. As masks become the norm, maybe we’ll see some changes,” he added. It’s also key that patients otherwise at high risk get the best preventive therapies.

The mounting evidence is consistent, Kloner concluded. “Every little bit of knowledge is helpful. And the arrows all point to one direction, which is: air pollution is bad for your heart.”