Air Pollution Particle Size Tied to Higher Stroke Mortality

Kelli Whitlock Burton

June 03, 2022

Poor air quality may increase the risk of in-hospital death among patients with ischemic stroke by up to 24%, depending on the size of air pollution particles, new research shows.

Investigators found that mortality risk was highest among ischemic stroke patients who had prolonged exposure to air pollution, especially to small particulate matter (PM), such as soot and smog.

The research adds to a growing body of evidence tying air pollution exposure to stroke outcomes and offers new information on the role of PM size in mortality risk.

"Air pollution has been previously linked to a greater risk of stroke, and stroke is a leading cause of death worldwide," study investigator Hualiang Lin, PhD, of Sun Yat-sen University in Guangzhou, China, said in a press release.

"What is lesser known is how the different sizes of particulate matter affect that risk. Our research found that the size of air pollution particles may affect a person's risk of dying from stroke."

The study was published online May 25 in Neurology.

Size Matters

As previously reported by Medscape Medical News, living in areas with high air pollution is associated with increased risk of stroke and stroke-related mortality. One study showed that air pollution may be implicated in up to one third of all strokes.

However, data on the type of air pollution and how particle size is involved in stroke have been lacking.

To explore that question, researchers used electronic medical records in China to identify more than 3.1 million hospitalizations for ischemic and hemorrhagic stroke; 32,140 people, or 1%, died of stroke while hospitalized.

The study assessed the role of three sizes of air pollution particulate matter: submicron particulate matter that is <1 μm in diameter (PM1), such as soot and smog; fine particulate matter that is <2.5 μm in diameter (PM2.5), such as fly ash from coal combustion; and respirable particulate matter that is <10 μm in diameter (PM10), such as cement dust.

Using participants' home addresses and records of daily concentrations of air pollution particulate matter, researchers identified individual levels of air pollution exposure for each participant. They then calculated 7-day air pollution exposure immediately before hospitalization for stroke.

For each 10 μg/m3 increase in annual average exposure to particulate matter, the risk of in-hospital stroke mortality increased.

Stroke mortality risk rose 24% with exposure to PM1 (adjusted odds ratio [aOR], 1.240; 95% CI, 1.217 – 1.265), 10.5% from exposure to PM2 (aOR, 1.105; 95% CI, 1.094 – 1.116), and 9% from exposure to PM10 (aOR, 1.090; 95% CI, 1.082 – 1.099).

The greatest risk of death from stroke was in people with ischemic stroke who had been exposed to the smallest air pollution particles.

"Our study includes measurements of PM1, which may be small enough to be inhaled deeply into lungs, pass through lung tissue, and circulate in the bloodstream," Lin said in the press release. "Obtaining a deeper understanding of the risk factors of all particulate matter sizes and the magnitude of their possible effects may help reduce the number of deaths and improve the outcomes for people with stroke."

Although the findings show that smaller air particles are linked with higher risk of stroke mortality, reducing exposure to larger particulate matter may have a larger effect on improving stroke outcomes, because the level of PM10 pollution was higher than levels of smaller particles, researchers found.

In fact, the study showed that reducing PM10 could reduce stroke mortality by 10% in the short term (95% CI, 8.3 – 11.7) and 21.1% over the long term (95% CI, 19.1 – 23).

"The results of counterfactual analyses indicate that ambient PM10 was associated with a larger burden of stroke in-hospital case fatality, compared to smaller-size PM," the authors note. "The results suggest that public health policy efforts should consider targeting reducing ambient PM10 instead of focusing on smaller-sized PM."

Mechanism Still Unclear

In an accompanying editorial, Enrique C. Leira, MD, director of the Division of Cerebrovascular Disease and the Comprehensive Stroke Center at the University of Iowa, Iowa City, and Julius G. Latorre MD, medical director of the Neurology-Stroke Service at Upstate Medical University, Syracuse, New York, note the researchers did not adjust for smoking history, stroke severity, or body mass index. They also note that there are no data on morbidity or mortality after hospital discharge. More importantly, the findings do not show a causal relationship between air pollution PM and stroke mortality.

"While this study provides interesting insights regarding the effect of different sizes of particles in stroke mortality, it does not inform about the likely causal mechanism," they write.

More research that explores a causal relationship is critical to efforts to implement effective environmental interventions, Leira and Latorre write. Still, they add, the new research offers detail that could move the field in that direction.

"There are already plenty of health reasons to improve the quality of the air, including stroke prevention. Studies like this that more accurately estimate a reduction in mortality should help regulatory bodies and policy makers contemplating the design and implementation of environmental interventions," they write.

The investigators and editorialists report no relevant financial relationships.

Neurology. Published online May 25, 2022. Abstract, Editorial

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