As levels of air pollution rise, so too does the risk for stroke, a new study suggests.
Researchers used data from the United States and China. These two countries are the biggest producers of greenhouse gases in the world, and are responsible for one-third of global warming, according to study lead author Dr. Longjian Liu.
“Cities with poorer air quality have significantly higher prevalence of stroke, compared with cities that have a better air quality,” said Liu, an associate professor of epidemiology and biostatistics at Drexel University in Philadelphia.
The differences are particularly striking in winter and summer, Lui added. “Winter and summer have higher concentrations of air pollution than spring and fall, and death from strokes is significantly higher in winter,” he said.
It’s important to note, however, that this study was only designed to look for an association between air pollution and stroke. It did not prove a cause-and-effect relationship.
The results of the study were scheduled to be presented Wednesday at the annual meeting of the American Stroke Association (ASA), in Los Angeles. Findings presented at meetings are generally viewed as preliminary until they’ve been published in a peer-reviewed journal.
Stroke is the fifth leading cause of death in the United States, killing nearly 129,000 Americans each year, and is a leading cause of disability, according to the ASA. In addition, stroke is the second leading cause of death worldwide, after heart disease.
For the study, Liu’s team collected data on air quality between 2010 and 2013. The data came from more than 1,000 counties in 49 U.S. states and from 120 cities in 32 provinces in China, the study authors said.
The researchers looked at a type of pollution known as particulate matter. These tiny bits of air pollution come from cars, power plants, forest fires and other sources, according to the U.S Environmental Protection Agency (EPA). Such particles that are less than 2.5 micrometers in diameter (PM2.5) pose the greatest health risks because they are so small — 1/30th diameter of a human hair — and can easily lodge in the lungs.
The investigators found that across the United States and China, the total number of stroke cases rose 1.19 percent for each 10 micrograms per cubic meter increase in PM2.5.
In addition, the researchers found regional differences in PM2.5 levels linked to the number of strokes.
The American South had the highest average annual PM2.5 levels, while the West had the lowest, according to Liu. The South, known as the “stroke belt,” had the highest prevalence of stroke at 4.2 percent, compared with the West, which had the lowest at 3 percent, Liu said.
Temperature also seemed to have an effect on air quality and the risk of stroke, he said. Seasonal variations in air quality can be partly attributable to climate changes, he explained.
“In the summer, there are lots of rainy and windy days, which can help disperse air pollution. High temperatures create a critical thermal stress that may lead to an increased risk for stroke and other heat- and air quality-related illnesses and deaths,” Liu said.
Although patients can’t control air quality, these study findings provide evidence for policy makers and public health officials to develop better models for monitoring and predicting climate changes so patients can better protect themselves, Liu suggested.
“Air pollution, extreme cold in winter or extreme heat in summer are risk factors for stroke,” he said. “Patients, specifically older adults, who live in areas with poor air quality should pay specific attention to the risk of stroke that may be caused by both air pollution and extreme cold or heat,” Liu said.
Dr. Gregg Fonarow, a professor of cardiovascular medicine and science at the University of California, Los Angeles, said, “While a number of studies have linked increases in heart attack and hospitalizations for heart failure to increases in pollution levels, there have been few studies looking at associations with the number of strokes.”
Certain types of pollution have been shown to increase inflammation in blood vessels, which in turn may increase stroke risk, he said.
Reducing air pollution may have a positive effect on cutting the number of strokes and heart disease, Fonarow added. “Efforts to improve air quality may translate to reductions in heart attack, heart failure and stroke,” he said.
For more about stroke, visit the American Stroke Association.