Stroke Prevalence Linked to Poor Air Quality

Nancy A. Melville

February 19, 2016

LOS ANGELES — Higher prevalence rates of stroke correspond to areas with poor air quality in the United States and China, with higher temperatures potentially exacerbating that relationship, new research shows.

"[This] study, using nationally representative data, is one of the first studies to address a positive and complex association between air quality and prevalence of stroke, and a potential interaction effect of temperatures on the air-stroke association," the researchers, with lead author Longjian Liu, MD, PhD, an associate professor of epidemiology and biostatistics at Drexel University in Philadelphia, Pennsylvania, conclude.

The results were presented here at the International Stroke Conference (ISC) 2016.

While many adverse health effects of poor air quality have been well documented, Dr Liu and his colleagues hypothesized that areas with high air pollution may have higher stroke rates.

For the study, the researchers looked at air pollution data from 49 states in the United States that included estimated measures of particulate matter from 2010 to 2013. They also looked at data from 120 cities in 32 provinces in China that had air pollution index (API) measures for 2012 to 2013.

The United States and China were selected because of their roles as "the world's two largest emitters of greenhouse gases and responsible for about one third of global warming to date," Dr Liu said in an American Stroke Association press statement.

Comparing the data with stroke measures, the researchers found that stroke prevalence rates in the United States did indeed correspond with pollution levels — increasing by 1.19% for every 10 μg/m3 of air increase in levels of particulate matter less than 2.5 μm in diameter (PM2.5), which is known to present the greatest health risk because of the small particulate size (P < .001).

Particulate matter levels in the United States varied significantly from state to state (P = .01), as well as from month to month, with the highest levels in July (10.2 μg/m3) and the lowest in October (7.63 μg/m3).

The highest average annual PM2.5 levels were seen in the southern region of the United States and the lowest were in the West, which correlate to the highest prevalence of stroke in the South (4.2%) compared with the lowest in the West (3%), the authors reported.

In China, stroke prevalence was also significantly higher in cities with higher API concentrations, while the associations between air quality and the risk for stroke were significantly mediated by temperatures.

The highest API in China was in December, with levels of 75.76 in 2012 and 97.51 in 2013, while the lowest levels were recorded in July, with levels of 51.21 in 2012 and 54.23 in 2013.

"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," Dr Liu said.

In addition, "patients with stroke are in danger of dehydration due to high temperatures in the summer and are in danger of suffering from pneumonia, influenza, and other respiratory diseases in winter. Women and the elderly also appear more vulnerable to stroke risk due to air quality and heat-related diseases," he said.

Asked to comment on the findings, Daniel T. Lackland, DrPH, professor of epidemiology and neurology at Medical University of South Carolina in Charleston, said the mechanisms linking air pollution with stroke are not well understood.

"Certainly we are looking for the reasons and factors associated with disease burden from stroke and we don't have all the answers," he told Medscape Medical News.

"Pollution has an ecological association but is still poorly understood. For example, the population exposed to air pollution may also have poor hypertension control, which leads to the excess disease risks."

Furthermore, important caveats include the difficulty in understanding the extent of exposure to poor air quality and stroke risk.

"What complicates this is the difficulty to quantify 'exposure'," Dr Lackland said.

"Most are not exposed as they spend most time in a 'climate'-controlled environment including air conditioning, et cetera."

The adverse health effects of PM2.5 have been acknowledged in an American Heart Association (AHA) Scientific Statement https://circ.ahajournals.org/content/121/21/2331.full.pdf+html issued in 2010, in which the AHA concluded, "PM2.5 exposure is deemed a modifiable factor that contributes to cardiovascular morbidity and mortality."

Exposure to PM2.5 even over the course of a few hours to weeks can trigger cardiovascular disease–related mortality and nonfatal events, the statement concludes, and longer-term exposure increases the risk for cardiovascular mortality to an even greater degree.

Likewise, "reductions in PM levels are associated with decreases in cardiovascular mortality within a time frame as short as a few years; and many credible pathological mechanisms have been elucidated that lend biological plausibility to these findings."

In terms of mechanisms behind the risk, evidence points to a link between PM2.5 and systemic inflammation and the elevation of proinflammatory biomarkers, suggesting a response that is not confined to the lung after inhalation of pollution.

The study was funded by a joint grant of Drexel (US)–SARI (China) Co-Research and Education on Low Carbon and Healthy City Technology Study. The authors and Dr Lackland have disclosed no relevant financial relationships.

International Stroke Conference (ISC) 2016. Abstract 36. Presented February 17, 2016.

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