Air pollution, even at levels considered safe, are anything but, according to a new study.
A report, published in the Feb. 14 issue of the Archives of Internal Medicine, by researchers from Beth Israel Deaconess Medical Center in Boston, found that exposure to ambient fine particulate matter, usually from vehicle traffic, was associated with a significantly higher risk of ischemic strokes on days when the Environmental Protection Agency’s air quality index for particulate matter was yellow instead of green.
The researchers concluded the odds of having a stroke were 34 percent higher on or following moderate air quality days.
Researchers analyzed more than 1,700 medical records of patients in the Boston area who went to the hospital for treatment of confirmed strokes.
They matched the onset of stroke symptoms in each patient to hourly measurements of particulate air pollution taken at the Harvard School of Public Health’s environmental monitoring station.
The particles, known as PM2.5, came from a variety of sources including power plants, factories, trucks and automobiles, and the burning of wood.
Researchers found that black carbon and nitrogen dioxide, two pollutants associated with vehicle traffic, were closely linked with stroke risks.
“The link between increased stroke risk and these particles can be observed within hours of exposure and are most strongly associated with pollution from local or transported traffic emissions,” said Dr. Murray A. Mittleman, senior author of the study.
Mittleman said the study suggests that the EPA may need to strengthen its language when describing the health consequences of moderate air quality.
“Any proposed changes in regulated pollution levels must consider the impact of lower levels on public health,” he said.
An ischemic stroke means blood flow to the brain has been interrupted. explained internal medicine physician Dr. Kevin Stigge, of Lakeview Hospital in Bountiful.
Ischemic strokes account for 87 percent of all strokes. The other 13 percent are hemorrhagic strokes, which cause bleeding in the brain.
Symptoms include weakness, tingling or a loss of feeling on one side of the face or body, vision problems, speech problems, difficulty talking, headache, problems understanding others, loss of balance and fainting.
Effects of pollution
Air pollution is known to cause a variety of effects on the overall health of the human body, said Stigge.
Bad air quality can cause compromised respiratory function — and can be especially hard on those with asthma.
“Further detrimental effects of air pollution, such as those referenced in this study, may very well exist,” he said. “While there’s a link between increased levels of air pollution and risk of a stroke, the exact mechanism has not yet been fully studied.
“Most likely, however, it is related to the known facts of how smoking can have both immediate and long-term changes in the body, to increased risk of both stroke and heart attack.”
On average, someone suffers a stroke every 40 seconds, Stigge said, and someone dies every four minutes. Each year, 795,000 people suffer a new or recurrent stroke.
Those at highest risk include smokers, those with previous strokes or a mini stroke, anyone with a mechanical heart valve or anyone diagnosed with diabetes, high blood pressure, high cholesterol, heart disease, vascular disease or obstructive sleep apnea.
Kristy Chambers, a coordinator of the stroke and chest pain center at Ogden Regional Medical Center, said that although continued research to study environmental factors related to stroke is needed, those at higher risk of suffering a stroke should closely monitor air-quality advisories and avoid exercising outdoors when the air-quality index is anything beyond satisfactory.
“Air pollution particles can lodge deep in the lungs and have been linked to difficulty breathing in asthmatics and heart attack patients,” she said.
Dr. RJ Bunnell, medical director of the stroke program at McKay-Dee Hospital, said the exact mechanism by which fine particles in the air increase stroke risk is unclear.
“In animal models, these particles have been associated with faster formation of cholesterol plaques and more unstable plaques that can rupture, causing clotting and potentially a stroke or heart attack,” Bunnell said. “Air pollution over the short- and long-term has been shown to increase the risk of stroke and heart attack.”
While researchers say results need to be done in other cities, they note that Boston is considered to have relatively clean air.