THURSDAY, Oct. 9, 2003 (HealthDayNews) -- Hospital admissions for stroke increase as air pollution levels go up, a Taiwanese study finds.
Data on more than 23,000 patients between 1997 and 2000 shows a close relationship between the incidence of strokes and two major air pollutants, small particles and nitrogen dioxide, says a report in the Oct. 10 issue of Stroke.
"The effects seem to be stronger on warm days," says a statement by Chun-Yuh Yang, director of the Institute of Public Health at the Kaohsiung Medical University.
"I am not surprised that there is a relationship between air pollution and stroke," says Dr. Marc R. Mayberg, chairman of the department of neurosurgery at the Cleveland Clinic Foundation and chairman of the American Heart Association stroke council. "I am surprised that they have been able scientifically to verify the relationship."
Previous studies trying to link air pollution to the risk of stroke have produced mixed results, Mayberg says. "It is a difficult thing to show because air pollution varies quite a bit, and it is not clear that air pollution on one day causes changes in the body that are evident a few days later. And different people have different stroke risks, so it is hard to isolate specific factors."
The Taiwanese researchers compared air pollution levels on the days when stroke patients entered the hospital with levels one week before and one week after those admissions.
Every 25 percent increase in fine particulate pollutants and nitrogen dioxide levels was associated with a 54 percent increase in hemorrhagic strokes, which occur when a blood vessel bursts. Hospital admissions for ischemic stroke, in which a clot blocks a blood vessel, increased by 55 percent for a 25 percent increase in nitrogen dioxide levels and 46 percent for a 25 percent increase in fine particulate pollutants.
The effect was strongest on days when the temperature was above 20 degrees Celsius, 68 degrees on the Fahrenheit scale.
A link between stroke and air pollution is to be expected because "there is pretty clear evidence that air pollution causes cardiovascular and respiratory disease," Mayberg says. "The association between stroke and the other two problems is high, so it is logical that conditions that cause heart and lung problems cause stroke."
The only downside to the study is that it doesn't lead to advice for people with specific factors that increase the risk of stroke, he says.
"When you do this kind of population study, you don't know anything about the patients, not even whether they are men or women," Mayberg says. "Also, you don't know what the link is between pollution and risk, what is it in air pollution that leads to stroke. That can happen in three or four ways."
Thomas Johnson, director of respiratory therapy at the Long Island University Brooklyn campus, suggests several mechanisms of damage. For example, "nitrogen dioxide alters arterial pressure and may stress already damaged blood vessels, especially in the brain," he says.
And pollutants can penetrate deeply enough to attack the alveoli, the tiny sacs where air exchange occurs in the lungs, Johnson says.
"But nailing down the actual mechanism is a little bit of conjecture at this time," he says. "What is clear is that people who are at risk of stroke should take proper precautions when air pollution levels are high, like filtering air to cut their exposure to pollutants."
The people who should take the most precautions, Mayberg says, are those with known stroke risk factors such as diabetes, high blood pressure, heart disease and smoking. "They should try to avoid exposure to polluted air," he says.
Stroke risk factors that require protective action can be found at the American Heart Association. Meanwhile, learn about how pollution affects your health in other ways from the American Academy of Family Physicians.