A second study in the same issue links air pollution to an increased risk of anxiety.
Both studies were observational and therefore do not offer direct proof that polluted air causes stroke or anxiety. But the findings add to a growing body of research that has identified air pollution as a major global health problem that requires urgent attention.
The stroke study
The association between long-term exposure to air pollution and an increased risk of cardiovascular disease, including heart attacks and strokes, is well established. Indeed, repeated exposure to a single form of air pollution — particulate matter — is believed responsible for more than 3 million deaths worldwide each year.
Less is known about the cardiovascular effects of short-term exposure to air pollution.
To investigate that lesser-understood link, a team of researchers from Edinburgh University conducted a meta-analysis, a type of study that analyzes pooled data from many different but similar studies. In total, the researchers analyzed 103 studies involving 6.2 million stroke-related hospitalizations and/or deaths in 28 countries.
They found that several types of pollution — nitrogen dioxide, sulfur dioxide, carbon monoxide and particulate matter — were associated with a substantially greater risk of stroke. Only ozone showed a minimal risk.
For example, the associated increased risk for sulphur dioxide was 1.9 percent per 10 parts per billion, while for small particulate matter (particles smaller than 2.5 micrometers — or 100 times thinner than a human hair) the increased risk was 1.1 percent per 10 micrograms of cubic air.
The study also found, not surprisingly, that the higher the pollution levels, the greater the risk. Furthermore, the analysis revealed that the highest risk for stroke occurred on the day of exposure, although the risk tended to persist for slightly longer when the exposure was to particulate matter.
In addition, although only 20 percent of the studies in the meta-analysis were conducted in middle- to low-income countries — mostly China — those countries demonstrated a stronger association between pollution and strokes than higher-income countries.
The reasons for an association between stroke and polluted air are not fully understood, although previous research has shown that pollution constricts blood vessels, increases blood pressure and raises the risk of blood clots.
The anxiety study
For the second study, researchers at Johns Hopkins University and Harvard University analyzed data collected from more than 71,000 American women, aged 57 to 85, who were participants in the ongoing Nurses’ Health Study.
First, they calculated the women’s exposure to particulate matter, using national geographic and meteorological data. They also calculated the distance between the women’s homes and the nearest major road — a commonly used indicator of exposure to traffic-related air pollution.
With that data in hand, the researchers then looked at whether women with higher exposure to particulate matter were more likely to have symptoms of anxiety. Based on questionnaires, about 15 percent of the 71,000 women in the study reported high levels of anxiety.
The analysis revealed a significant association between exposure to small particulate matter (less than 2.5 micrometers in diameter) and high levels of anxiety. The more recent the exposure — especially if it had occurred within the past month — the higher the risk.
The findings held even after controlling for such varied factors as age, a pre-existing heart condition and region of the country.
The researchers found little support, however, for an association between anxiety levels and large particulate matter. Nor did they find a strong association between anxiety levels and proximity to a major road. For, although women who lived between 54 and 218 yards of a busy road were more likely to report high levels of anxiety than those who lived more than 218 yards away, women who lived within 54 yards of a major road were not.
‘An urgent need’
In a commentary that accompanies the two studies, Michael Brauer, a professor of population and public health at the University of British Columbia, writes that these findings “confirm the urgent need to manage air pollution globally as a cause of ill health.”
“One of the unique features of air pollution as a risk factor for disease is that exposure to air pollution is almost universal,” he adds. “While this is a primary reason for the large disease burden attributable to outdoor air pollution, it also follows that even modest reductions in pollution could have widespread benefits throughout populations.”
Both studies and the commentary can be read in full on the BMJ website.