SOURCES: Melinda Power, Sc.D., postdoctoral fellow, epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore; Michael Brauer, Sc.D., professor, University of British Columbia, School of Population and Public Health, Vancouver, Canada; March 24, 2015, BMJ
WEDNESDAY, March 25, 2015 (HealthDay News) -- Air pollution may take a toll not only on physical health, but mental well-being as well, two new studies suggest.
In one, researchers confirmed a long-studied connection between air pollution and cardiovascular health -- finding evidence that dirty air may help trigger strokes in vulnerable people.
The other study looked at a newer question: Could air pollution also affect mental health? The answer, it found, is "possibly." Among over 70,000 U.S. women in the study, those who lived in relatively polluted areas were more likely to report multiple anxiety symptoms.
The studies, published online March 24 in the BMJ, only link these factors; they do not prove that air pollution is the direct cause of either strokes or anxiety.
There could be other explanations, said Melinda Power, a researcher at Johns Hopkins University, in Baltimore, who led the anxiety study.
She said her team included the other possible factors that they could account for, such as whether women lived in a big city, or had heart or lung conditions.
"But you can't account for everything," said Power, who was with Harvard University at the time of the study.
"I think some of the most likely alternative explanations would be other forms of pollution," Power said. Chronic noise -- from traffic, for example -- is one possibility, she noted.
It's too soon to declare that better air quality could help ease anxiety symptoms, Power stressed. "But it's an interesting finding," she said. "And studies need to look further into the association between air pollution and mental health."
If a connection is confirmed, then reducing air pollution could have an "important impact" on mental health on the broader scale, according to Michael Brauer, a professor at the University of British Columbia, in Vancouver, Canada. Brauer is also the author of an editorial in the same issue of the journal.
That's because both anxiety disorders and pollution are common, worldwide problems, Brauer said.
But, he stressed, it's too early to call air pollution a risk factor for anxiety. "This is early research," Brauer said. "It's an intriguing finding, but you can't make conclusions from a single study."
In contrast, the idea that air pollution can contribute to strokes, or heart attacks, has a large body of research behind it, Brauer pointed out.
The new stroke study is actually an analysis of about 100 previous studies spanning 28 countries. And overall, the researchers reported, there was a "clear association" between air pollution and people's short-term risk of having or dying from a stroke.
Researchers believe that in vulnerable people -- including the elderly, and people with existing artery disease -- spikes in air pollution may trigger a heart attack or stroke by causing inflammation in the blood vessels.
The American Heart Association already recommends that people at risk of heart attack or stroke try to limit their time outdoors on days when air quality is poor.
But the risks go beyond the short-term, Brauer noted. Other studies, he said, have suggested that long-term exposure to dirty air contributes to the development of clogged arteries in the first place.
For the anxiety study, Power's team used data from a long-term health study of over 71,000 U.S. women aged 57 to 85. The women were asked some standard questions about anxiety symptoms -- looking at whether they had certain phobias or tended to be worriers in general.
Overall, 15 percent showed "high symptoms" of anxiety -- though, Power said, that does not necessarily mean they had an anxiety disorder.
The researchers then estimated the women's exposure to air pollution based on where they lived.
In general, the study found, the women's risk of anxiety symptoms increased along with their exposure to fine particle pollution. Those particles are released into the air when fossil fuels are burned, so car exhaust and industrial sources, such as power plants, are big contributors.
How would air pollution feed anxiety symptoms? One possibility, Power said, is through an indirect effect -- by worsening heart or lung disease, for example. But, she said, this study suggests that chronic physical conditions are not the missing puzzle piece.
A more speculative explanation is inflammation, Brauer said. Some lab research has suggested that inflammation affecting the brain could contribute to anxiety.
"So it's biologically plausible," Brauer said. "But we need more research looking at the potential mechanisms."
For now, he said, "I don't think people should avoid exercising outside because they're afraid of anxiety symptoms."
On the other hand, Brauer added, there are already reasons to limit your exposure to air pollution. So it's still wise, he said, to listen to local air-quality reports and heed warnings about hazardous pollution levels.
The U.S. Environmental Protection Agency offers updates on local air quality.