Air Pollution Linked to Anxiety, Increased Stroke Risk

Liam Davenport

March 26, 2015

Symptoms of clinically meaningful anxiety are increased by exposure to fine particulate air pollution, regardless of the presence of major comorbid conditions, results of a large observational study indicate.

The results suggest that fine particulate air pollution increases the risk for high anxiety symptoms by 12% to 15%, with the association stronger with exposure in the previous 1 to 3 months.

"Our study suggests that higher exposure to fine particulate matter, especially higher recent exposure, is associated with an increased risk of high symptoms of anxiety," the investigators, led by Melinda Power, ScD, postdoctoral fellow in epidemiology, John Hopkins Bloomberg School of Public Health, in Baltimore, Maryland, write.

The study was published online March 24 in the BMJ, alongside an article indicating that air pollution is also associated with an increased risk for stroke, particularly in developing countries.

Intriguing Findings

Dr Power and colleagues note there is "a small but growing body of literature" suggesting a link between air pollution and mental health outcomes. They add that there is a paucity of research examining this association in humans.

To investigate the association between exposure to particulate air pollution and anxiety symptoms, the researchers examined data from the Nurses' Health Study.

This study included up to 71,271 women from throughout the United States whose average age was 70 years. Valid estimates of exposure to particulate matter were available for these women during multiple periods averaging up to 15 years.

In addition, prevalent symptoms of anxiety were assessed in 2004 using the phobic anxiety subscale of the Crown-Crisp index. A score of 6 or more was considered a meaningfully high level of anxiety.

The team determined that 15% of women had high symptoms of anxiety. Exposure to fine particulate matter was significantly associated with high symptoms of anxiety, at adjusted odds ratios of 1.12 for exposure in the previous month, 1.13 for the previous 3 months, 1.14 in the previous 6 months, and 1.15 in the previous 12 months (P = .0001, P = .0004, P = .002, and P = .001, respectively).

There was no evidence that the association was affected by age, previous anxiety levels, metropolitan statistical area residence, region of country, or the presence of respiratory or cardiac conditions or major comorbidities.

In addition, there was no association between high anxiety symptoms and exposure to coarse particulate matter or with proximity of residence to a major road.

"I think what I'd really emphasize is that this is an observational study, with very intriguing findings, but it definitely does need to be replicated in other settings and with other methods before we make any strong recommendations about what people should do to influence their anxiety levels," Dr Power told Medscape Medical News.

The second study, conducted by investigators at the University of Edinburgh, in the United Kingdom, is a systematic review and meta-analysis of 103 studies.

In this study, researchers found that increases in the levels of the gaseous pollutants carbon monoxide, sulphur dioxide, and nitrogen dioxide increased the risk for stroke-related hospital admissions or death, at a 1.5% increased risk per 1 ppm, 1.9% per 10 ppb, and 1.4% per 10 ppb, respectively.

Global Health Concern

In an accompanying editorial, Michael Brauer, ScD, MD, a professor at the School of Population and Public Health, University of British Columbia, Canada, notes that "the findings of these two studies support a sharper focus on air pollution as a leading global health concern."

"The two linked papers in this issue confirm the urgent need to manage air pollution globally as a cause of ill health and offer the promise that reducing pollution could be a cost effective way to reduce the large burden of disease from both stroke and poor mental health," Dr Brauer writes.

Both Dr Power and Dr Brauer believe the mechanism underlying the association beween anxiety and exposure to air pollution may be linked to inflammation and oxidative stress.

"It's the most plausible pathway based on what we understand from how air pollution affects other organ systems. What we know is that air pollution affects the heart, and that seems to be via a pathway that involves inflammation," said Dr Brauer.

"Once you invoke that, things like effects on the brain are plausible, but that's really about all we can say at this stage," he added.

Dr Brauer concluded by observing that it is important to study environmental risk factors for mental health issues.

"The reason that air pollution may be important is not that it is likely to be a very strong risk factor for anxiety but that collectively, because everyone is exposed to air pollution, everyone has the potential to be affected by it," he said.

"The flip side of that is that, if this really is a causal link, if you make improvements in air quality, there's a potential for really getting wide population benefits."

The authors and Dr Brauer report no relevant financial relationships.

BMJ. Published online March 24, 2015. Full text, article 1; Full text, article 2; Editorial

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