Does Air Pollution Cause Anxiety?

Power MC, Kioumourtzoglou MA, Hart JE, Okereke OI, Laden F, Weisskopf MG. The relation between past exposure to fine particulate air pollution and prevalent anxiety: observational cohort study. BMJ. 2015 Mar 24;350:h1111. 

This observational cohort study’s purpose was to determine whether higher past exposure to particulate air pollution can be associated with high symptoms of anxiety.

The researchers selected 71,271 women enrolled in the Nurses’ Health Study (NHS) who lived in the contiguous United States and for whom valid data of particulate matter exposure were available during the time periods of interest. Ages ranged between 57 and 85 years (mean: 70 y).

Using home addresses, which were updated every 2 years as part of the NHS, the researchers used latitude and longitude data to estimate particulate air pollution exposure measured via levels of particulate matter (PM); this pollution was characterized by standard size categories (PM2.5 or PM10) during the 1-month, 3-month, 6-month, 1-year, and 15-year time periods prior to an assessment of the participants for anxiety symptoms. Distance of residence from major roads 2 years before anxiety assessment was also determined. 

Anxiety levels were assessed using the Crown-Crisp index’s phobic anxiety subscale.

Higher exposure to particulates in the PM2.5 range (<2.5 μm in diameter) was significantly associated with increased odds of high anxiety symptoms over multiple time periods. As an example, every per 10 µg/m³ increase in the prior 1-month average of PM2.5 increased odds of high-level anxiety by 12% (odds ratio [OR]:1.12; 95% confidence interval [CI]:1.06-1.19). This same increase in exposure to PM2.5 over the previous 12 months increased odds of high anxiety only slightly more, by 15% [OR:1.15, CI: 1.06-1.26]. Short-term exposure appeared more relevant than long-term exposure, with more recent exposures potentially more relevant than more distant exposures. Neither the larger particulate size PM10 (2.5 µm-10 µm in diameter) nor proximity of residence to major roads appeared to be associated with anxiety. 

The potential link between particulate levels and anxiety levels is surprising. Until now, we have associated high levels of particulate matter mainly with cardiovascular disease (CVD) and respiratory illness; as Walter Crinnion, ND, notes in this issue of the Natural Medicine Journal, particulate matter is “associated with adverse effects in the respiratory, cardiovascular, and nervous systems, in addition to stimulating oxidative damage and inflammation.”

 

The idea that PM pollutants might impact mood is relatively new. The majority of such papers have focused on depressive symptoms. In a 2012 article on premenopausal women in rural India, Bannerjee et al reported a strong correlation between depression and cooking with biomass pellets made of reprocessed organic material. The explanation offered to explain this association was the high PM levels in the participants’ homes, the result of this cooking method.¹ Likewise, in a 2014 publication, Cho reported a significant association between air pollutant levels in Korea with the number of emergency room visits for depressive complaints.²

In a January 2015 review, Tzivian et al reported on 15 articles that related to the long-term effects of air pollution and ambient noise levels on cognitive and psychological functions in adults. Their conclusion: “Both exposures were separately shown to be associated with one or several measures of global cognitive function, verbal and nonverbal learning and memory, activities of daily living, depressive symptoms, elevated anxiety, and nuisance.”³ Unfortunately no study examined both exposures at the same time, and it is often hard to separate the 2 factors.³ For example, an April 2015 study tells us that traffic wardens in Pakistan have above-average levels of depression, stress, public conflict, irritation, behavioral problems, speech interference, hypertension, loss of concentration, hearing impairment, headache, and CVD. The authors of this study blamed high noise levels for these cognitive effects, though they neglected to report PM exposure levels.⁴ One would suspect such exposure was high.

 

A March 2014 study that sought an association between particulate matter levels and depression in Boston was unable to prove one⁵; this article was immediately criticized for its methodology.⁶

 

The smallest of the airborne particles (PM0.1 or smaller) are small enough to cross from lungs to blood and then across the blood brain barrier to reach the brain. In addition, larger particulates (PM2.5 and PM10) can carry small molecules such as solvent residue, which then traverses the alveoli and enters the bloodstream directly. This is probably why air pollution is associated with stroke and depression in adults and why children exposed to pollution “show significant systemic inflammation, immunodysregulation at systemic, intrathechal and brain levels, neuroinflammation and brain oxidative stress, along with the main hallmarks of Alzheimer and Parkinson’s diseases.”⁷

 

This current report of a significant association with PM and anxiety should not come as a surprise. The only surprise is that we hadn’t considered this possibility until now. These results certainly have a clear clinical implication. We should consider the potential impact of air quality on any patient with anxiety symptoms. 

 

Potential improvement in anxiety symptoms might be achieved if patients simply use an air filter at home. Few medical interventions will come with a lower risk profile than this, a consideration often important for anxious patients. The potential side effects of using an air filter are all desirable, particularly reduced CVD risk and according to a March 2015 paper, reduced risk of stroke.⁸