Air pollution irritates our eyes, noses, and throats; and it can upset our emotional equilibrium as well. Over time, pollution exposure causes breathing difficulties and symptoms of asthma and heart disease and a variety of diseases including cancers, immune, neurologic, reproductive, and respiratory problems.

Now it appears that air pollution can also make us anxious.

Researchers from Harvard and Johns Hopkins studied the connection between anxiety and air pollution in a sample of over 70,000 women between the ages of 57 and 85.

They computed each woman's exposure to air pollution based on where they lived in relation to busy streets and meteorological measurements of levels of particulate matter (pollution) in their environment and used a standardized anxiety rating scale to measure their emotional distress.

Exposure within the preceding month had the strongest effect.

They followed the women over time to track the connection between their degree of particulate exposure to symptoms of anxiety.

About 15% of the women experienced symptoms of high anxiety. Those who were exposed to pollution were at higher risk for anxiety than their less exposed peers. This remained true when education, age, marital status, and socioeconomic status were the same.

Recent exposure to air pollution was significantly related to high anxiety: Exposure within the preceding month had the strongest effect. Proximity mattered too — women who lived between 55 yards to 220 yards from a major road were more likely to have high anxiety symptoms than those who lived farther away. But being closer than 55 yards to a major road did not make the risk of anxiety any greater.

The authors offer two explanations for why pollution may increase anxiety. Exposure to particulate matter may lead to increased physiologic stress on the body by causing increased inflammation and oxidative stress. Both happen when the body is unable to keep up with the demand to neutralize or counteract the effect of damage-causing compounds that enter the body from the environment.

Animal studies have shown that oxidative stress increases biochemical measures of stress and anxiety-like symptoms in test animals, supporting this hypothesis.

A second possibility is that exposure to pollutants promotes or aggravates existing chronic diseases such as respiratory illnesses and heart disease and this, in turn, produces stress responses and anxiety.

Anxiety is a common and often debilitating illness which is associated with reduced productivity, increased medical care costs, and increased risk of suicide.

The authors suggest that, based on their study results, reducing exposure to pollution through changes to regulations and changes in people's behavior may be an effective way to decrease the individual and societal burden of this condition.

The study is published in the BMJ.