It’s pretty obvious that air pollution makes people sick and shortens lives. Microscopic particulate matter, sulfur dioxide, and nitrogen oxides emitted by things like coal plants worsen respiratory conditions as well as heart diseases. But do the pollutants that can cloud the sky cloud your mind, too?

A study led by Beijing Normal University’s Xin Zhang and Yale’s Xi Chen took advantage of a powerful dataset to expand our knowledge on this question. Many previous studies have focused on students, comparing school testing results with regional air quality, for example. The new study used the results of a massive Chinese survey of more than 50,000 people who took standardized tests in 2010 and then again in 2014. Such “longitudinal” studies that follow individuals over time are excellent because you can compare a person to their own previous results. That's better than using two groups of people whose differences you hope will average out.

The tests included a set of increasingly difficult math questions and verbal/language tasks. Subjects continued answering questions until they missed three in a row and were assigned a score based on how far they got.

The date, time, and location of each test was recorded, so the researchers were able to match the results with local air quality measurements. If air pollution inhibits your ability to think, you could expect to see some correlation between air quality and scores. And if long-term exposure causes permanent damage, you might see people living in the most polluted areas getting lower scores in 2014 than they got back in 2010.

The results are striking but a little complex. When the researchers calculated air quality, they did so for a range of time spans from just the day of the test to the three years preceding the test. If you just look at the day of the test, you don’t see much of a correlation between air pollution and scores. That means the short-term impact of a “bad air day” seems to be limited.

But for longer time spans, a clear pattern emerges. Verbal test scores show a significant correlation with air quality that grows with time, reaching its maximum at the three-year air quality average. Math scores show a similar pattern but with a much smaller impact that is just barely statistically significant for longer time periods.

If you separate men and women, however, each have slightly different trends. Women’s verbal scores were less affected by air pollution, but their math scores were slightly more affected.

While the researchers emphasize that they can’t speak to the precise physiological processes behind these trends, they do point out some basic differences between white and gray matter in the brain that could help explain the gender split. They write, “Air pollution has a stronger effect on white matter (required more by verbal tests) than on gray matter (required more by math tests). Since men have a much smaller amount of white matter activated during intelligence tests, their cognitive performance, especially in the verbal domain, tends to be more affected by exposure to air pollution.”

The test data can also be broken down by age and education level. The verbal scores of older men show a stronger connection to air pollution, as do the verbal scores of less-educated men.

The researchers suggest that their results tell us we’ve done an incomplete job of accounting for the health and economic impacts of pollution. Many analyses have calculated things like health care costs for respiratory and heart conditions affected by air quality or productivity lost as employees miss work. Apart from the disturbing loss of personal cognitive skill, it could be that some portion of the health care costs of other diseases should also be included.

The researchers write, “Cognitive decline or impairment are risk factors of Alzheimer’s disease and other forms of dementia for elderly persons. As the most expensive form of cognitive decline, Alzheimer’s disease alone costs $226 billion of health services and 18 billion labor hours of unpaid caregiving in 2015. Moreover, given that senior citizens have to make a host of complex high-stake economic decisions, such as purchasing health insurance and planning retirement, the decay in cognitive ability induced by air pollution will likely impair the quality of the important decisions.”

If any of that is due to air pollution, the costs certainly aren’t reflected in the price of goods or energy produced with fossil fuels. Some of us pay the price, nonetheless.

PNAS, 2018. DOI: 10.1073/pnas.1809474115 (About DOIs).