Researchers investigating the effects of air pollution conducted math and verbal tests over the course of multiple years on more than 25,000 people in 162 Chinese counties. They matched those results with pollution conditions at the time of each test, and found sobering results.
In paper published Monday in Proceedings of the National Academy of Sciences, scientists found that pollution is linked to a significant decline in cognition, and that the impact increases with age.
The findings underscore the need for China to clean up the air, says study co-author Xi Chen, a professor of health policy and economics at the Yale School of Public Health.
"If the air pollution improves from China's level to the American EPA standard level, that means that would improve everyone's education by around one year," Chen said, referring to an annual EPA standard measurement for particulate matter that was used until 2006.
The researchers tested the same subjects in 2010 and 2014, and the math and verbal tests were meant to show "different dimensions, representative of the functioning of the brain," Chen said. The data allowed the researchers to compare the effect of pollution across ages, from children to the elderly.
"We can say that the bigger impact is toward the older adults," Chen added.
The scientists found both short-term and cumulative effects of air pollution on cognitive performance. Pollution's impact on verbal test performance became worse as people aged, particularly among men and people with less education.
People with lower education levels are likely to experience more harm, Chen says, because they work outside more often and are exposed to higher levels of pollution.
He says exposure to pollution could make elderly people less effective in making major financial and medical decisions.
Could the link between cognitive decline and pollution be caused by a another factor? Chen says the study tries to overcome that issue by testing the same people over time.
It's not clear precisely how pollution impacts cognitive decline, though Chen suggests that pollution may have a negative effect on the brain's white matter, which could vary between men and women. As NPR has reported, "the brain's white matter coordinates communication among brain regions."
James Hendrix, the director of Global Science Initiatives at the Alzheimer's Association, was skeptical of the claim that pollution impacts white matter. "I think that's speculation. I think we don't have a direct cause and effect that can be proven," he said.
He says that establishing a causal link between pollution exposure and cognitive decline is challenging: "I would say that it probably increases your risk — how much is difficult to say." He says there are other factors that people should be aware of — such as diet, exercise, cognitive stimulation and social interaction — and could possibly change.
"All of these things play a role in an overall risk of Alzheimer's," Hendrix says.
China has a major pollution problem that has resulted in red alerts in dozens of cities, and the government is actively trying to tackle the issue.
Last year tens of thousands of factories were shut down so that inspection crews could determine who was breaking environmental regulations. As NPR's Rob Schmitz reported, "China's government may be finally getting serious about enforcing its environmental laws."