Deteriorating air quality around the globe has been linked to declines in physical health, including lung cancer, heart disease, stroke, and overall life expectancy. However, new research that published on Monday in Proceedings of the National Academy of Sciences of the United States of America suggests that high levels of pollution can lead to a decline in cognitive ability, too.
The study analyzed scores on cognition tests taken by nearly 32,000 participants across China, searching for demographic trends that may be associated with pollution levels. And they found them.
“Long-term exposure to air pollution impedes cognitive performance in verbal and math tests,” Xiaobo Zhang, lead author on the study, told Eos. Zhang is a professor in the National School of Development at Peking University in Beijing and a senior research fellow at the International Food Policy Research Institute in Washington, D. C.
“The negative impact on verbal scores was more pronounced for men than women,” he said. “The damage increases as people age.”
The researchers note that most cities in developing nations, including China, fail to meet international air quality standards, so this study may have implications beyond China’s borders.
“The damage on cognitive ability by air pollution also likely impedes the development of human capital,” Zhang explained. “Therefore, a narrow focus on the negative effect on health may underestimate the total cost of air pollution.”
Sifting Through Scores
Zhang’s team mined cognition test results gathered in 2010 and 2014 by the China Family Panel Studies (CFPS), a national demographics survey conducted by Peking University. Among other questions, the survey included 24 standardized math questions and 34 word recognition questions of increasing difficulty. The surveys gathered responses from 162 counties spread over China and are representative of the Chinese population, according to CFPS.
Demographic data collected with the survey allowed researchers to split participants on the basis of personal factors like age, gender, and education level, which in China likely determines whether a person works predominantly outdoors and breathes unfiltered air. The data also supplied researchers with background information on test participants such as where they have lived and for how long.
By isolating these individual factors, the researchers could classify a benchmark value for each group. Assuming that a person’s scores started at the benchmark when they were young, the researchers began to ponder the factors that could have led to changes in a person’s cognition over time.
Given respondents’ age distribution as well as the spatial distribution of scores over much of China, the factor the researchers kept circling back to was air pollution.
Pollution on the Brain
Personal experience led Zhang to consider the health effects of air pollution. Zhang told Eos that when he returned to China from the United States in 2012, he immediately began experiencing headaches and found it hard to concentrate on research on days when Beijing had heavy air pollution.
Past studies, the team noted in the paper, had looked at how air pollution affects children’s test scores in school, so Zhang wondered if the effects were the same for everyone. He became curious how low air quality might affect different subpopulations in China. Do air pollution’s effects differ for older populations or for men and women or for those who work primarily outdoors?
So Zhang instructed his team to learn more about air pollution at the locations and times that the CFPF tests were administered. Fortuitously, the survey recorded the precise time and location of the tests, which the researchers used to gather local air quality data for the testing period. Specifically, the team examined each location’s air pollution index, a metric recorded by the Chinese Ministry of Environmental Protection that accounts for levels of sulfur dioxide, nitrogen dioxide, and inhalable particulate matter smaller than 10 micrometers, such as smog, smoke, ash, and dust.
High Pollution, Low Scores
Several areas in China are hot spots of poor air quality as defined by the air pollution index. The country’s northeastern coast from Shanghai to Beijing, in particular, has consistently been a source of unhealthily high pollution.
Most research into the cognitive impacts of air pollution “have focused on the U.S. or Europe, where ambient air pollution levels are relatively low,” said Dave Marcotte, a professor of public administration and policy at American University in Washington, D. C. “Because China has relatively high levels of air pollution, it is a setting highly relevant for other parts of the globe, including south and southeast Asia and urban areas in Africa.” Marcotte, who was not involved in this research, has studied educational impacts of air pollution.
When mapping these locations to test scores, the researchers found that, overall, lower verbal test scores matched times with heavy air pollution, regardless of age, gender, or education level. This effect was stronger than what was seen in math scores. The researchers also found that 1 week of exposure saw a roughly 0.3-point drop in verbal scores, but 3 years’ exposure saw a 1.1-point drop.
Looking at gender, men’s verbal test scores dropped nearly twice as much as women’s after 3 years of exposure to heavy pollution when compared to the benchmark. For men, but not women, the decrease in verbal cognition after exposure to heavy pollution was more pronounced with increasing age and even more so for men with less than a middle school education. Math scores of the two genders dropped roughly the same amount in each age group in those with the highest exposure to pollution.
The researchers speculate that the gender gap in verbal scores corresponds to differences in how white matter and gray matter are activated in men’s and women’s brains during testing. Past research has shown that white matter and gray matter, used for more verbal and more math cognition, respectively, have different sensitivities to air pollution.
Marcotte called this research “an excellent and convincing study” and lauded that unlike previous research that focused mainly on pregnancies and infants, this study expands our understanding into pollution’s cognitive impacts on other groups. He added that future work should also seek to account for personal behaviors, like smoking, which may affect cognitive performance.
Reduce Air Pollution to Save Cognitive Function
Cognitive decline with age, the team noted, is a risk factor for Alzheimer’s disease, which costs hundreds of billions of dollars in health services each year and affects almost 2% of the adult population over 65. “The damage on the aging brain by air pollution likely imposes substantial health and economic costs,” the team wrote, “considering that cognitive functioning is critical for the elderly for both running daily errands and making high-stakes decisions.”
Reducing air pollution in China to meet the air quality standards published by the U.S. Environmental Protection Agency could increase verbal scores by 2.41 points and math scores by 0.39 point nationwide in future populations, the team estimated. Had these standards already been in place, men older than 64 who have been exposed to long-term heavy pollution might have seen average verbal scores more than 9 points higher than currently reported, or the equivalent of moving from the median score to the 87th percentile.
“Investment in cleaning up air pollution is not only good for health,” Zhang said, “but also for the intellect of society at large.”
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer