Air Pollution Tied to Cognitive Decline

Batya Swift Yasgur, MA, LSW

August 30, 2018

Air pollution can impair cognitive performance, especially in older people, men, and those with less education, new research suggests.

Investigators studied the verbal and mathematical skills of more than 25,000 people in China over a 4-year period. They found a correlation between worse air quality and cognitive decline, especially on verbal test scores and particularly in older men who were less educated.

"Long-term exposure to air pollution impeded cognitive performance in verbal and math tests, and the negative impact was more pronounced for men than women, with the damage increasing as people age," senior author Xiaobo Zhang, PhD, National School of Development, Peking University, Beijing, China, and the Division of Development Strategy and Governance, International Food Policy Research Institute, Washington, DC, told Medscape Medical News.

"Investing in cleaner air not only improves human health but also cognitive capital, so when evaluating the impact of air pollution, the hidden cost on intellect should not be ignored," Zhang added.

The study was published online August 27 in the Proceedings of the National Academy of Sciences.

"Hard to Concentrate"

Significant research has demonstrated that air pollution harms human health in a variety of ways, but little research has focused on its impact on cognitive abilities, especially in older people, the authors write.

Earlier research has been imprecise in matching exposure to local environmental stressors at the exact time of cognitive testing with individual cognitive performance.

Additionally, although these studies considered either the effects of transitory exposure or cumulative exposure to air pollution, few studied both effects simultaneously.

"I returned from the US to Peking University to teach in 2012. Immediately, I felt a headache, and [it was] very hard to concentrate on research in days with heavy air pollution in Beijing," Zhang said.

"Keen to understand the impact of air pollution on cognitive function," he collaborated with lead author Xin Zhang, PhD, of Beijing Normal University, and Xi Chen, PhD, assistant professor of public health (health policy), the Institution for Social and Policy Studies, Yale University, New Haven, Connecticut, to conduct the investigation.

He noted that air pollution "became increasingly serious during the period of 2012 to 2014 in Chinese cities, with a lot of media reports covering the problem."

To investigate these issues, the researchers studied the cognitive scores of 25,486 individuals older than 10 years. The information was drawn from a dataset of cognitive scores taken in 2010 and 2014 in a nationally representative survey of Chinese families and individuals.

In both waves (2010 and 2014), a cognitive ability module consisting of 24 standardized mathematical questions and 34 word-recognition questions was used.

To assess air quality, the researchers used the air pollution index (API), which is calculated on the basis of daily readings of levels of sulfur dioxide, nitrogen dioxide, and particulate matter <10 μm (PM10). They obtained daily API observations from city-level air quality reports published by China's Ministry of Environmental Protection.

In addition, the investigators analyzed the weather on the date of cognitive testing. This enabled them to distinguish the impact of air pollution from the impact of general weather patterns.

They then used an econometric formula to calculate the relationship between cognitive performance, air quality, and weather in participants of both sexes at various ages and levels of education.

Declining Verbal Skills

Of the observations conducted in both 2010 and 2014, the researchers matched 31,944 persons to API and weather data.

Most cognitive testing was conducted in the afternoon and evening. The authors note that among the three pollutants, PM10 was dominant throughout the day.

Because most interviews were conducted during summer months, the researchers used a weighted regression formula to ensure the findings were robust, despite seasonal differences at the time of testing.

The researchers employed seven windows of exposure to air pollution: 1 day, 7 days, 30 days, 1 year, and 3 years. They found that air pollution inhibited respondents' test performance, particularly when a longer window of exposure measure was used.

An increase in the 7-day mean API by 1 standard deviation (SD) was found to lower verbal test scores by 0.278 points (0.026 SD); a 1-SD increase in average API over the 3 years before the interview was associated with a 1.132 point (0.108 SD) decline in verbal test scores.

In comparing subgroups of men and women, the researchers found that air pollution was associated with lower verbal test scores in both sexes, with longer duration of exposure associated with a larger effect.

However, the effect was much stronger in men than in women, with a 10% significance level in those between the ages of 55 and 64 years and a 1% significance level in those older than 65 years.

"As a result, the gender gap in the decline of verbal skills widens as people age," the authors write. They note that the pattern was "less noticeable" for math than for verbal test scores.

White Matter Damage?

The effect for men older than 44 years whose education was at the primary school level or lower was "highly negative." Among the more educated subsample, the negative effect appeared only in men aged 65 years or older.

Educational level did not play a similar role in women.

When 3-year mean API was decreased by 1 SD, less educated men younger than 64 years showed an increase in verbal test scores by 9.18 points (or the movement of people from the median to the 87th percentile in the verbal test distribution) relative to the cohort that was younger than 25 years.

The effect remained sizable for more educated older men: a 1-SD decrease in 3-year mean API was associated with an increase in verbal test scores of 1.88 points (or the movement of people from the median to the 69th percentile in the verbal test distribution) for this group, relative to their younger counterparts.

"We suspect that pollutant damage is most likely accumulating in the white matter of the brain, which is associated with the functioning of verbal ability," Chen told Medscape Medical News.

"The activated white matter is in greater shortage within men's brains than women's, therefore demonstrating the large gender gap, especially in verbal skills, that we observed — ie, men performed much worse in verbal tests, relative to women, on the same polluted day," Chen said.

"More research is needed to understand this linkage," he added.

“Huge” Societal Loss

Commenting for Medscape Medical News, Masashi Kitazawa, PhD, associate professor, Center for Occupational and Environmental Health, Department of Medicine/Occupational and Environmental Medicine Program, Public Health University of California, Irvine, said that the study "nicely unveils the substantial impact of air pollution, particularly on aged men with less education."

He noted that the study has an important take-home message for practicing clinicians.

"While we still do not know the exact causes of dementia, this study clearly highlights that air pollution could be one of the risk factors to consider," said Kitazawa, who was not involved with the research.

From a public health perspective, "we have to be aware of the risk of air pollution on our cognition, since decreased cognitive reserve or human capital will be a huge loss for our society as a whole," he said.

Zhang suggested that people should try to "avoid making big decisions that require great mental power during the days of heavy air pollution."

"For practicing clinicians, it can be important to take necessary actions to avoid pollution, such as installing air filters in health facilities," Chen added.

The study was funded by the Yale Macmillan Center Faculty Research Fund, the US Federal PEPPER Center, the NIH/National Institute on Aging, the China Postdoctoral Science Foundation, and the Fundamental Research Funds for the Central Universities. The study authors and Dr Kitzawa have disclosed no relevant financial relationships.

Proc Natl Acad Sci. Published online August 27, 2018. Abstract

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