Chronic exposure to air pollution can cause harm to cognitive performance, a new study reveals.
Researchers believe that the negative impact increases with age, and affects men with less education the worst.
Over four years, the maths and verbal skills of some 20,000 people in China were monitored by the US-Chinese study.
Researchers believe the results have global relevance, with more than 80% of the world's urban population breathing unsafe levels of air pollution.
The study was based on measurements of sulphur dioxide, nitrogen dioxide and particulates smaller than 10 micrometres in diameter where participants lived. It is not clear how much each of these three pollutants is to blame.
Carbon monoxide, ozone and larger particulates were not included in the study.
Described as an invisible killer, air pollution causes an estimated seven million premature deaths a year worldwide, according to the World Health Organization.
"We provide evidence that the effect of air pollution on verbal tests becomes more pronounced as people age, especially for men and the less educated," the study published on Monday in the Proceedings of the National Academy of Sciences said.
Pollution also increases the risk of degenerative diseases such as Alzheimer's and other forms of dementia, the study suggests.
Exposure to high levels of polluted air "can cause everyone to reduce their level of education by one year..., which is huge," one of the co-authors, Xi Chen of the Yale School of Public Health, told the Guardian newspaper.
Previous studies found air pollution had a negative impact on students' cognitive abilities.
In this study, researchers tested people of both sexes aged 10 and above between 2010 and 2014, with 24 standardised maths questions and 34 word-recognition questions.
One of the reasons the researchers suggest older men with less education were worst affected by chronic exposure to air population is because those subjects often work outdoor manual jobs.
"Our findings about the damaging effect of air pollution on cognition," the study concludes, "particularly on the aging brain, imply that the indirect effect on social welfare could be much larger than previously thought."
The study suggests that while the research findings are specific to China, it can shed light on other developing countries with severe air pollution.
The authors point to the 98% of cities with more than 100,000 people in low- and middle-income countries that fail to meet WHO air quality guidelines.
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