Chronic exposure to air pollution can cause major harm to cognitive performance, a joint study by United States and Chinese researchers has shown. The researchers monitored the math and verbal skills of 20,000 people in China over four years to form their conclusions, BBC reported on Tuesday.

People of both sexes above the age of 10 were subjects of the study, which was conducted between 2010 and 2014. They were asked 24 standardised math questions and 34 word-recognition questions.

The researchers believe that the study has global relevance, because 80% of the world’s population breathe polluted air. They also say that the impact of air pollution worsens with age, and affects men with low education the most – as they usually work manual jobs outside home.

The study was based on measurements of sulphur dioxide, nitrogen dioxide and particulates smaller than 10 micrometres in diameter, in areas where the participants lived, BBC reported. Measurements of carbon monoxide, ozone and larger particulate matter were not considered.

The study, published in the Proceedings of the National Academy of Sciences on Monday, added that pollution increases the risks of Alzheimer’s disease and other forms of dementia. The scientists also discovered that language ability is more harmed than mathematical ability, and men are more susceptible than women, The Guardian reported.

“Our findings about the damaging effect of air pollution on cognition, particularly on the aging brain, imply that the indirect effect on social welfare could be much larger than previously thought,” the researchers said.

“Polluted air can cause everyone to reduce their level of education by one year, which is huge,” Xi Chen of the Yale School of Public Health told The Guardian. “But we know the effect is worse for the elderly, especially those over 64, and for men, and for those with low education. If we calculate the loss for those, it may be a few years of education.”