Air pollution is causing a "huge" reduction in our intelligence, according to a new study.
Toxic air can cause everyone's level of education to reduce by one year – and could extend to a few years' worth of education for those aged over 64.
Dr Xi Chen, from the Yale School of Public Health in the US, said this can have serious consequences for the elderly, as this is usually when the most critical financial decisions of their lifetime are made.
The study also suggests that high levels of air pollution have a short-term impact on intelligence, meaning students may perform worse if they take important exams on polluted days.
Dr Chen warned the damage to intelligence is likely to increase as the world's air becomes more polluted, as every 1mg rise in pollution equates to losing a month of education.
The research, published in The Guardian, was carried out in China but could have ramifications worldwide, as figures suggest 95% of the world's population is breathing unsafe air.
Dr Chen warned: "There is no shortcut to solve this issue. Governments really need to take concrete measures to reduce air pollution.
"That may benefit human capital, which is one of the most important driving forces of economic growth."
As part of the study, scientists analysed the language and arithmetic tests of 20,000 people, and compared the results with records of nitrogen dioxide and sulphur dioxide.
The longer a person was exposed to toxic air, the worse the damage was to their intelligence.
Language ability suffered more than mathematical ability and men were harmed more than women – likely because there are differences in how male and female brains work.
2:37Dr Chen said toxic air was most likely to be the cause of the intelligence loss rather than a correlation in the results.
Scientists accounted for the gradual decline in brain function seen as people age, and ruled out subjects being more uncooperative or impatient during tests when pollution was high.
The team's study followed the same individuals as air pollution levels changed from year to year, meaning that many other possible causes such as genetic differences were accounted for.
The research has been published in the Proceedings of the National Academy of Science journal.
Past studies have concluded that air pollution harms cognitive performance in students, but the more recent findings are the first to examine people of all ages and the difference between men and women.
Toxic air reportedly causes more than seven million premature deaths a year but the harm to people's cognitive abilities is not as well understood.
Results published in January found that air pollution was linked to "extremely high mortality" in people with mental disorders.
Findings released in 2016 found that toxic air was linked to increased mental illness in children – and in 2017, scientists warned that living near heavy traffic increases the risk of dementia.
More from UK
Air pollution is falling in China but levels remain three times above World Health Organisation's limits.
The body states that 20 of the world's most polluted cities are in developing countries.
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