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Being exposed to air pollution over a long period of time can harm intelligence, a new study finds. One of its authors, Dr Xi Chen from Yale School of Public Health, said: "Exposure to air pollution affects people's cognitive skills and ability not only in the short-term, but also in the long-term". Over four years, the maths and verbal skills of some 20,000 people in China were monitored. Researchers believe the results have global relevance, with more than 80% of the world's urban population breathing unsafe levels of air pollution. (Image: A cyclist wearing an anti-pollution mask on London's Oxford Street. Credit: Â鶹Éç).