Air pollution could impede cognitive ability as people get older, according to a study from researchers in China and the U.S.



The research, published online in the Proceedings of the National Academy of Sciences on Monday, found that long-term exposure to air pollution impeded people's performance in both verbal and math tests.



The skills of thousands of people in China were monitored for the study over a period of more than four years, according to reports. The impact of air pollution on verbal tests, researchers found, became more pronounced as people got older. This was especially the case for men and the less educated.



"The damage on the aging brain by air pollution likely imposes substantial health and economic costs, considering that cognitive functioning is critical for the elderly for both running daily errands and making high-stake decisions," the paper said.



Air pollution poses a serious problem to people all over the world. 4.2 million people die each year from exposure to outdoor air pollution, according to the World Health Organization (WHO), while 91 percent of the planet's population live in areas where the air quality exceeds WHO guideline limits.



"Polluted air can cause everyone to reduce their level of education by one year, which is huge," the Yale School of Public Health's Xi Chen, one of the report's authors, said.