Islamabad - Chronic exposure to air pollution could be linked to poor cognitive performance, a new study in China suggests. 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 .

However, while establishing a link between pollution and lower test scores, the study did not prove cause and effect.

The study  published in the Proceedings of the National Academy of Sciences  -  includes researchers from Beijing’s Peking University and Yale University in the US - 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 the study stated. Described as an invisible killer, air pollution causes an estimated seven million premature deaths a year worldwide, according to the World Health Organization. Pollution also increases the risk of degenerative diseases such as Alzheimer’s and other forms of dementia, the study suggests. “Our samples enable us to examine the impact of air pollution as people age. So our results across the life course are quite new,” one of the co-authors, Xi Chen of the Yale School of Public Health, told the BBC.