Chronic exposure to air pollution could be linked to 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 – which 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.
Carbon monoxide, ozone and larger particulates were not included in the study.
Described as an invisible killer, air pollution causes an estimated seven million premature deaths a year worldwide, according to the World Health Organization.