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

Over four years, mathematics and verbal skills of some 20,000 people in China were monitored by researchers from Beijing's Peking University and Yale University in the US. Researchers tested people of both sexes aged 10 and above between 2010 and 2014, with 24 standardised maths questions and 34 word-recognition questions.

Researchers believe the results have global relevance, with more than 80% of the world's urban population breathing unsafe levels of air pollution.

The study was based on measurements of sulphur dioxide, nitrogen dioxide and PM10(particulates smaller than 10 micrometres in diameter) where participants lived. Carbon monoxide, ozone and larger particulates were not included in the study. However, while establishing a link between pollution and lower test scores, the study did not prove cause and effect.

The study which was published in the Proceedings of the National Academy of Sciences said: "We provide evidence that the effect of air pollution on verbal tests becomes more pronounced as people age, especially for men and the less educated." 

Many pollutants are thought to directly affect brain chemistry in a variety of ways – for instance, particulate matter can carry toxins through small passageways and directly enter the brain. The study also suggests that air pollution also increases the risk of degenerative diseases such as Alzheimer's and other forms of dementia. 

The researchers also suggest older men with less education were worst affected by chronic exposure to air pollution is because those subjects often work outdoor manual jobs.

"Our findings about the damaging effect of air pollution on cognition," the study concludes, "particularly on the aging brain, imply that the indirect effect on social welfare could be much larger than previously thought."

"For older persons (in our study those age 55-65 or 65+) the effects can be very difficult to mitigate given the long term cumulative exposure," Mr Xi says.

The study suggests that while the research findings are specific to China, it can shed light on other developing countries with severe air pollution.

The authors point to the 98% of cities with more than 100,000 people in low- and middle-income countries that fail to meet WHO air quality guidelines.

The study can be found here.