The study was based on a survey conducted by Yale and Peking University researchers who tested over 25,000 participants living around China, Shanghaiist reported.
In the study, residents were given standardized math and language questions during two exams, first in 2010 and again in 2014.
The scientists then compared the test results with the relative levels of pollution (sulfur dioxide, nitrogen dioxide, and PM10) where the participants were located.
The findings suggest that air pollution causes a “huge” negative effect on human intelligence. It was estimated that participants lost an average of one year of education due to inhaling pollutants.
The Yale School of Public Health health policy and economics professor Xi Chen, one of the study’s co-authors, noted that the effect is even more evidenced among the elderly population, who, in comparison, may have lost several years of education from inhaling the polluted air.
Old men without much education who are more likely to be exposed to higher levels of pollution while working outdoors are even more affected, according to the study.
While the study did not specify how pollution actually affects intelligence, it speculated on the possibility of pollution harming the brain’s white matter.
When the study was published, many expressed criticisms and stated that it did not necessarily establish cause and effect, NPR reports. The study’s critics noted that many other factors that influence a person’s intelligence were largely ignored in the survey.
In response, Chen explained that the study was able to account for other factors since the same people were tested four years apart.
Earlier this year, NGO Greenpeace East Asia found that concentrations of PM 2.5 — the smallest polluting particles which pose the greatest health risks — were 54% lower in the Chinese capital of Beijing during the fourth quarter of 2017 than during the same period of 2016.
Meanwhile, concentrations of PM 2.5 in 26 cities across northern China, were reportedly a third lower.