That is according to a recent article published in PNAS by Zhang, Zhang and Chen (2018). As reported in The Economist:
…chronic exposure to pollution lowered the scores on the verbal tests, and that the higher the pollution levels were the more the scores dropped. On average, an increase of 13.23 units (one standard deviation) in the pollution index over the course of three years resulted in a reduction of 1.36 points for men and 0.91 points for women, on the 34-point verbal exam…The effects were particularly dramatic in older men who had no education beyond primary school. The data showed that these men lost an average of 9.18 points on the verbal exam if they were exposed to an increase of 13.23 units of pollution over three years.
Note that only verbal test scores–but not mathematics–were affected by pollution. Thus, further research is needed, but the results merit a closer examination of the true costs of pollution.