If there is one scientific principle that the media and their audiences should know, it is that:
It’s very difficult to prove a causal relationship without some type of complex dataset, abstract analysis or an interventional study, which is sometimes impossible depending on the subject. It very well could be that pollution makes people stupid, but this study has not shown that. It has simply shown that areas with high air pollution tend to have students who perform poorly on tests. It’s also just as likely that pollution is highest in low income areas, which also tend to have poor academic performance (so income is the auxiliary variable that may cause both pollution and poor test scores). Its cheap to build polluting factories in shitty areas, and they also find cheap labor among people with the lowest socio-economic status, which also correlates with poor academic achievement.
So remember, if there is a relationship between A and B, it could be that A causes B, or B causes A. Or it could be that something unknown (C) causes A and B. Or it could be something more complex.
The authors try to claim they have shown a causal relationship, but they only sampled 4 years, and while they can suggest causation, they rely on significant assumptions to do so and they have certainly not proven it with any kind of scientific standard:
Chen said air pollution was most likely to be the cause of the loss of intelligence, rather than simply being a correlation. The study followed the same individuals as air pollution varied from one year to the next, meaning that many other possible causal factors such as genetic differences are automatically accounted for.
**tl;dr: Just because there is a relationship between two things doesn’t mean one causes the other. Correlation does not equal Causation!