Worrying; not a huge surprise, there's been other (epidemiological) evidence suggesting this kind of link in the past; on the basis of this article, not necessarily conclusive. E.g., they tried to control for other things known to affect IQ, but that's very hard, and they could, for example, have missed something that caused both the high pollution indicators and the low IQ. It's also not clear to me from this why they think it's a prenatal influence rather than an early childhood influence: surely the same children heavily exposed in utero were heavily exposed as babies and young children? As an illustration of possible confounding factors - just as an illustration, I'm not arguing that it's likely - it could be, from this, that the children with highest pollution exposures live in areas with the heaviest traffic, and therefore get the least opportunity to run around and be boisterous on their early outings. If that limited their IQ (as I believe is the case with baby rats!), we'd see this kind of effect without pollution being anything to do with it. I fear, though, that it's probably exactly what it looks like: pollution damaging the developing brain.
