There are many environmental poisons that affect brain development, consumption of lead, for instance. Also there are many diseases that can effect brain development, such as congenital syphilis.
Overall exposure is lower, sure, that doesn't mean it is insignificant in impact on IQ (and other health issues, notably cardiovascular disease.)
No, lead exposure from water systems, unabated older housing, continued use of leaded aviation fuel, and other sources continues to be a nontrivial issue in the US and some other parts of the first world.
https://www.ncbi.nlm.nih.gov/pubmed/2628311
https://www.sciencedaily.com/releases/2004/11/041117005027.h...