You can however lower IQ with environmental changes very effectively.

Getting rid of parasites improves educational outcomes of children. Possibly their IQ as well.

Who knows how much smarter would we be if flus, colds and other duseases didn't ravage bodies of our children.

My understanding is that removing lead from a variety of infrastructure has done wonders for people's intelligence, as has enabling better family planning.

Which failures are you talking about?

I consider those to be in the category of not lowering people's IQ. Making someone dumber is fairly easy: just drop them on their heads as a child, malnourish them, and expose them to lead and lots of parasites. And make sure they don't get enough iodine. But in rich countries we've already fixed those things, and there doesn't seem to be much else we can do.