>Probably the most likely reason has been the removal of lead from gasoline, significantly reducing the degree of low-level lead poisoning
Blue on black. Favorite Daughter No. 1 started teaching six or seven years ago after majoring in ed. Wife just retired after 33 years of teaching high school science. I asked them about this because I thought that for sure it would be noticeable by now. They report seeing nothing. Apparently some other substance in the soup of our existence stepped into the breach. (Arsenic from lignite power plants? Estrogen mimicking plasticizing additives?)
Looking at what I see in the media, social media, and workplaces I interact with it would be a plausible hypothesis that dose makes the poison and at some small level lead is a nutrient of some sort.