Low doses of certain chemicals at specific days of fetal development can cause permanent problems that often only manifest later in life. The same dose at a different time may have no measurable effect at all.
Tiny doses are often toxic to some people.
We are all exposed to mixtures of hundreds of chemicals, and no one knows the dangers of these mixtures in our blood. Not only that, no one can ever know. To test just 100 chemicals in combinations of 3 would require 162,000 tests. And that is just for one effect, say cancer. For nervous system toxicity, it would take another 162,000.