Exactly, scientific research established that DDT was perfectly safe, too! Scientists even used to eat it to "prove" that it was safe.

In reality it depends, being biology, "safe level" is also very relative since you don't know every effect the substance has on the body.

That's why pesticides and other chemicals such as bisphenols are regularly phased out, since effects can appear long after "scientific research" established it was "safe". Or it can affect certain populations, such as farmers, who get a high dose, or children, who are more sensitive than adults.

Others, such DDT, lead or cadmium, are accumulated in the body over a long period, and then start to show effects, even when the person has stopped eating it. Or can find their way later the food chain: Inuits would get poisoned when eating polar bear's meat, that was full of DDT from fields on the other side of the globe.