Person 1: I once or twice got a small drop of a dangerous chemical on my skin in 1982

Person 2: I bathe in that same chemical for 30 minutes every single day. It’s no different than 1982. The risk of harm is the same.

In this case Person 2 has no duty to prove that bathing in it is fine or that getting exposed to a small drop is dangerous.

If Person 1 is claiming that there are harmful effects, they are required to prove that they exist and explain why bathing in it causes those effects while being exposed to it otherwise does not.