appeal to nature fallacy. Just because something is natural doesn't mean that it is harmless. In the same way, just because something doesn't exist in its final form in nature doesn't mean that it's going to kill you.
One more thing I'd like to point out is your use of the word "chemical." Everything is a chemical. We are made of chemicals, the drugs that we take are made of chemicals, and the food that we eat is made of chemicals. Chemicals aren't inherently bad.