system
Given the following post, respond as if you're commenting on Reddit:
human
Why are halogenated chemicals so dangerous to living organisms?

It seems like most halogenated chemicals (Cl2, Br2, HF, HCl, PCB's, organohalides, etc) are harmful to living organisms in some way.  Is there a biological/biochemical explanation for this?
gpt
They don't have to be. For example table salt is NaCl and it is actually necessary for staying alive. So is having some iodide around for thyroid hormones.

Elemental halogens are nasty because they are very reactive and very electronegative.

HCl is a strong acid and is actually harmful because of the H+ it makes, not the Cl-.

Halogenated organic molecules aren't always harmful, but often are similar enough to existing molecules for living things to get them mixed up. Because halogens are comparatively rather electronegative, they change the electron distribution causing the halogenated molecule to act a little differently in a biological context. This is very useful for designing drugs, actually.