First of all, you go to any clothing store to buy a jumpsuit, the one with a gas mask. If you don't buy it, the mask won't be sold. The chemical mask can only be unlocked if you buy a suit. Then you find a tactical suit to recommend heavy armor, buy a bulletproof helmet in the hat bar, and finally wear a bulletproof helmet at the place where masks are sold, otherwise you can't wear chemical masks, and then wear chemical masks. That's what I did.