Formalin should be stored in a well-ventilated area away from oxidizing agents. Formaldehyde gas can form explosive mixtures in air when the concentration exceeds 7%. This could occur in the event of a fire.
Formalin should not be stored in the same cabinet as hydrochloric acid or chlorine-containing solutions, including bleach (sodium hypochlorite). Chemical reactions between these substances could occur that would form the potent carcinogen, bis(chloromethyl) ether.