Hazard Summary: Highly toxic by ingestion, inhalation, and skin absorption. Known carcinogen. Strong irritant. LD50 100 mg/kg. TLV 0.3 ppm, PEL 0.75 ppm as formaldehyde gas. Use only in a fume hood.
Shelf Life: 1-3 years. A flaky white precipitate eventually forms in all bottles of formaldehyde. This is paraformaldehyde, a polymer of formaldehyde also known as trioxane. Over time, this does reduce the concentration of formaldehyde in solution. Methanol is added to aqueous formaldehyde solutions to retard polymerization, but the process cannot be prevented.