Lye (sodium hydroxide)
This highly caustic alkaline chemical cures fish and olives, straightens hair, and bleaches wood faster than you can say "Ow! That stuff really burns!" But unless your house's previous owner had a taste for Scandinavian lutefisk, lye would probably be lying around for one reason: drain cleaning. While the chemical is legal, it's still as dangerous as ever—especially if moisture hits the dry, granulated version. Inhaling lye fumes can cause your throat to swell, inflame your lungs, and also affect your esophagus. It can also burn skin on contact or blind you if it gets in your eyes; if ingested, it can eat holes in the digestive tract and even induce a coma.