In the late 19th century, German scientists discovered that an easy-to-make acid could rather neatly peel off a layer of skin cells. They promptly marketed it for “skin rejuvenation.” Today, that compound, trichloroacetic acid, or TCA, is widely used by dermatologists both to brighten up aging faces and to remove damaged skin cells, including precancerous ones. You don’t have to get a chemical peel to be exposed to TCA. It is used as an antiseptic, a soil sterilizing agent and a reagent in pharmaceutical manufacturing; it turns up in drinking water as a byproduct of chlorination, and as a metabolite of some industrial solvents and some medications, like chloral hydrate, a common sedative. Research with animals shows that it can be absorbed both in drinking water or through dermal exposure, seeping through the skin into the bloodstream.