A Warning on Chemical Peels --- 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. --- But scrutiny intensified after a 2011 review by the federal Environmental Protection Agency, which cited study after study in lab mice linking TCA to liver cancer; one research group called it a “complete carcinogen.” More, NYTimes