If you buy only one fruit or vegetable organic, make sure it's this: apples. Year after year, the nonprofit Environmental Working Group has listed apples as the No. 1 most contaminated produce. Eighty percent of apples sampled by the USDA tested positive for a chemical called DPA (or diphenylamine), which farmers use to prevent the skin from darkening. While it extends the shelf life of apples, it has the potential to be a serious carcinogen. And since the DPA is sprayed on apples after they are harvested, its concentration is highest when consumers buy them. Buy organic, or at the very least, wash apples thoroughly and peel them before eating.