Arsenic has long been recognized as a poison and a contaminant in drinking water, but now concerns are growing about arsenic in foods, especially in fruit juices that are a mainstay for children. Controversy over arsenic in apple juice made headlines as the school year began when Mehmet Oz, M.D., host of “The Dr. Oz Show,” told viewers that tests he’d commissioned found 10 of three dozen apple-juice samples with total arsenic levels exceeding 10 parts per billion (ppb). There’s no federal arsenic threshold for juice or most foods, though the limit for bottled and public water is 10 ppb....