According to Olga Naidenko, PhD, a senior scientist for the Environmental Working Group (via this article in Prevention):
The problem: Chemicals, including perfluorooctanoic acid (PFOA), in the lining of the bag, are part of a class of compounds that may be linked to infertility in humans, according to a recent study from UCLA. In animal testing, the chemicals cause liver, testicular, and pancreatic cancer. Studies show that microwaving causes the chemicals to vaporize—and migrate into your popcorn. “They stay in your body for years and accumulate there,” says Naidenko, which is why researchers worry that levels in humans could approach the amounts causing cancers in laboratory animals. DuPont and other manufacturers have promised to phase out PFOA by 2015 under a voluntary EPA plan, but millions of bags of popcorn will be sold between now and then.
The solution: Pop natural kernels the old-fashioned way: in a skillet. For flavorings, you can add real butter or dried seasonings, such as dill weed, vegetable flakes, or soup mix.
Budget tip: Popping your own popcorn is dirt cheap.