You might have heard recently that microwave popcorn isn't safe because it contains chemicals that can increase your risk of cancer. The most common rumor is that it is associated with lung cancer, but in reality, there's no direct link. There is a minor link, however, between microwave popcorn and certain other cancers.
While there's no truth to the rumor that microwave popcorn contains chemicals proven to cause lung cancer, there is a chemical used in the nonstick coating on the inside of the popcorn bags that decomposes, producing a compound called perfluorooctanoic acid. This chemical has been associated with increased risk of certain cancers, including liver and prostate cancer. Dr. Frank Gilliland and colleagues reported in a 1993 article in the "Journal of Occupational Medicine" that factory workers exposed to the chemical had increased cancer mortality.
Though not a type of cancer, there's a lung disease that's also associated with microwave popcorn. A chemical called diacetyl, used in the flavoring agent added to microwave popcorn, increases rates of an otherwise relatively rare lung condition called bronchiolitis obliterans in factory workers, notes Dr. Frits van Rooy and colleagues in a 2007 article in "Respiratory and Critical Care Medicine." The lung condition results from scarring of small airways in response to chemical irritants, and makes breathing difficult.
You can't do much about your exposure to perfluorooctanoic acid if you use pop-in-bag microwave popcorn, though you won't be exposed to the chemical if you choose to use free-kernel corn in an microwave popping apparatus. Diacetyl, on the other hand, has a boiling point somewhat below that of water. Your exposure to inhaled diacetyl is greatest when the popcorn is hottest, meaning if you allow your popcorn to cool before you open it, you'll breathe less of the chemical.
Generally speaking, even if you don't let your popcorn cool before you open the bag, you're not likely to suffer ill health effects from eating reasonable quantities of microwave popcorn. The studies that have demonstrated harm as a result of chemical exposure have been in cases of factory workers who are exposed to much larger quantities of the chemicals over a much longer period than you would be, preparing and consuming microwave popcorn at home.