Microwave popcorn uses an artificial butter flavoring called diacetyl. Numerous studies have now linked diacetyl fume inhalation to a rare condition called bronchiolitis obliterans, in which the bronchioles of the lungs get blocked by masses of fibrous tissue. The problem has been known about since 1999, but so far federal agencies haven’t done anything. There’s now a bill in California proposing to ban diacetyl by 2010. The Delve Special episode “Food for Thought” just seems more and more relevant as the years go by.