Regulators and health professionals have known of this risk for decades, but always assumed that it would only affect people breathing in especially high concentrations in factory settings. Then in 2007, a man who regularly ate two bags of microwave popcorn every day was diagnosed with popcorn lung, indicating that Diacetyl enters the air and lungs when microwave popcorn is cooked. Anxious to reassure consumers, most microwave popcorn companies phased out Diacetyl -- only to replace it with chemicals that have the same effects.
Today, you can still find Diacetyl in many flavored snack foods and even in some so-called "natural" foods. Make sure you read the ingredients of any food you intend to consume, and make sure it contains no Diacetyl (and no "yeast extract" for that matter, either).
If you love popcorn, stick with organic kernels that you can pop yourself in a kettle and douse with healthy ingredients like coconut oil, grass-fed butter, and Himalayan pink salt.