This was posted on reddit over a year ago but is still relevant today. If you are new to e-liquid give this a read... This post is intended to be stickied and work as a caution about the flavouring that might be used in some e-liquids and flavourings called Diacetyl. Diacetyl is a butter flavoring used in some food products like butter, butterscotch, butter flavoured popcorn and some alcoholic beverages. It has also found it's way into the ingredients of some e-lquids. The FDA and the EU regard it as GRAS (Generally recognized as safe) and permit it for consumption though it has come to light it might not be suitable for inhaling: The United States National Institute for Occupational Safety and Health has suggested that diacetyl, when used in artificial butter flavoring (as used in many consumer foods), may be hazardous when heated and inhaled over a long period. Workers in several factories that manufacture artificial butter flavoring have been diagnosed with bronchiolitis obliterans, a rare and serious disease of the lungs. The cases found have been mainly in young, healthy, non-smoking males. There are no known cures for bronchiolitis obliterans except for lung transplantation. While several authorities have called the disease "Popcorn Worker's Lung," a more accurate term suggested by other doctors may be more appropriate, since the disease can occur in any industry working with diacetyl: diacetyl-induced bronchiolitis obliterans. After the workers filed a lawsuit against the manufacturers, the United States Environmental Protection Agency began an investigation into the chemical properties of microwave popcorn butter flavoring. In March 2004, former microwave popcorn plant employee Eric Peoples, of Joplin, Missouri, was awarded $20 million for permanent lung-injuries sustained while on the job. On July 19, 2005, jurors awarded $2.7 million to another popcorn plant worker in Missouri for his claim of diacetyl-induced respiratory problems. Dr. Cecile Rose, pulmonary specialist at Denver's National Jewish Medical and Research Center, in a letter, warned federal agencies or regulators that consumers, not just factory workers, are in danger of contracting bronchiolitis obliterans from fumes produced by microwaving popcorn containing diacetyl. David Michaels of the George Washington University School of Public Health first published Rose's letter on his blog. However, the only sample data known-to-date is the case where a consumer, who ate at least two bags of buttery microwave popcorn daily for 10 years, became diagnosed with the same disease affecting workers exposed to the substance, bronchiolitis obliterans. His lung problems were linked to breathing the vapors; although rare, the reported man's kitchen also had diacetyl levels comparable to those in popcorn plants. There are currently two bills in the California Legislature to ban the use of diacetyl For further reading, check the wikipedia entry linked at the top where this is taken from. Diacetyl often comes up in e-cig communities and some vapers like me are against using it as a precaution. Some suppliers too refuse to stock any liquid that contain it, but there are a few that do. Particular caution should be taken when ordering bottles of flavourings to add to e-liquid as these products' primary purpose is to be added to foodstuffs. It's only recently that a second market of vapers has been opened up to them and they might not be aware that their flavouring will be inhaled or that this perhaps is dangerous. Dekang who are a large e-liquid manufacturer in China (and supply a lot of the suppliers around the world) have promised that their liquids do not contain Diacetyl but to err on the side of caution I avoid any flavour I think might contain it. It is always good to ask a supplier too. Liberty Flights is one site that sells flavourings from a business called Flavour Art. Some of their flavourings definitely contain Diacetly. At the bottom of this page there is a FAQ about Diacetyl by Liberty flights but more importantly there is a list of the flavourings they sell which contain Diacetyl and the amount they contain. A cached mirror of that page is here and I will reproduce the list below. This list does not appear to found on their US site. Apple pie 0.4 Beer 0.001 Butter 1.5 Buterscotch 0.012 Beef boiled 0.00005 Bell pepper 0.01 Chestnut 0.03 Champagne 0.0016 Coconut 0.5 Croissant 0.074 Cheese Erdammer 0.09 Cheese Emmenthal 0.018 Cheese Parmigiano 0.29 Coffee 0.27 Caramel and butterscotch 0.07 Corn 0.2 Cream 0.078 Egg yolk 0.015 Fried 0.01 Golden syrup 0.09 Hazelnut 0.01 Maple 0.012 Milk condensed 0.5 Malt 0.01 Melon 0.01 Nut wild mix 0.9 Olive 0.01 Potato 0.00012 Pistacchio 0.0015 Rum 0.022 Soy sauce 0.35 Salmon 0.02 Truffle black 0.1 Tiramisu 0.05 Vanlla Madagascar 0.04 Vanilla Tahiti 0.08 Yogurt 1.6 I wholly recommend avoiding these flavourings and asking your e-liquid supplier whether their liquids contain diacetyl if it is not clear. There's a thread here about it with some more info.