The bottom line is that I believe, and its my opinion, there is really no cause for concern.
The reasoning is simple. Looking in detail at what's been published about the rise of "popcorn lung" and related lung disease its pretty clear that, given eighty years of diacetyl used in commercial food preparation contexts, the incidence of "popcorn lung" in some very particular types of manufacturing facilities are likely related to either A) some other causal agent or B) some specific means of manufacturing flavoring that may have involved diactyel.
Please read through the material below and draw your own conclusions:
Here is a very detailed document created by the CDC describing the prevention of lung disease as it relates to the manufacturing of flavors: "Preventing Lung Disease in Workers Who Use or Make Flavorings". This is dated from December 2003.
Some details are provided, starting on page #3, of "Case Clusters" which describe each of the circumstances on which the entire "diacetyl" (DA) controversy is based.
Cluster #1: "... mixers of a heated soybean oil, salt, and butter flavoring mixture; the butter flavoring was poured by hand from open buckets into open mixing tanks ..."
Cluster #2: "... a 38-year-old worker who became short of breath and started coughing within seconds after adding 30 gallons of acetaldehyde to a flavoring mixture ..."
So let's note here that acetaldehyde (from Wikipedia) "... is toxic ..." "... is an irritant of the skin, eyes, mucous membranes, throat, and respiratory tract ..." and "... is a probable or possible carcinogen in humans."
Cluster #3: "...worked in a room where liquid and powdered flavorings were combined with starch and flour in large mixers..."
Cluster #4: "...became noticeably worse when he used a new butter flavoring mixture..."
Cluster #5: "...mixing and holding tanks for heated oil and butter flavoring..."
Cluster #6: "...mixing and holding tanks for heated soybean oil and flavorings..."
Firstly, while diacetyl is mentioned throughout these discussions (and below) its unclear exactly what mixtures were being used, how long they were used, what amounts of diacetyl were involved, and, to my mind most important, was there a pattern of usage involving diacytel with other chemical elements that triggered the problems.
Note, for example, that a lot of these workers were mixing diacytel and other, undefined "buttery flavoring" with oils. Diacetyl is water solulable - so one might question the relationship between mixing it with water and oil to the problems found.
Diacetyl has been manufactured since the 1930's and used in food production extensively since then with little apparent issue.
Why all of a sudden in popcorn plants?
Second, I have not been able to discover any documentation of what the other "buttery flavors" used were. So, again, its entirely possible that diacetyl is being singled out incorrectly. What were these other flavors? Its clear from the cluster report that known, problematic chemicals like acetaldehyde were also being used.
Interesting this point never seems to be questioned.
The results provided by this document are common sense: ventilate, be careful, and so on.
But if we look at the studies done at the time we see some interesting things. (Note, I have not read all the studies, I am not any sort of official "scientist" or anything like that. I do, however, make a living tracking down bugs and preparing for issue they cause so I am pretty good at determining causal links and also pretty good at figuring out when linkages between things or events are not causal.)
General detailed data on these cases are available by googling ("Parmet and Von Essen 2002"):
At the end of the linked PDF you see the following: "Any conclusions pertaining to causation are based on the assumption that diacetyl is either causal or correlated with other chemical constituents of butter flavourings.
In conclusion, increased exposure to a reactive substance such as diacetyl by itself or in combination with other constituents of butter flavouring likely increases the risk of obstructive lung diseases including BO in susceptible individuals."
Let's understand this: "conclusions pertaining to causation," i.e., cause is dependent on an assumption diacytel is the actual cause or correlates, e.g., is present with, another, apparently unknown, causal agent.
Effectively "we don't know what caused it but it seems like it might be diacetyl related or not."
Then we see these assumed components "likely increases" the risk of obstructive lung disease.
But what is risk?
Risk in this context is not a predictor of something and not cause something (see this). Instead risk "represents the numerical chance something might happen based on examination of a large group."
So, in this case, "increases the risk" means that your chance of obstructive lung disease might increase if the assumption diacetyl, or other things, might be the cause is true and they, infact, cause a probem - which is not known.
In fact, "we have no idea," would have been more accurate.
Now, based on past history it seems clear that
A) Diacetyl has been around a long time, probably centures, and, in fact, its a natural product of fermentation. During that time it apparently has not caused problems outside of the "popcorn lung" context as far as I can tell.
B) Today there's a lot of vaping-related or popcorn-lung-related studies that say to some degree "diacetyl causes substantial damage to airway epithelium."
But these two facts seem somewhat incongruous.
My personal belief, and I do vape and I do not concern myself with diacetyl, is that diacetyl is around food and beer manufacturing: it has been and always will be.
So does vaping create an "exposure" concern?
I'd have to say no based on what I have seen so far.
My guess is that what's dangerous are flavoring manufacturing labs like those uncovered in the popcorn-lung studies. But its totally unclear what, in a chemical sense, is going on in there.
And no one seems interested in studying it.
And then, too, suddenly the "problems" all vanished...
Probably because the manufacturing process was changed.
But "buttery flavoring" is still around...
So what's different?
(EDIT: More links: