It’s the e-cigarette debate: “popcorn lung” edition.

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The headline hit the news sites last week that certain flavouring agents in e-cigarette liquids contain potentially dangerous compounds. Well, that’s what it would have said. Instead it read (rather breathlessly in my opinion, because we’ve known for a little while now that this is the case):

E-cigarettes contain flavouring chemical linked to deadly ‘popcorn lung’

The long and short of it is that there’s a chemical, diacetyl, that’s been implicated in a disease called Obliterative Brionchiolitis (OB) or “popcorn lung” because it was suffered by a number of workers at a microwave popcorn plant. Diacetyl is a chemical that’s part of the fake “butter flavouring.” It’s totally safe for human consumption, at least at the levels we consume it at, but we don’t know how great it is to inhale it. While, as far as I can tell, it was never directly implicated in causing the disease (there was, according to the current Harvard study, a strong correlation between exposure to the flavouring mix, of which diacetyl “was the most prominent chemical,” and OB, but nothing ruling out the other compounds specifically or interactions between them), animal model studies show that chronic exposure to moderate to high levels of the stuff sure isn’t likely to be good for you.

This study from 2008, for example, “Respiratory Toxicity of Diacetyl in C57BI/6 Mice,” found that exposure to various concentrations for varying amounts of time could cause both inflammation and lesions in the nose, throat, and lungs, but the levels were chosen to be relevant to the popcorn factory workers’ experiences, not to the (much lower) exposures relevant to people who use e-cigarettes. For example, the first batch of mice, who saw the worst response (and they were very bad responses) they were exposed to 200 and 400 ppm for six hours a day for five days. This, for the record, kills mice. They also tested lower doses — for example 100 ppm for six hours a day for twelve weeks — and also saw rather negative results. But the negative results tapered off with less exposure. According to the study, when they dropped down to one hour a day for two and four weeks at a time: “The most obvious difference between these mice exposed intermittently for 1 h/day to 400 or 200 ppm, and the mice exposed sub-acutely for 6 h/day, was the greatly diminished necrosis and ulceration of the epithelium in the intermittently exposed 400 ppm mice and the absence of necrosis and ulceration in the 200 ppm mice.”

They also tested “subchronic” exposure levels of 25, 50, and 100 ppm — finally getting into the vaping range — but, again, for six hours a day, and found negative results (inflammation mostly) which seems to have reversed itself, at least in part, after the study.

The real question is, then, just how dangerous are these vapours to “vapers”? And it seems to depend on what you compare it with. This study from 2014 found e-cigarette exposures of 26-278μg/mL (i.e. ppm). The new Harvard study found a range from as little as they could detect to 239μg per e-cigarette. These sound like they could be at least a little dangerous, especially to heavy users of e-cigarettes, who in theory could possibly, by smoking an hour a day of the most diacetyl-laden liquids, receive at least a comparable exposure.

[Note also that I’m referring to it in terms of exposure and not dose, because if we’re measuring dose by weight, well, humans are a lot heavier than mice. But I think in this case, where it’s about reactions in the lungs rather than metabolization, exposure to similarly-laden air might be a better approach. However, human lungs are also larger than mouse lungs… well, I’m assuming someone will point out how wrong I am shortly, and I’ll be back to fix this.]

However, as those who sell e-cigarettes like to point out, smoking actual cigarettes exposes you to more diacetyl. For example this 2014 study found the average range of diacetyl in cigarette smoke to be 250-361 ppm, and also points out that smoking cigarettes hasn’t been shown to be a risk factor for OB. Now, it’s possible that something in cigarette smoke has some kind of mitigating effect on the diacetyl we know is present in the smoke as well, but at face value it would appear that chronic exposure to low levels of diacetyl at least might not be that harmful (I mean, compared with smoking, which, you know, gives you cancer).

The studies also look at other similar compounds — acetoin, for instance (which, if I understand correctly has been used as an alternative to diacetyl and may break down into small amounts of it), and 2,3-pentanedione (another alternative), but the link between them and OB has never been fully argued. The Harvard study refers to them simply as “compounds of interest,” because they were present in the cocktail of flavouring agents the popcorn workers were exposed to, and it was that cocktail of compounds that was so linked with OB. (This doesn’t mean they don’t cause OB, either, of course.)

So what does this mean for people who smoke e-cigarettes? Well — with all the usual disclaimers about me not being a medical professional and if you want actual medical advice you should maybe not get it from a blogger — you’re probably still better off smoking e-cigarettes than actual cigarettes. Purveyors of e-cigarettes, furthermore, seem to be lining up to tell you how their e-cigarette liquids don’t contain diacetyl (among other compounds) — so if you’re concerned (and I’m not going to say at least a little concern isn’t warranted) you can always get those products.

Probably the best thing would be not to smoke tobacco or e-cigarettes. It’d probably be best not to drink too much alcohol, either, and not to eat bacon or spam. But since I do some of these things, I’d be a hypocrite to demand you not do them. Many things we do carry with them some risk, so measure yours as best you can and make your own choices. I just thought it’d be worth taking a closer look at the latest “e-cigarettes are bad for you” news article.

Thanks for reading.

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Richard Ford Burley is a writer and doctoral candidate at Boston College, as well as an editor at Ledger, the first academic journal devoted to Bitcoin and other cryptocurrencies. In his spare time he writes about science, skepticism, feminism, and futurism here at This Week In Tomorrow.