By Gaurav Khanna, MD

Every few weeks, someone asks me about "popcorn lung." And nearly every time, one thing is clear: the story of vaping has been flattened into a scary headline, and the real picture is both more nuanced and, in some ways, more concerning. So, here's what we actually know, because as a pulmonologist I think the details are worth getting right.

The Origins of “Popcorn Lung”

In the early 2000s, workers at a microwave popcorn plant developed a serious, irreversible lung disease traced to inhalation of diacetyl — the flavoring chemical that delivers the buttery taste — which they were breathing in at occupational concentrations day after day.¹

What they developed is bronchiolitis obliterans — constrictive bronchiolitis, histologically: peribronchiolar inflammation, smooth-muscle hypertrophy, and progressive fibrosis that obliterates the lumen of the terminal bronchioles.⁸ Translation: the smallest airways scar shut, permanently. The result is fixed airflow obstruction — chronic cough, dyspnea, wheeze — that doesn't respond to bronchodilators and doesn't reverse. There is no disease-modifying therapy; once it's established, the job shifts from treatment to managing decline.

When diacetyl turned up in e-liquids, people did the obvious math: vaping → diacetyl → popcorn lung. The nickname stuck.

The Diacetyl Data — And the Honest Answer

The diacetyl concern isn't hypothetical. In a 2016 Harvard study published in Environmental Health Perspectives, Allen and colleagues tested 51 flavored e-cigarette products

And yet, to date, there is no confirmed, biopsy-proven case of bronchiolitis obliterans from vaping in the medical literature. The closest we have is a 2019 CMAJ report of a 17-year-old who developed near-fatal bronchiolitis with persistent airflow obstruction after heavy flavored-liquid vaping, which the treating team flagged as possible vaping-associated bronchiolitis obliterans.³ The accompanying editorial went further, calling it potentially the first direct evidence of the lung disease most expected to result from e-cigarette use.⁴ But “possible” is doing real work in that sentence — there was no tissue diagnosis.

And skeptics have pushed back, arguing that diacetyl exposure from e-cigarettes is low compared to conventional cigarettes,⁵ and that combustible tobacco, despite delivering plenty of diacetyl, has never been strongly linked to constrictive bronchiolitis either.

Before we breathe a collective sigh of relief: dose and route matter, and so does time. The popcorn workers had years of exposure before disease declared itself. Widespread adolescent vaping is barely a decade old. And the flavoring industry itself has acknowledged that these ingredients are evaluated for safety through ingestion — findings that cannot be extrapolated to inhalation.⁶ Your gut has enzymatic machinery your alveolar epithelium simply does not. “Generally recognized as safe” on a food label says nothing about that molecule aerosolized and delivered to the terminal bronchioles several hundred times a day.

The consensus position, stated honestly: no confirmed cases, genuine biological plausibility, documented chemical exposure, insufficient longitudinal data. Absence of evidence is not evidence of absence — particularly this early in the natural history.

EVALI: The Outbreak That Actually Happened

While the popcorn lung debate simmered, a real acute lung injury epidemic hit in 2019. By the CDC's final update in February 2020, 2,807 hospitalized cases of e-cigarette, or vaping, product use-associated lung injury (EVALI) had been reported across all 50 states, with 68 confirmed deaths.⁷ Median patient age was 24; two-thirds were male, and 82% reported using THC-containing products.⁹

These patients presented with hypoxemia, bilateral ground-glass opacities on CT, and in the sickest cases frank respiratory failure requiring mechanical ventilation. The investigation identified vitamin E acetate — a thickening agent in illicit THC cartridges — as strongly linked, found in both product samples and patients' bronchoalveolar lavage fluid.¹⁰ It appears to disrupt surfactant function at the air-liquid interface. That molecule has no business in an alveolus.

EVALI cases have declined substantially since the outbreak, but the condition remains a live concern whenever cartridges come from unverified or informal sources.⁷ A patient vaping THC from a black-market cart in 2026 is running the same experiment those 2,807 patients ran in 2019.

What Are You Actually Inhaling?

Diacetyl and vitamin E acetate get the headlines, but they're two entries on a longer list. The base liquid in nearly every product is propylene glycol and vegetable glycerin, and neither is inert in the airway.

Propylene glycol appears to damage the airway epithelium and impair cellular repair — effects likely amplified in patients with existing COPD — while vegetable glycerin disrupts normal nasal mucosal function.¹¹

Then there's what happens when you heat that base. Thermal degradation of PG and VG generates carbonyl compounds — formaldehyde, acetaldehyde, and acrolein — and output climbs with coil temperature, wattage, and coil age.¹² Formaldehyde is an IARC Group 1 carcinogen; acrolein is one of the most potent respiratory irritants we know of. Levels are generally lower than in combustible cigarette smoke, but “lower than a cigarette” is not the same as “fine.” The heating coils themselves shed metals into the aerosol — nickel, chromium, lead, and tin have all been detected — and the aerosol is delivered as ultrafine particulate that reaches the distal airway efficiently.¹²

Nicotine itself is a bioactive exposure, not just the addiction driver: it promotes airway inflammation, impairs mucociliary clearance, and — in adolescents — disrupts a prefrontal cortex that is still wiring itself.^12,15

Meanwhile, functional lung imaging is now showing small-airway abnormalities in vapers⁸ — often before symptoms prompt evaluation. In pulmonary medicine, that's the recurring theme: the histology precedes the complaint, sometimes by years.

So, What's the Actual Consensus?

This is where most articles fail readers: the honest consensus is two-sided, and people report the half that suits them.

Side one: for an established adult smoker who switches completely to vaping, e-cigarettes are almost certainly less harmful than continuing to smoke. The National Academies' 2018 review concluded that completely substituting e-cigarettes for combustible cigarettes reduces exposure to numerous toxicants and carcinogens¹³, and the Cochrane living review — 88 trials, more than 27,000 participants — found high-certainty evidence that nicotine e-cigarettes help more smokers quit than nicotine replacement therapy does.¹⁴ That's a real tool for a real population.

Side two: “less harmful than cigarettes” is an extraordinarily low bar. Cigarettes kill roughly half of their long-term users. Every major body — the Surgeon General, the National Academies, the American Heart Association, the WHO — also agrees that e-cigarettes are not safe, that the long-term data simply doesn't exist yet, and that no non-smoker or adolescent should start.^13,15 The Cochrane authors themselves flag that evidence on adverse effects remains inconclusive beyond about two years.¹⁴

My own specialty society is more conservative than that. The American Thoracic Society's official clinical practice guideline on treating tobacco dependence recommends varenicline as first-line pharmacotherapy and does not recommend e-cigarettes as a cessation tool, citing insufficient long-term safety and efficacy data and the risk of simply trading one nicotine dependence for another.¹⁶ 

So the consensus, stated fairly: a plausible harm-reduction tool for adult smokers who have failed everything else — though not one my own society is ready to recommend — a clear net harm for everyone else, and an open question about what a decade or two of daily use does to the lungs. Those three statements are all true at once. Hold them together.

Dr. Khanna is a board-certified specialist with clinical expertise in general pulmonary critical care, interstitial lung diseases, and advanced robotic bronchoscopy and lung cancer. He currently directs the pulmonary clinic at Sharp Memorial Hospital in San Diego, California, and is a partner at Chest Medicine & Critical Care Medical Group.

References

1. Kreiss K, Gomaa A, Kullman G, et al. Clinical bronchiolitis obliterans in workers at a microwave-popcorn plant. N Engl J Med. 2002;347(5):330-338.
2. Allen JG, Flanigan SS, LeBlanc M, et al. Flavoring chemicals in e-cigarettes: diacetyl, 2,3-pentanedione, and acetoin in a sample of 51 products. Environ Health Perspect. 2016;124(6):733-739.
3. Landman ST, Dhaliwal I, Mackenzie CA, et al. Life-threatening bronchiolitis related to electronic cigarette use in a Canadian youth. CMAJ. 2019;191(48):E1321-E1331.
4. Stanbrook MB. Vaping-associated lung illnesses highlight risks to all users of electronic cigarettes. CMAJ. 2019;191(48):E1319-E1320.
5. Pierce JS, Abelmann A, Finley BL. Comment on “Flavoring chemicals in e-cigarettes: diacetyl, 2,3-pentanedione, and acetoin in a sample of 51 products.” Environ Health Perspect. 2016;124(6):A100-A101.
6. Risk assessment of inhaled diacetyl from electronic cigarette use among teens and adults. Sci Total Environ. 2021.
7. Centers for Disease Control and Prevention. Outbreak of lung injury associated with the use of e-cigarette, or vaping, products — final update. February 25, 2020.