Acetylated Cannabinoids: Why Vaping THC-O Releases Toxic Ketene Gas

There is a category of products sold in hemp shops and online under names like THC-O, HHC-O, delta-8-O, and “acetate” cannabinoids. They are marketed as more potent alternatives to delta-9 THC. They are largely unregulated. And when you heat them in a vape device, they produce ketene — a toxic gas that industrial chemists handle with serious respiratory precautions.

This is not a theoretical concern. It has been confirmed in peer-reviewed chemistry journals, and a 2026 study published in Analyst quantified the conversion rate: the acetate group on these cannabinoids decomposes into ketene with greater than 99% efficiency during vaping. Not most of the time. Nearly every time.

Here is what you need to know.

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What Ketene Actually Does to Lungs

Ketene (CH₂=C=O) is a highly reactive cumulated diene — an industrial chemical used in the synthesis of acetic anhydride and other compounds. In industrial settings, it is handled under controlled conditions with ventilation systems, because even low concentrations cause severe pulmonary damage.

At sub-toxic concentrations, ketene acts as a potent irritant to mucous membranes and airways. At higher concentrations, it crosses from irritation into tissue destruction. The primary mechanism is direct chemical reaction with biological molecules — ketene is a highly electrophilic compound that reacts rapidly with water, proteins, and lipids in lung tissue. Inhaled ketene causes pulmonary edema (fluid accumulation in the lungs), alveolar damage, and hemorrhagic inflammation. At high concentrations, it can cause acute respiratory distress syndrome and, if exposures are severe enough, death.

The occupational exposure limits that exist for ketene exist precisely because this damage is well-established. The chemical is classed alongside other acylating agents — a category that includes phosgene, the chemical weapon used in World War I — as a severe respiratory hazard.

When you vape an acetylated cannabinoid, you are generating this gas and inhaling it directly.

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The Acetate-on-Cannabinoid Chemistry

To understand why this happens, you need a brief chemistry detour.

“Acetylation” means adding an acetate group (CH₃CO–) to a molecule. Chemists do this routinely to modify a compound’s properties — bioavailability, solubility, potency. Aspirin is acetylated salicylic acid. Heroin is diacetylmorphine — morphine with two acetate groups added.

For cannabinoids, producers figured out they could acetylate the hydroxyl group on compounds like delta-8 THC, delta-9 THC, HHC, CBD, and CBN. The result is an ester: THC-O-acetate, HHC-O-acetate, CBN-O-acetate, and so on. The claim was that these acetylated forms were more potent or produced a longer-lasting effect.

What the sellers did not appear to account for — or did not disclose — is basic organic chemistry. Ester groups are thermally unstable. When you heat an acetate ester past a certain temperature, it undergoes pyrolysis: the ester bond breaks, releasing an alcohol and ketene. This is not a fringe reaction — it is a textbook transformation. Acetic anhydride is industrially produced via ketene. Any chemist with a second-year organic chemistry background knows acetate esters do this when heated.

Vape coils heat to 200–400°C routinely. That is enough.

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The 2026 Study: Nearly Quantitative Ketene Production

The clearest data came in April 2026, when Evans, Corley, Hunsaker, Nicklisch, Ehrlich, and Nguyen published “Effect of O-acetylation on ketene and carbonyl yields from conventional and emerging cannabinoids” in Analyst (DOI: 10.1039/D6AN00190D).

The research team used an optimized impinger collection method with HPLC high-resolution mass spectrometry to measure ketene and carbonyl production from a panel of cannabinoids vaped on a commercial cannabis e-cigarette. They tested:

• Acetylated: Δ8-THCO, Δ9-THCO, CBD-di-O-acetate, CBG-di-O-acetate, HHC-O-acetate
• Non-acetylated controls: Δ8-THC, Δ9-THC, CBD, CBG, HHC

The results were unambiguous. The acetate group on every acetylated cannabinoid decomposed into ketene with greater than 99% efficiency during vaping. Ketene was not detected in the unvaped distillates — this is not a storage issue. And it was not detected during vaping of the non-acetylated cannabinoids.

The conversion is essentially complete. When you hit a THC-O cart, the acetate group does not travel intact into your lungs. It converts to ketene. That is where it goes.

Ketene and carbonyl production yields from acetylated cannabinoids ranged between 1–4% and 0.1–1.5% respectively, by cannabinoid mass. CBD-di-O-acetate (which has two acetate groups) produced the highest combined toxicant output. The researchers found that airflow through the device affected the distribution — opening a vent reduced ketene but increased carbonyl production — meaning there is no “safe” device adjustment. You are trading one toxicant mix for another.

The paper’s conclusion: exposure levels “far exceed safety limits for pulmonary effects,” and “further stringent product regulation is required.”

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This Was Predictable — and Confirmed Before 2026

The Evans et al. 2026 study is the most rigorous quantification to date, but it was not the first warning.

In 2022, Munger, Jensen, and Strongin published “Vaping Cannabinoid Acetates Leads to Ketene Formation” in Chemical Research in Toxicology (Vol. 35, pp. 1202–1205, DOI: 10.1021/acs.chemrestox.2c00170). They tested delta-8-THC-O, CBN-acetate, and CBD-acetate. Ketene was consistently detected in vaped condensates from all three. They also purchased a commercial delta-8-THC-O product online and tested it — ketene there too.

Also in 2022, Benowitz, Havel, Jacob, O’Shea, Wu, and Fowles published “Vaping THC-O Acetate: Potential for Another EVALI Epidemic” in the Journal of Medical Toxicology (2023, Vol. 19, pp. 37–39, DOI: 10.1007/s13181-022-00921-3). The authors — toxicologists including researchers from UCSF — ran theoretical energy profiles showing the pyrolysis mechanism for THC-O and drew the direct parallel to vitamin E acetate, warning explicitly of an EVALI-class event.

The industry had ample warning. The chemistry was not obscure. The products continued to be sold.

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The EVALI Echo: We Have Seen This Before

In 2019 and 2020, 2,807 Americans were hospitalized with e-cigarette or vaping product use-associated lung injury (EVALI). Sixty-eight died. The outbreak was concentrated in illicit and gray-market THC vape carts — and the culprit identified by the CDC was vitamin E acetate (tocopheryl acetate), used as a cutting agent.

The vitamin E acetate mechanism was established by Wu and O’Shea in a 2020 study in PNAS: under vaping temperatures, the acetate group on vitamin E acetate undergoes pyrolysis and releases ketene. Same ester chemistry. Same toxic product.

EVALI was not caused by cannabis itself. It was caused by an acetate ester used as a cutting agent in gray-market vape products. The vaping industry learned nothing from it — or rather, some parts of it learned nothing, because acetylated cannabinoids then entered the hemp market and scaled.

THC-O, HHC-O, and their analogs are not vitamin E acetate. They are not cutting agents. They are the active ingredient. But the chemistry is the same: acetate ester plus heat equals ketene.

The EVALI outbreak is the clinical proof of concept that this mechanism causes mass lung injury in real people using real products under real conditions.

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What “Vaping Temperature” Actually Means

A common defense of acetylated cannabinoids is that vaping temperatures are lower than laboratory pyrolysis conditions. This defense does not hold up.

Vape cartridge coils typically reach 200–400°C depending on voltage, resistance, and draw style. The Strongin lab at Portland State confirmed ketene formation from cannabinoid acetates at temperatures as low as 250°C using a dab platform. Standard cartridge vaping sits squarely in that range.

The 2026 Evans et al. study used a commercial cannabis e-cigarette — not a laboratory pyrolysis apparatus — and still achieved greater than 99% acetate-to-ketene conversion. There is no practical vaping temperature where the acetate group stays intact during inhalation.

Dab rigs operate at higher temperatures (often 400–700°C or above at the nail) and can generate higher peak ketene concentrations, but they are not the primary concern here. The primary concern is the hundreds of millions of standard vape cartridge hits taken by consumers who believe they are inhaling a cannabinoid.

They are. Plus ketene.

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Products to Avoid

If it has any of the following in the name or on the COA, do not vape it:

• THC-O (THC-O-acetate, THCO, delta-8-O, delta-9-O)
• HHC-O (HHC-O-acetate, HHCO)
• CBN-O (CBN acetate)
• CBD-O (CBD acetate, CBD-di-O)
• CBG-O (CBG acetate)
• Any cannabinoid with “-O-acetate”, “-OA”, or the suffix “-O” denoting an acetate ester

These products are sold through hemp market channels — gas stations, smoke shops, online retailers — because they are derived from hemp and occupy regulatory gray zones in many states. They are not available in licensed cannabis dispensaries because the regulated market has largely rejected them on exactly these grounds.

The hemp market operates with minimal testing requirements in most jurisdictions. There is no federal agency actively screening these products for ketene precursors. There is no required warning label. The COA you might find on a website typically only confirms cannabinoid percentages — not what that cannabinoid becomes when heated.

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What Is Actually Safe to Vape

This is the important distinction: the ketene risk is specific to acetylated cannabinoids. It does not apply to standard cannabis vape products.

Delta-9 THC, delta-8 THC, CBD, CBG, HHC — the non-acetylated forms — do not produce ketene when vaporized. The 2026 Evans et al. study confirmed zero ketene detection from the non-acetylated control compounds under the same vaping conditions that produced near-quantitative ketene from the acetylated versions.

Regulated cannabis market vape products — distillate cartridges, live resin carts, rosin cartridges from licensed dispensaries — do not contain acetylated cannabinoids. They are required to be tested for pesticides, residual solvents, heavy metals, and in many states potency. They are not cutting agents. The cannabinoids in them are not ester-form analogs.

Strains like Blue Dream, OG Kush, and Gelato — vaped as standard distillate or live resin from licensed dispensaries — involve THC, CBD, and the terpene profiles native to the plant. Terpenes like myrcene, pinene, and eucalyptol are present in many of these products; none of them are acetate esters. Vaporizing them is categorically different chemistry from vaporizing THC-O.

The effects you might seek from cannabis — relaxation (see our Relax family of strains), balance (see the Balance family) — are achievable with non-acetylated cannabinoids from regulated sources. You do not need an acetate ester product.

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Harm Reduction: If You Have Used These Products

Most single or occasional exposures to acetylated cannabinoid vapor will not cause immediately noticeable symptoms. Ketene is a severe lung toxin at meaningful concentrations, but brief exposures at the concentration range from a few vape hits may produce subclinical effects that take time to manifest.

If you have vaped THC-O, HHC-O, or similar acetylated products:

• Stop using them. The exposure risk is ongoing and cumulative.
• Watch for symptoms: Cough that worsens over days, shortness of breath, chest tightness, low-grade fever, or hypoxia (low blood oxygen). EVALI-type presentations often developed over days to weeks after repeated exposures — not immediately after a single session.
• If symptomatic, see a doctor. Tell them you have been vaping, and specifically that you may have used acetylated cannabinoid products. Clinicians need that information to consider EVALI-pattern lung injury in the differential diagnosis.
• A chest X-ray or CT scan can identify ground-glass opacities or other vaping-associated pulmonary findings even in patients who feel only mildly symptomatic.

If you have experienced respiratory symptoms after using hemp-market vape products and they are not resolving, do not wait. The 2019 EVALI outbreak hospitalized people who initially felt only moderate symptoms that progressed to acute respiratory failure.

This is education, not a substitute for medical advice. If you are experiencing post-vape respiratory symptoms, see a doctor.

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Why This Matters Beyond THC-O

The Evans et al. 2026 study also noted that higher airflow altered the toxicant distribution rather than eliminating it. This suggests the broader principle: any acetate ester vaped at the temperatures common to consumer e-cigarettes is a candidate for this chemistry.

Flavor compounds in conventional vape products sometimes include ethyl acetate and other acetate esters. The Strongin lab demonstrated ketene formation from ethyl acetate and geranyl acetate under dabbing conditions. The acetate-to-ketene chemistry is not limited to cannabinoids. The cannabinoid acetates are the most concentrated exposure source in the hemp market, but the problem is structural to the category of acetate esters in heated aerosols.

The deeper failure here is regulatory. The hemp market, as it currently operates in most of the United States, does not require pre-market toxicological review of novel cannabinoids. A compound can be synthesized, branded, and placed on a shelf without anyone having tested what it produces under heat. THC-O was sold for years before the first peer-reviewed ketene quantification papers appeared. The products predated the safety data by a wide margin.

For additional context on how hemp-derived cannabinoids have reached consumers — including children — without adequate safety review, see our related coverage: hemp-derived THC sending kids to the ICU, hemp THC products sold to minors, and the 101 cannabis tips every adult consumer should know.

The pattern of vaping-related lung injury is not unique to this category — if you want background on how chronic consumption affects the body, our coverage of cannabinoid hyperemesis syndrome covers another underreported harm. Different mechanism, same theme: the harm often comes from what is being consumed, not cannabis itself.

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Knowing What Is In Your Cart

The informed cannabis consumer asks one question that the hemp market makes deliberately difficult to answer: what is actually in this, and what does it become when heated?

A COA that lists “delta-8-THC-O: 85%” tells you the concentration of a ketene precursor, not a safe cannabinoid dose. The label “made from hemp” is a regulatory category, not a safety certification. The word “natural” means nothing chemically.

Knowing what is in your cannabis — the specific cannabinoid profile, whether any acetate ester analogs are present, the tested COA from a licensed laboratory — is the difference between informed adult use and rolling the dice on your lung health. The High IQ app helps adult users find regulated, COA-verified products from licensed dispensaries, so you know what you are actually consuming.

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Sources

1. Evans RL, Corley CN, Hunsaker HC, Nicklisch SCT, Ehrlich AK, Nguyen TB. “Effect of O-acetylation on ketene and carbonyl yields from conventional and emerging cannabinoids.” Analyst. 2026 Apr 13. DOI: 10.1039/D6AN00190D (Primary source)

2. Munger KR, Jensen RP, Strongin RM. “Vaping Cannabinoid Acetates Leads to Ketene Formation.” Chemical Research in Toxicology. 2022;35(7):1202–1205. DOI: 10.1021/acs.chemrestox.2c00170

3. Benowitz NL, Havel C, Jacob P, O’Shea DF, Wu D, Fowles J. “Vaping THC-O Acetate: Potential for Another EVALI Epidemic.” Journal of Medical Toxicology. 2023;19(1):37–39. DOI: 10.1007/s13181-022-00921-3

4. Wu D, O’Shea DF. “Potential for release of pulmonary toxic ketene from vaping pyrolysis of vitamin E acetate.” Proceedings of the National Academy of Sciences. 2020;117(12):6349–6355. DOI: 10.1073/pnas.1920925117

5. Munger KR, Anreise KM, Jensen RP, Peyton DH, Strongin RM. “Mechanistic Rationale for Ketene Formation during Dabbing and Vaping.” JACS Au. 2024;4(6):2403–2410. DOI: 10.1021/jacsau.4c00436

6. Bone CB, Klein C, Munger K, Strongin RM, Kruger DJ, Meacham MC, Kruger JS. “Reviewing the Risk of Ketene Formation in Dabbing and Vaping Tetrahydrocannabinol-O-Acetate.” Cannabis and Cannabinoid Research. 2024;9(5):e1404–e1409. DOI: 10.1089/can.2023.0094

7. Wang P, Jacob P, Wang ZM, Fowles J, O’Shea DF, Wagner J, Kumagai K. “Conditions Leading to Ketene Formation in Vaping Devices and Implications for Public Health.” Chemical Research in Toxicology. 2024;37(8):1415–1427. DOI: 10.1021/acs.chemrestox.4c00190

8. CDC EVALI investigation data: cdc.gov/tobacco/basic_information/e-cigarettes/severe-lung-disease.html