Abstract

Propylene glycol (PG) and polyethylene glycol (PEG) cannabis liquidizers should be evaluated as components of noncombustion delivery systems against the methods and formulations they replace. Human studies show that cannabis vaporization can maintain or increase delta-9-tetrahydrocannabinol (THC) delivery while reducing measured combustion exposure. Analytical comparisons also demonstrate fewer pyrolytic compounds and lower modeled risks from selected gas-phase toxicants. Carrier research supplies additional reasons to reject the presumption that glycol free or botanical alternatives are necessarily better: PG has favorable controlled exposure findings and outperformed phytol in a direct animal comparison. Prominent aldehyde claims have also been challenged by work examining aversive heating and exposure realism. The evidence therefore supports a comparative, formulation-specific case for PG and PEG liquidizers rather than categorical rejection based on ingredient names or isolated heating tests. This review examines those arguments, the limits of their application, and competing additives. It distinguishes the broader PG research record from the more limited PEG record. No clinical ranking of every carrier or original product testing is presented.

1. The comparison begins with combustion

For adults choosing an inhaled cannabis product, evaluating a vape only against clean air leaves the substitution question unanswered. Smoking cannabis burns material to obtain cannabinoids and other constituents. A noncombustion system instead seeks to transfer those constituents to an inhalable aerosol. The scientific question is whether it can accomplish that task with a lower burden of selected toxicants per delivered cannabinoid dose.

This paper critically evaluates evidence relevant to that question and the use of PG and PEG carriers. It is a targeted narrative review, not a systematic review or new experiment. Sources were selected purposively for direct comparisons of inhalation methods, carrier toxicology, heating conditions, and human exposure. Source verification used PubMed, publisher records, and official agency documents through 15 October 2026; availability ranged from abstracts to full reports. No exhaustive search, formal risk-of-bias scoring, or meta-analysis was undertaken. PEG grades differ in average molecular weight and cannot be treated as interchangeable exposures. Its position is that blanket condemnation of PG or PEG from isolated emissions findings does not resolve comparative harm and can obscure the benefits of replacing combustion. It also distinguishes three questions: whether vaporization improves delivery relative to smoking, whether particular heating tests represent intended use, and whether a particular carrier improves the performance of a complete formulation.

2. THC delivery without the combustion exposure

Abrams and colleagues compared smoked cannabis with Volcano dry-herb vaporization in 18 participants. Peak plasma THC and six hour THC exposure were similar, while expired carbon monoxide was lower with vaporization. This is direct human evidence that THC delivery can be maintained while reducing a measured combustion exposure. The study examined acute use of a dry-herb device. [1]

Spindle and colleagues subsequently compared smoking and vaporization in 17 adults using cannabis infrequently. At an equal starting dose containing 25 mg THC, mean peak blood THC was 10.2 ng/mL after smoking and 14.4 ng/mL after vaporization. Vaporization also produced stronger effects and impairment. This supports greater delivery efficiency in that experiment; the additional THC exposure is not itself a health benefit. The harm reduction opportunity is obtaining the intended effect without requiring the same amount of burned material, a proposition that depends on actual use and dose adjustment. [2]

Fischedick and colleagues provide complementary chemical evidence. In three cannabis varieties, measured THC volatilization efficiencies were 22.0–39.5% for the tested vaporization method and 15.9–29.8% for smoking. Smoke contained additional pyrolytic products, whereas vapor showed substantially less degradation in their analysis. Terpenoids were present in both. This supports delivery of cannabinoids and terpenoids without the same degree of combustion chemistry; it does not establish that every terpene is delivered more efficiently by every vaporizer. [3]

The studies above concern dry-herb vaporization. They establish why noncombustion delivery is useful, rather than a carrier-specific advantage. A liquidizer is intended to make a concentrate suitable for a cartridge delivery system; whether a particular mixture retains the exposure advantages must be assessed in that system.

3. Compare toxicant burden at a comparable THC dose

Meehan-Atrash and colleagues analyzed cannabis concentrate vaping and dabbing and compared modeled gas-phase risks with cannabis smoke. Their exposure assumptions were matched to approximately equivalent daily THC delivery. The estimated cancer and non-cancer risks from the assessed gas-phase constituents were lower for vaping and dabbing than smoking, although thermal degradation products remained detectable. The cartridge mixture contained THC and terpenes rather than PG or PEG. [4]

This comparison shows why detecting a carcinogen cannot, by itself, establish that a vape is the worse alternative. The amount matters, as does the amount of cannabinoid delivered alongside it. A per puff result answers a different question from toxicant mass per milligram of delivered THC. A complete comparison must also account for constituents outside the measured gas-phase and actual consumption behavior.

Gieringer and colleagues identified substantially fewer pyrolytic compounds in Volcano vapor than combusted cannabis, including a reduction in the range of detected polycyclic aromatic hydrocarbons (PAHs). Their gas-phase composition percentages must not be confused with THC recovery efficiencies. Vaporization recovered 36–61% of starting THC, while their laboratory pipe recovered 78% under conditions that minimized sidestream losses. This supports a chemical exposure advantage in that experiment, not the eightfold yield advantage inferred in the earlier companion paper. [5]

Terms such as tar require a defined measurement. Total collected aerosol mass can include the intended cannabinoids and carrier droplets; it should not automatically be equated with the tar produced by combustion. For PG or PEG cannabis liquids, a claim of lower tar or carcinogen exposure should specify the measured compounds or particulate definition and the comparator. The cited cannabis comparisons establish the rationale for avoiding combustion; they do not directly measure a Terps USA PG or PEG mixture against smoke.

4. A demonstrated methodological challenge to high aldehyde claims

The 2015 Jensen report projected elevated cancer risk from formaldehyde-releasing compounds measured from an e-cigarette at a high voltage setting. It was a chemical exposure calculation, not an observation of cancer in users. Its interpretation depended on operating conditions, consumption assumptions, and how the measured formaldehyde-related compounds were treated toxicologically. [6]

A 2017 replication by Farsalinos and colleagues tested the same equipment and liquid while checking sensory responses in 26 experienced users. Most identified dry puffs by 4.2 V or below, and the investigators selected 4.0 V as the upper realistic setting. Formaldehyde measured 19.8 micrograms per 10 puffs at 4.0 V and 718.2 at 5.0 V. This is a material challenge to presenting the high setting as representative exposure: operating conditions substantially changed the result. It concerns that device and liquid, not a universal voltage limit. [7]

The analytical dispute did not end there. Salamanca and colleagues subsequently reported concerning formaldehyde-related emissions at an intermediate setting described in earlier work as nonaversive. They also argued that different collection and analytical methods did not quantify the same formaldehyde-related species equally. Thus, sensory acceptability and analyte recovery both require examination. The supported methodological criticism is that high setting results cannot be generalized without exposure validation; it is not that every adverse finding was disproven. [8]

5. What the cannabis thinning agent study can establish

Troutt and DiDonato tested isolated PG, vegetable glycerin (VG; glycerol), PEG 400, and medium-chain triglycerides (MCT) at a device setting of 230°C and measured carbonyls. PEG 400 produced particularly high formaldehyde and acetaldehyde, and PG produced more formaldehyde than VG or MCT. The reported protocol used 4 second, 55 mL puffs at 30 second intervals in three blocks of 25, with at least ten minutes between blocks. [9]

The central limitations concern generalization. It did not test the marketed cannabis mixtures, measure illness, or compare methods at equivalent THC delivery. It did not report independent coil temperature verification or a user assessment demonstrating whether exposure was aversive. These gaps limit claims about routine use of complete products. They do not establish that the experiment overheated, and 230°C alone does not demonstrate an unrealistic condition. The measurements are evidence about the tested isolates and apparatus, rather than a clinical ranking of all liquidizers. [9]

Chen and colleagues demonstrated why liquid supply belongs in this analysis. In their top-coil system, temperatures varied from approximately 110–185°C under fully wet conditions to 145–334°C with liquid supplied through the wick, and were considerably higher when dry. A device setting, measured coil temperature, and aerosol temperature are therefore different quantities. There is no universal temperature threshold that establishes realistic operation across devices and formulations. [10]

A Swiss government-commissioned experimental report found no measured PAHs or phenols in emissions from three dry-herb vaporizers and four electronic liquid-vaping systems, unlike smoke. The liquid experiments included PG/glycerol and PEG/glycerol cannabis formulations. This directly supports reductions in these measured combustion toxicants, without establishing lower exposure to every carcinogen or clinical risk. [11]

The Swiss report also associated high aldehyde emissions from a viscous PEG/glycerol formulation with poor wicking and overheating, with device-dependent results. Its authors favored PG/glycerol formulations and recommended considering a prohibition on PEG. These findings support separating carrier-specific concerns from the advantages of avoiding combustion and assessing hardware compatibility; they do not endorse PEG. The report did not measure human illness. [11]

6. Evaluate PG and PEG against actual alternatives

PG has favorable findings that deserve the same attention as adverse emissions results. Dalton and colleagues found no objective pulmonary function changes after the tested acute PG aerosol and vapor exposures in 20 healthy adults, although some symptoms were reported. Boulay and colleagues likewise found no significant changes in assessed lung function after brief nicotine-free, flavor-free PG and glycerol aerosol use in healthy and asthmatic participants. These are human tolerability findings for defined short exposures. The Dalton study had industry consortium funding. [12, 13]

There is also a direct comparison with a proposed botanical substitute. Schwotzer and colleagues reported severe lung injury and mortality in rats exposed to phytol, while PG produced no observed adverse reactions under its reported 14 day exposure. The study was commissioned by Canopy Growth. It supports PG over phytol under those experimental conditions. Health Canada emphasized the high animal exposures and differences from device generated vaping aerosols, which prevent treating it as a human risk ratio. [14, 15]

For PEG, the evidence needs to be assessed separately. The much cited PEG 3350 rat study used an aqueous aerosol and a different molecular weight material from heated PEG 400. It reported macrophage, lung weight, and other exposure-related changes. Those differences limit its direct relevance to low molecular weight cannabis liquidizers, while also preventing a claim that PEG inhalation research found no effects. Neither oral use history nor food status resolves inhalation performance. [16]

MCT based products also require evidence rather than a presumption of gentleness. Bhat and colleagues found greater pulmonary effects in mice and greater human cell toxicity from a cannabidiol (CBD) and MCT product than from a nicotine and PG/VG product tested through the same device. Active ingredients and flavorings differed, so the comparison cannot isolate MCT as the cause. It nevertheless supplies a counterexample to assuming that an oil-based cannabinoid product necessarily has a better experimental profile than a glycol-based formulation. [17]

Terpenes can assist formulation and contribute aroma, but their thermal chemistry also depends on delivery conditions. A 2021 study found increasing beta myrcene raised certain hazardous products during dabbing, while the tested cartridge mixtures showed lower degradation with higher myrcene content. Thus, neither a blanket rejection of terpenes nor a blanket preference for them over glycols follows. Concentration, delivery mode, and power determine the relevant comparison. [18]

The comparison with other ingredients must remain specific. Removing PG or PEG does not eliminate the thermal degradation of THC and terpenes, as the concentrate experiments demonstrate. Conversely, showing that a particular replacement performs poorly does not prove that every PG or PEG mixture performs better. A favorable formulation is an experimentally assessable combination of ingredients, concentration, hardware, and exposure. [4]

7. The case for formulation-based carrier selection

The evidence supports a positive case for choosing noncombustion cannabis delivery over smoking when the objective is maintaining cannabinoid delivery while reducing particular combustion exposures. It also supports challenging the inference that the presence of PG or PEG, or the detection of carbonyls in an isolated agent test, establishes that a product is a worse option than smoking.

For a PG or PEG cannabis vape, these findings provide a rationale for its intended use as a noncombustion delivery option. They do not establish that the carriers themselves create the advantages demonstrated in dry-herb or terpene-based studies. The appropriate product question is whether the complete PG or PEG mixture retains the exposure advantages in compatible hardware at its intended cannabinoid delivery.

Biological and clinical evidence still sets boundaries. Kim and colleagues reported airway effects of PG and VG aerosols in human cell cultures and sheep. Small samples and repeated measurements constrain interpretation: aggregated sensitivity analyses did not retain most significant findings, although the ratio of two mucus proteins remained significant. Hariri and colleagues described small airway fibrosis and constrictive bronchiolitis in four people with vaping histories, all former smokers, without identifying a particular carrier as the cause. These findings warrant consideration without assigning unproven PG- or PEG-specific causation. They do not directly rank a specified cannabis liquidizer against smoking. [19, 20]

8. Vitamin E acetate and what the CDC actually concluded

Vitamin E acetate was strongly linked to e-cigarette, or vaping, product use-associated lung injury (EVALI): Blount and colleagues detected it in lung fluid from 48 of 51 patients and none of 99 healthy comparators. The US Centers for Disease Control and Prevention (CDC) nevertheless stated that contributions from other chemicals could not be ruled out. This supports identifying vitamin E acetate as a major outbreak-associated additive. It does not support the earlier version's statement that CDC explicitly cleared PG, VG, or PEG. The 2017 thinning agent experiment also predates the outbreak. [21, 22]

9. Interpreting the broader record consistently

The PG evidence includes favorable results and measurable effects. Langston and colleagues reported little evidence of adverse pulmonary toxicity in a 13 week rat exposure study, but also observed minimal nasal mucous cell hyperplasia that resolved during recovery. Its authors included tobacco industry researchers. Chu and colleagues reported limited conventional toxicity in a 28 day PG and VG experiment, alongside reduced male body weight and molecular changes. Neither should be summarized as showing no biological effects, nor does either establish long-term clinical outcomes for a cannabis mixture. [23, 24]

Polosa and colleagues followed nine daily e-cigarette users who had never smoked and 12 controls for 3.5 years. They detected no significant deterioration in the measured outcomes. This is favorable observational evidence, but the small cohort cannot establish the absence of uncommon or delayed effects, or isolate PG's role. Its November 2017 publication followed the initial publication of the thinning agent paper. [25]

Styles and colleagues demonstrated antiviral activity of PG in laboratory and animal systems. That supports an antimicrobial application under the studied conditions; it is not a clinical assessment of chronic heated cannabis aerosol use. Likewise, PG's food generally recognized as safe (GRAS) status concerns food uses rather than a US Food and Drug Administration (FDA) clearance for inhalation. These distinctions allow the favorable PG findings to stand on direct evidence instead of a generalized assertion of pharmaceutical safety. [26, 27]

Flash point and oral lethal dose values do not define repeated inhalation tolerability or aerosol carbonyl production. A general safety data sheet (SDS) instruction to avoid overheating cannot establish what a device emits. These physical and route distinctions strengthen the methodological argument by focusing it on the exposure that was actually measured.

The same standard applies to THC and terpene preservation claims. Repka's polymer processing study did not demonstrate that 29.1% of starting THC converts to cannabinol (CBN) during a dab at 200°C; that percentage referred to CBN formation relative to the degraded fraction. Nor does detecting terpenoids in both smoke and vapor establish a binary clinical entourage effect. Direct delivery experiments and chemical measurements provide the stronger basis for the noncombustion case. [3, 28]

Conclusion

PG and PEG liquidizers deserve evaluation as functional components of cannabis delivery systems, rather than rejection through a chemical versus natural distinction. The strongest supporting case combines the demonstrated advantages of avoiding combustion, efficient cannabinoid delivery, favorable PG findings, and the absence of an automatic advantage for substitute carriers. Methodological criticism of adverse studies is warranted where operating conditions, analytical methods, or generalization do not support the broader claims made from them.

The evidence supports formulation-based comparison: the right question is what a complete product delivers, what exposures accompany that delivery, and how it compares with smoking or another formulation at a relevant cannabinoid dose. This framework provides a defensible basis for considering PG and PEG liquidizers within noncombustion cannabis delivery while identifying the product measurements that can strengthen specific comparative claims.

Revision note and disclosure

This Version 2.0 revises The Science of Safety (WP-2026-02, Version 1.0, May 2026) and corrects related scientific statements in Why Settle: The War on Vapor (TUSA-WP-2026-03, Version 1.2, May 2026). It replaces universal temperature and dry puff assumptions, the claim of explicit CDC carrier clearance, the incomplete PEG 3350 summary, and flash point and oral toxicity inferences. It also corrects THC recovery and CBN percentage interpretations, the transfer of a tobacco risk estimate to cannabis, overbroad terpene claims, publication chronology, and bibliography. The PG history is limited to what the cited evidence establishes; methods replace criticism based on credentials. These corrections do not retract the comparative rationale for avoiding combustion.

The earlier companion paper's use of a 95 percent tobacco harm reduction estimate does not establish a corresponding cannabis risk reduction; that estimate is not applied here. Its Colorado discussion should identify PEG, vitamin E acetate, and MCT in the named prohibited additive list, rather than include PG in that list. Its other financial and regulatory arguments, including claims about coordinated policy motives, are outside this scientific review and are not validated by it. [29, 30]

This review was supported by Terps USA. Kenneth Fry is the founder of Terps USA, which sells liquidizers and has a commercial interest in the subject. No original Terps USA product testing is presented. Generative AI assisted drafting, source identification, and editorial preparation; it is not an author. This publication is a company-supported narrative review, not an independently peer-reviewed product evaluation.