Vapor pressure, not boiling point: what determines the loss of terpenes from dried flower

What is the difference between the temperature set on the vaporizer and the temperature of the dried flower?

The setting on the device describes the temperature of the heating element or the airflow, not the temperature of the raw material itself. Ground dried flower absorbs heat unevenly, and with each inhale, the air drawn through the chamber cools it down. No one has publicly measured the scale of this discrepancy, so no numerical correction can be provided.

The heating element reaches the set value faster than the raw material surrounding it. Heat must still penetrate into the portion, and it conducts poorly, so the edges can be hotter than the center. Each inhale passes cool air through the chamber and lowers the temperature of the material for a few seconds, after which the heater warms it back up. Therefore, the number displayed describes the operating point of the device, not the state of the raw material at that moment.

Eyal's team additionally points out that the vapor cools on the way to the mouth, so the inhaled mixture corresponds to different conditions than those prevailing in the chamber (Cannabis and Cannabinoid Research 2023, PMID:35442765). The very separation of the setting and the temperature of the substrate is analyzed. a separate page of this cluster. Here it is only needed to clarify what the number refers to, around which tables have formed: the device, not the plant. This difference is not a technical curiosity, as it determines what the repeated value in circulating tables refers to.

Does a terpene evaporate only at its boiling point?

No. Evaporation occurs at any temperature at which a molecule gathers enough energy to detach from the surface of the liquid. The boiling point only means that the vapor pressure has equaled the surrounding pressure, and it is not a threshold for activation. The rate of loss is determined by the vapor pressure.

The distinction is physical, not linguistic. Above every liquid, there is vapor at a specific pressure, and it is this pressure, not any threshold, that indicates how quickly molecules escape from the surface. Boiling is the moment when vapor forms throughout the entire volume because its pressure has equaled that of the surrounding environment. Below this temperature, nothing happens less; it happens more slowly.

Eyal's team reviewed the documentation reaching patients and their caregivers and found false data and a confusion of three separate phenomena: boiling, vaporization, and evaporation. A separate item on this list includes the boiling points of cannabinoids provided incorrectly. This review is based on the team's own material and literature, supplemented with step-by-step composition markings throughout the entire processing (PMID:35442765). The authors set a different criterion: how quickly a given component decreases results from comparing its vapor pressure with that of the others, not from where its boiling point lies.

This misunderstanding underlies the entire popular targeting of a single terpene. Since everything evaporates at a given temperature, only at different speeds, no setting releases a single compound or retains the others. This same work adds one more thing: during vaporization, terpenes reach the lungs before cannabinoids, and monoterpenes do this the earliest.

What is vapor pressure and why does it rise so steeply?

Vapor pressure is the pressure exerted by vapor in equilibrium with its liquid. It rises steeply with temperature because it depends on it exponentially, not proportionally. Therefore, a small increase in heat can significantly accelerate evaporation, even though no threshold has been crossed along the way.

This steepness has a visible effect on composition. The vapor rising above the heated dried flower does not replicate the composition of the raw material; it is shifted towards compounds with higher vapor pressure at that moment. The lighter the molecule, the earlier and more abundantly it enters the gas phase, while heavier ones remain in the material longer. Therefore, the order of evaporation results from comparing vapor pressures among components, not from ranking their boiling points.

For dried hemp, this results in a repeatable order at every stage of thermal processing: first monoterpenes, then sesquiterpenes, and finally cannabinoids. This order does not change depending on whether the heat comes from drying, curing, or the heating chamber. Only the rate changes, along with how much remains in the raw material for later. The relative vapor pressure is the only criterion that predicts anything here.

The boiling point itself describes one point of this relationship, additionally lying above the conditions under which the dried flower is usually heated. The rest of the process remains outside the table, although it determines the rate of loss.

Why are terpene temperature tables not comparable?

Because a significant portion of the circulating numbers does not have the pressure stated, and without it, the boiling point describes nothing. We checked in PubChem all eleven terpenes described in this project. Only six of them have records along with atmospheric pressure, while two have no boiling point in the database.

The division came out as follows. Atmospheric pressure stated directly: caryophyllene, limonene, linalool, myrcene, pinene, and terpinolene. Exclusively measured under reduced pressure: humulene. A number without any information about pressure, thus not comparable to the previous ones: nerolidol and ocimene. Without a boiling point in the database: bisabolol and farnesene.

Humulene shows what the harm is. The only record that the database holds for it is 99-100 degrees Celsius measured at a pressure of 3 mm Hg, thus almost in a vacuum. Taken out of this context and placed in a table next to atmospheric values, such a number ceases to be a rounding and becomes a falsehood. The same mechanism is seen with caryophyllene, which appears in the database twice: 257.5 degrees at atmospheric pressure and about 130 degrees at a pressure of 14 mm Hg.

The compilations circulating online mix these four cases in one column and do not indicate which row comes from where. Why the discrepancy persists even when the pressure has been provided, describes a separate page of this cluster.

When does terpene loss from dried flower begin?

Much earlier than during heating before inhalation. Losses begin when the harvested material dries and matures, and intensify during decarboxylation. Monoterpenes disappear the fastest, so the raw material reaching the patient has fewer of them than the plant from which it originated. This difference is inherent in the process itself.

Eyal's review work bases this on measurements taken after each step of processing. Finished hemp products consequently have fewer terpenes than the plants from which they started, with the lightest fraction being the most diminished. Hence the authors' criticism of terms like full spectrum: no product replicates the composition of the plant, so the name promises something that is not present inside.

For reading tables, the conclusion is inconvenient. The boiling point describes a pure compound in a flask, not the content of the chamber where the dried flower ends up after harvesting, drying, and months of storage. Even a correctly measured number thus refers to something different than the material the patient actually has in hand.

The state of the raw material therefore depends on its thermal history, which is not provided to the patient. Two portions of the same strain, dried and stored differently, enter the chamber with different reserves of volatile components, and the temperature comparisons remain silent on this difference.

Will the terpene survive heating in unchanged form?

Not every terpene and not entirely. Among the five examined monoterpenes, the proportion of molecules leaving from heating in unchanged form ranged from 97 to 98 percent at one end to 11 to 28 percent at the other. The range between the extremes is nearly ninefold.

This number says nothing about the share in the dried flower profile. In the denominator is the amount of substance introduced for measurement, in the numerator that part which remained unchanged. The rest did not evaporate without a trace: it transformed into oxidized derivatives with masses of 134, 150, or 152, and among the identified ones, probable p-(1-propenyl)-toluene was mentioned, alongside beta-pinene and fenchol.

The most resistant in this measurement was myrcene, while the least stable was terpinolene. The measurements were performed using thermal desorption combined with gas chromatography and mass spectrometry, within the temperature range encountered during vaporization (PMID:35773373). The authors add a caveat in both directions: no one has checked whether inhaling these derivatives is harmful, while allergies and irritations from oxidized oils in this group have already been described multiple times.

Hence the second correction to the tables. Even if the boiling point were correctly provided and with pressure, it would describe an unchanged compound, not what actually rises from the chamber. Part of the material changes along the way into something else, which no temperature comparison mentions.

What does this imply for the patient?

It means that the decision about the method of administration is made by the attending physician, not by a compilation found online. Dried hemp is a pharmaceutical raw material dispensed by prescription in the Rpw category, so the choice of method of intake remains part of the therapy. This page describes the state of knowledge, providing no numbers as guidance.

From the two discussed works, no value emerges for prescribing to a device. What emerges is something else: the order in which components leave the dried flower, and that some of them do not reach the vapor in unchanged form. Both things occur under any thermal conditions, just at different speeds, so there is no threshold beyond which one begins and the other ends.

It works the other way around as well. Lack of evidence is not evidence of absence: just because no one has measured how much terpene reaches the bloodstream does not mean that none reaches at all. It only means that the claim about the effect of such or such heat selection currently has no basis in measurement in humans.

A list of items currently available in Polish pharmacies is maintained by the hub of this cluster. Store category drought is a separate compilation, maintained outside this list.

What has not been studied about the vaporization of terpenes from dried flower?

The most important link. No one has measured in humans what portion of the terpene leaving the heated substrate reaches the lungs and the bloodstream. The evidence base for this project does not contain a single study involving humans on this axis, and both discussed works conclude before the boundary of the organism.

The review paper describes the composition of the material at various stages of processing, that is, what happens to the raw material. The laboratory measurement describes the composition of the vapor produced in the apparatus, that is, what comes out of the chamber. Neither goes further to absorption, concentration in the blood, and the effect experienced by the patient.

This gap is practically overlooked. Circulating compilations provide temperatures with an accuracy of one degree and assign effects to them, although there is a lack of a link connecting the number to anything measured in humans. The declared precision is inversely proportional to the data coverage.

Nothing has also been shown to justify selecting heat for a single terpene from the compilation. The order of evaporation is known, the percentage of the compound that survives heating has been measured under laboratory conditions, while the further fate of what is inhaled remains unexamined.

Popular guides close this gap with one sentence: a given terpene evaporates at a given temperature and therefore produces a given effect. No one has measured the second half of this sentence in patients.

Frequently Asked Questions

Does a terpene not evaporate at all below its boiling point?

It does evaporate. The boiling point is the temperature at which the vapor pressure equals the surrounding pressure, not the threshold for starting evaporation. Below it, evaporation occurs more slowly, but it happens all the time, including during drying and storing the raw material.

Why do two tables provide different boiling temperatures for the same terpene?

Most often because one number comes from a measurement at atmospheric pressure, while the other is from a reduced pressure. In PubChem, caryophyllene has both records: 257.5 degrees at atmospheric pressure and about 130 degrees at a pressure of 14 mm Hg. Without the specified pressure, such numbers cannot be compared.

Does heating break down terpenes?

Partially. Among the five examined monoterpenes, the share of molecules that come out unchanged from heating ranged from 97 to 98 percent at one end to 11 to 28 percent at the other. The resulting oxidized derivatives had masses of 134, 150, or 152.

Can one terpene be isolated by selecting heat?

No. At a given temperature, all components evaporate, only at different speeds depending on the vapor pressure. There is no threshold that would release one compound while retaining the others, so selecting heat for a single entry from the table has no basis in the physics of evaporation.

What has not been shown in studies on the evaporation of terpenes from dried flower?

No study has been published that would measure in humans how much terpene from heated dried flower actually reaches the lungs and the blood. It is known in what order components evaporate and what portion decomposes along the way, but it is unknown how much of this reaches the body or whether it has any measurable effect. It has also not been shown that setting the vaporizer to the temperature listed in the table as the boiling point of a specific terpene leads to inhaling that particular terpene.

Who decides on the method of consuming dried flower?

The attending physician. Dried hemp is a pharmaceutical raw material dispensed by prescription in the Rpw category, so the choice of administration route and inhalation parameters remains a therapeutic decision. This page describes the state of evidence and does not replace consultation.

Do terpenes leave the dried flower along with cannabinoids?

No. A review paper from 2023 states that during vaporization, terpenes reach the lungs before cannabinoids, with monoterpenes doing so the earliest. The composition of the vapor thus changes during the inhalation itself and between successive inhalations.

The material is for informational purposes and is not a therapeutic advice or encouragement to use. Dried hemp is a pharmaceutical raw material dispensed by prescription in the Rpw category. Editorial text: redakcja ubucha.pl.