Vaporising versus combustion: what the available evidence says

Burning plant matter generates compounds that were not in the plant: carbon monoxide, tars, polycyclic aromatic hydrocarbons and irritant aldehydes. The available literature describes heating without reaching combustion as reducing the presence of several of those by-products in the aerosol, which is why harm-reduction services treat combustion as a distinct risk factor. What does not change is everything else: the effects of the substance on the body, the legal framework and, in Spain, the weight of the tobacco it is habitually mixed with.

It is worth pinning down exactly what that comparison means before going into detail. “Fewer combustion by-products” is a relative statement between two forms of use: it does not amount to “harmless”, it does not describe a safe threshold, and it does not turn consumption into a health-neutral matter. The general framework this discussion sits within is explained in the article on harm reduction.

What combustion produces

Combustion is a chemical process before it is a habit: the flame breaks down plant matter at high temperatures and generates compounds that were not in the plant, produced by the act of burning organic matter itself. Hence cannabis smoke and tobacco smoke share irritant and toxic substances, even though the active compounds of the two plants are different.

The respiratory effects that research associates with smoking cannabis are attributed mostly to those products of burning and not to the cannabinoids. What is described most consistently are airway symptoms: persistent cough, sputum, wheezing, episodes of bronchitis and greater irritation of the mucous membranes. They come from observational studies, with the limitations set out further down, but they recur regularly in the literature.

What changes when heating without burning

The difference between one route and the other is chemical: in one case there is combustion, and in the other heat generates an aerosol without a flame being produced. Controlled studies — generally small, short and carried out in a laboratory — have measured lower concentrations of carbon monoxide and of several combustion by-products in that aerosol than in smoke. That is, in essence, the entire verified content of the phrase “vaporising produces fewer combustion by-products”: a comparative result, measured in what is inhaled, which does not describe any route as safe.

Two families of devices that do not raise the same problems are often mixed together in the same conversation. On one side are those that heat dry plant material; on the other, the cartridges and pens containing concentrates with excipients and additives whose behaviour on heating is not always known. It is this second family that was involved in the outbreak of severe lung injury recorded in the United States in 2019: the Centers for Disease Control and Prevention (CDC), the federal health agency that investigated the cases, identified vitamin E acetate — an additive present in cartridges coming from the illicit market — as the substance most closely associated with the outbreak.

What does not change

The route of administration changes what is inhaled alongside the cannabinoids, not what those cannabinoids do once in the body. The acute effects on attention, working memory, reaction time and motor coordination are described irrespective of the device, and they are what explains why traffic rules deal with the matter: roadside tests detect the presence of the substance without asking how it got there, as set out in the article on cannabis and driving.

Nor does the potential for dependence change — the literature links it to the pattern of use and not to the technique employed — nor the set of risk factors discussed in cannabis and mental health, a field where it is worth separating what is established from what remains under debate. And the legal framework is not drafted in terms of devices: the administrative rule refers to consumption in public places and does not mention the technique used, so assuming that the device takes the case outside that provision is a risky reading. The consequences laid down for that provision are described in the fines for consumption in public.

Finally, no route of administration turns consumption into a therapeutic route, or an association into a health resource. An entity of this kind is a non-profit association of adults — in Spanish, an asociación sin ánimo de lucro — as explained in what a cannabis club is, and the people who run it perform no clinical role.

Mixing with tobacco: the factor that weighs most in Spain

In Spain, as in much of Europe, the usual way of smoking cannabis is mixed with tobacco. It is a habit that is widespread here and not universal, so visitors from places where cannabis is smoked on its own often do not expect it, and it has consequences that add to those of combustion and that the literature documents far more solidly than any comparison between devices.

The first is that it adds the entire toxic profile of tobacco smoke to what is inhaled. The second is nicotine: a substance with a well-established dependence potential that becomes part of the habit without the person necessarily identifying as a tobacco smoker, and that makes it harder to stop using either of the two. The third is methodological and explains many arguments: since a large share of the people studied in Europe use both, it is difficult to attribute the observed respiratory harms to one or the other.

Hence a reasonable conclusion in the Spanish context: tobacco is here a documented and separable risk factor, whereas the difference between devices rests on much weaker evidence. In enclosed spaces there is the added exposure of other people, a question that falls to the internal rules of each premises and is addressed in the article on house rules; how these private premises fit into national anti-smoking legislation has been the subject of argument, and each entity sets its own rules.

The limitations of the current evidence

The comparison between vaporisation and combustion rests on a body of studies smaller than the confidence with which it is usually repeated would suggest. Listing its limits does not invalidate the comparison, but it does bound what can be asserted on the strength of it.

• Most of the work involves small samples, short durations and laboratory conditions far removed from real-world use.
• They measure what is in the aerosol, not health outcomes over the years, which is what actually matters and what is still not available.
• Devices are heterogeneous and there is no standard: results obtained with one device do not transfer automatically to another.
• People who vaporise tend to differ from people who smoke in age, pattern of use and tobacco use, which makes it hard to separate the effect of the method from the effect of the person.
• Without a regulated market, the composition of what is heated is certified by nobody, and that variable weighs more than the technique.

That is why these findings admit only a comparative and provisional formulation. A measured reduction of certain by-products is a data point; “it is safe” is not a reading the available evidence permits.

What harm-reduction services say

The harm-reduction approach does not consist of presenting one form of use as advisable, but of describing what is documented as risk. Applied to this comparison, what is left is little and fairly concrete: combustion and, above all, mixing with tobacco are added and documented risk factors; products of unknown origin, especially concentrates and cartridges, raise the problem that nobody certifies what they contain; and no device alters the legal framework, what a roadside test detects, or the acute effects on a person’s capacities.

Two further questions get lost in the technical debate. The first is that the inhaled route is not the only one with risks of its own: the digestive route raises different problems, described in the article on edibles and precautions. The second is that any persistent respiratory symptom, or any sign that use has become hard to control, is a matter for the health system and not for a forum or a members’ premises; the usual itinerary, from primary care to the public addiction network, is described in what to do about problematic use.

The list of entities collected in the directory of associations reflects the information each one publishes about itself, including its internal rules where it makes them public, and can be consulted city by city, for example in Barcelona or Madrid. Admission of socios (members) and the rules of the premises are always decided by the association.

Frequently asked questions

Is vaporising safe?

That is not a statement the available evidence supports. What is documented is that fewer by-products of burning are measured in the aerosol than in smoke, a comparative and limited assertion. There is no threshold described as safe, and the effects of the substance on the body are the same by one route and the other.

Which weighs more, the device or the tobacco mix?

In the Spanish context, tobacco. Its harms have been established for decades, and its presence in the usual mix adds toxicity and dependence on nicotine, whereas the comparison between devices rests on scarce and short-duration studies.

Does anything change at a roadside drug check?

Not in essence: the checks detect the presence of the substance in the body and the rule deals with that presence, without the device used forming part of what is verified. Which consequences apply in each case, and through which channel, is explained in the article on cannabis and driving.

Is smoking allowed inside an association?

It depends on the internal rules of each entity, which are what set out what is permitted on its premises and in which areas. How these private spaces fit into national anti-smoking legislation has been the subject of argument, and the directory collects what each association publishes about its rules where it does so publicly.