what are vapor, aerosol, particles, liquids, and such?

Here is much confusion about states of matter as they relate to e-cigarette “vapor”. This post is intended to explain what you need to know about them.

The word “vapor” has become a generally accepted term for “what comes out of an e-cigarette”. That means what it means, and that is fine. Common language often takes technical terms and uses them in a way that is “incorrect” if interpreted as if it were the technical term. But this can create confusion.

Vapor (in the normal sense of the term) refers to the gas phase of something that is “normally” a liquid. So the bits of H2O that evaporate into the air are “water vapor” (not to be confused with fog — keep reading). There is no actual technical definition for what subset of gasses are called vapor. The “normally” in my definition is obviously a fuzzy term that basically means, “in everyday settings, within Earth’s biosphere, in non-extreme weather, it is a liquid (e.g., water, ethanol, propylene glycol, to name a few favorites) but the particular molecules in question are in gas phase.” (I found a nice exposition of this “normally a liquid” point here if you are interested.)

Let me step back a second: Any chemical can be in gas, liquid, or solid phase, depending on the temperature and pressure. These are defined as follows (phrased casually, but basically the technical definitions): Solid is the phase in which something maintains its shape rather than conforming to the space it occupies; the molecules of a solid mass mostly stay where they are in relation to each other. Anything that is not a solid is a fluid, which means that the molecules move freely with respect to each other, which produces the notable feature that it conforms to the shape of the space it occupies. That word is often misused as if it means “liquid”, but it actually means “liquid or gas”. A gas is a fluid where the molecules are separated across a lot of space and mostly “ignore” each other, and as a result it has no definite size — i.e., it expands to fill a space or can be compressed, as in a tire. A liquid is a fluid where the molecules are packed together, and thus a given amount of liquid has a defined size, but the molecules are not stuck together and so it does not have an inherent shape.


Liquids contribute some gas molecules to the air (evaporation). So there is some water vapor in the air, as well as some gasoline vapor, some propylene glycol vapor, etc. There will be more of those vapors in the air if there is more of the liquid locally or if conditions are hotter. The air will “accept” more molecules of the liquid up to a point, but then it will saturate. (That is enough detail for present purposes. If you want more details, search the phrase “partial pressure”.) This means that e-cigarette “vapor” really does contain some vapor. But it is mostly in the liquid phase. That is, it is tiny little droplets of the liquid, suspended in the air. “Suspended” means that they are not in the gas phase, and thus part of the air, but are physically mixed with the air. That is the difference between dirt in water (little bits that are suspended) and salt dissolved in water (where it becomes part of the liquid). That suspension in air is called an aerosol, not a vapor.

At this point, we still have some complications. Many definitions of “aerosol” include solid particles suspended in the air, not just liquid droplets. That is, smoke would also be called an aerosol (it contains both liquid and solid bits suspended in the air). Thus, if we want to be very clear about what e-cigarettes produce, we should call it a liquid aerosol. There are a few detectable solid particles (some flavoring agents are suspended solids in the liquid rather than dissolved in it; minuscule quantities of solid contaminants come off of the hardware), but very few — not enough to invalidate this characterization.

Additionally, most definitions of aerosol include some requirement that it is in a fairly stable state. Foggy weather consists of an aerosol of liquid water droplets in the air in a stable state, so the cloud persists for a long time. By contrast, e-cigarette aerosol usually contains many droplets that are large enough that gravity will pull them out of the cloud (down to the bottom of your lung or the floor) over a relatively short period. Also, under normal circumstances (e.g., pretty much anywhere other than a vape meet), the amount of the chemicals in gas phase in the air is low enough that the droplets will quickly evaporate. These are why the visible cloud of e-cigarette exhalate normally disappears quickly. Thus, e-cigarette “vapor” is arguably not really stable enough to technically qualify as an aerosol.

That said, when scientists study something like e-cigarette “vapor”, they call it an aerosol even when it is not so stable under present conditions. So by that practical definition, it is an aerosol.

What it definitely is not is “particulate matter”. Again, we run into the difficulty of imprecise definitions. The common-language definition and everyday scientific definition (setting aside particle physics) of the word “particle” are “a very tiny solid object”. But in some contexts, it is locally defined as “anything suspended as an aerosol” which, given the option of defining aerosol to include suspensions of solids rather than just liquid, means that droplets get called particles. (This usage seems to be concentrated among people who do lab work using machines that cannot distinguish between particles and droplets that are suspended in the air, for obvious reasons.) But calling liquid droplets “particles” is grossly misleading in the context of health science. It is like referring to table salt as “metallic poison” or to acetaminophen as a “mind altering substance”, either of which may be technically true depending on your definitions, but wildly misleading except in very odd contexts.

Breathing tiny solid objects can be unhealthy, even if they are fairly inert (obviously it can be a lot worse still if they are also chemically toxic). Concern focuses on the really small ones, typically discussed under the rubric “PM2.5″ (particulates in the air that are smaller than 2.5 microns across), which are produced by combustion. There is actually extensive disagreement on just how harmful such exposure is, and there is the same “public health” ideological bias in that area as there is in discussions of tobacco, declaring the worst-case estimates to be settled science when clearly they are not. (I touch on this and other details in previous posts on this topic.) But magnitude of the risk aside, inhaling the wrong particles is not good for you. Smoke is unhealthy not just because of the chemically harmful molecules in it (though that is most of it), but also because of particles themselves. Rock dusts (silica, asbestos) are harmful to breathe because of the effects of the physical particles themselves rather than their chemistry.

Breathing droplets is not at all similar to this. Particles are solids, retaining their shape and size, and thus the particle itself can cause damage. Droplets are liquids, which do not maintain their shape and size. Whether a given volume of liquid enters the mouth and lungs in the form of big droplets or small makes no difference in terms of any harms it might cause. When the droplets land, they are just a layer of liquid that does not in any way “remember” the droplet size distribution it once had.

This is not a difficult concept, though it seems to elude the ANTZ. Throw a bunch of broken glass on the ground and you have a hazard, but the same number of SiO2 molecules in the form of sand poses no risk. Size and shape matter. But whether you spray a fine mist of a liquid on the ground or pour the same number of molecules from a bucket, the result is the same. The original size and shape do not matter. The presence of the liquid might still be a health hazard, of course (if it is propylene glycol, no problem; if it is gasoline, that could be a problem). But if there is a hazard, it is completely unaffected by the size of the particles that created the puddle.

For exactly the same reason, the health impacts on the body of e-cigarette vapor are not affected by the size of the droplets in which it arrives. Smaller droplets are different only in that they find their way to more of the lung surface, and thus form a thinner layer of liquid that is absorbed faster (think about spraying a liquid across a wide area of dry ground, rather than creating a small puddle by pouring a bucket) — this is normally considered to be a feature, not a bug. In theoryit could be that a liquid is more harmful to deep lung tissue than it is to other airway tissue, and thus smaller droplets (which reach the deep lungs) could have different effects. This is something that appears to be true for tiny (solid) particles. But there is simply no reason to believe this is the case for e-cigarette liquid deposition, and those who suggest otherwise are just engaged in the typical “public health” game of presenting an unsupported and wildly unlikely hypothesis as if it were a genuine reason for concern (see also: gateway).

That is pretty much what you need to know. Reading 1600 words has made you more expert on the topic than most of the faux-experts who are out there talking about this matter(!).