This was originally a submission to /u/skiddlzninja/ 's new community wiki project, but considering how many new people seem not to know much of this stuff, I thought I'd let it out into the wild before the wiki is complete.

##Safety

What's in e-juice, and how does our body deal with it? (A primer for understanding why certain things are less safe than others)

 

E-juice, whether commercial or DIY, consists of some combination of vegetable glycerin (VG), propylene glycol (PG), nicotine, and flavorings.

 

VG and PG used for e-juice should be USP grade, which means they are essentially pure (around 99.7%). Both are considered GRAS (Generally Recognized As Safe) for use as food additives and in other consumer products. PG is known to be an inhalation irritant for certain people.

 

Nicotine is sold to consumers suspended in a solvent, typically VG, PG or some combination of the two. Safe handling of nicotine is discussed in the Nicotine section.

 

The last potential ingredient, flavorings, is the subject of almost all the safety concerns regarding e-juice. Flavorings used in e-juice are made up of volatile compounds, suspended in a solvent. The solvent is most commonly PG, although VG and ethyl alcohol are also used. These volatile compounds are small organic molecules that have high vapor pressure at room temperature and will evaporate or sublimate into the air spontaneously. The human olfactory sense can detect many of these molecules in extremely low concentrations. This is the basis for the sense of smell, and contributes to the sense of taste. Safety concerns arise because there are so many of these compounds approved for use in food flavors. Food flavors are designed to pass through the digestive tract, not the lungs. Often, the specific compounds used in a given flavor are a trade secret, and some are irritating or dangerous when inhaled in high concentrations. The most famous example of potentially dangerous compounds is the diketones, including diacetyl and acetylpropionyl. These two volatiles provide a rich, buttery flavor that is difficult to replicate. Most "V1" or "original" custard or buttery flavors contain one or both. They are both also known to cause damage to the lungs after long term sustained exposure at high levels. Many flavor vendors have introduced replacement versions of these flavors, typically labeled V2 or DX, which replace diketones with butyric acid. Some people, however, prefer the original flavor despite the concerns.

 

So, flavors are bad?

 

No! Flavors are an important part of e-juice. They help ex-smokers continue vaping instead of going back to cigarettes. The concentrations known to be irritating or dangerous are typically much, much higher than those found in e-juice, even in the case of diketones. In fact, cigarettes contain diketones and other volatiles at these higher concentrations, which may contribute to their damaging effects. However, the exposure from vaping is similar to working around foods all day. Your body is capable of dealing with volatiles at these levels.

 

Ok, so what happens to vaporized e-liquid after I inhale it?

 

Most of it is exhaled, some of it is trapped in the lungs. It is important to understand that vapor from e-juice is technically an aerosol, which means lots of little tiny droplets. In well mixed e-juice, each droplet should contain a small amount of each of the original ingredients. Each droplet passes through your trachea and at least part of your bronchial tree. The "bronchial tree" is just a description of the way the passages in your lungs split over and over forming smaller and smaller passages the further down you go, sort of like an upside down tree. The inside surfaces of your trachea and bronchial tree are coated with tiny hairlike structures called cilia. On top of the cilia, rides a continuous layer of mucous. This mucous and cilia layer is the primary defense your lungs have against anything that's not a gas, including dust, debris, bacteria, aerosol droplets, and any other foreign matter. Many droplets will become trapped on this layer. In healthy lungs, the cilia are constantly in motion. This coordinated action pushes mucous containing trapped matter up the bronchial tree and trachea until it reaches the back of the throat, where it is swallowed, or expelled through the mouth. Source.

 

The aerosol created by vaping is very fine, and some droplets will never come in contact with any surfaces, and simply be exhaled. Many of the droplets will get past the mucous defense system and make to the alveoli. The alveoli are what's at the end of the smallest branches of the bronchial tree. They are clusters of specialized structures that handle gas exchange, taking in oxygen and expelling carbon dioxide. This is also where nicotine passes into the bloodstream. The alveoli do not have the cilia and mucous system to defend themselves from foreign matter, because all the cells there are dedicated to either gas exchange, producing a surfactant that weakens surface tension to help facilitate gas exchange, or a more active type of defense. Specialized cells called alveolar macrophages surround, envelop, and try to either destroy or digest anything that doesn't belong in the alveoli. In the case of e-juice, droplets that get trapped in the alveoli would be absorbed and processed or carried out to the cilia. Unfortunately, certain chemicals and substances cannot be handled properly by the macrophages. They either die in attempt to deal with the substance, or cannot remove it for mechanical reasons ^note. This leaves the substance in place, where if enough accumulates, it begins to interfere with gas exchange, either by blocking surface where the exchange takes place, or damaging the cells directly.

 

^note Most notably, asbestos cannot be digested by the macrophages, and the shape of asbestos particles pierces the cell wall when they try, killing them. Eventually, scar tissue is formed around the particles. E-juice should not contain any particles with these properties.

 

What things shouldn't be vaped? aka What can't the macrophages handle?

 

Oils (true oils which contain certain lipids), some organic compounds (including common food additives such as food coloring or thickeners), amino acids, combustion products of sugars (including products from incomplete combustion) and plant matter are all things that are known to be unsafe to inhale. Because of this, it is recommended that ONLY flavors known to contain only volatiles and a solvent such as PG, VG, or ethyl alcohol be used for vaping.

 

How do I know which flavors are safe?

 

The best way to know is check to see if an MSDS is availability for your flavor. This will list any unwanted components such as food coloring or thickeners. Several manufacturers, including CAP, TFA, FA, INW, and FLV are safe to use for vaping in most cases. FW and LA both have many safe flavors, however, both have some questionable ones. FW has some flavors that contain fructose or other sugars, which have a low oxidation temperature. This means combustion byproducts may be present in vapor generated by these flavors, dependent on coil temp. LA has a number of flavors containing food coloring, and still others containing oil. It is generally unwise to use other flavorings designed for food unless the contents have been vetted by the vaping community. Many people, when first considering mixing their own juice, assume any food flavor will work fine. McCormick's vanilla extract is a common example. People assume incorrectly that it is safe to vape because it is safe to swallow. The ingredients list reads "vanilla bean extractives in water, alcohol (35%), and corn syrup". "Vanilla bean extractives in water" certainly contains plant matter other than the raw volatiles that make up the vanilla odor, so it is unsafe to vape. Corn syrup is a sugar with a low combustion temperature, and so it is also unsafe to vape.

 

What about essential oils?

 

While essential oils may have a pleasant aroma, the safety of using them as a flavoring in e-juice is highly dependent on the method used to obtain the essential oil. Steam distillation results in a product that contains primarily volatiles in water. This is technically not an oil and will be water soluble. This type is the least likely to be harmful. Other essential oils are true (hydrophobic) oils obtained by pressing or solvent extraction. Most citrus essential oils fall into this category. These are generally not safe to use as a flavoring in e-juice due to the possible presence of lipids. In general, essential oils are a poor substitute for commercially available flavorings in e-juice, especially because flavorings with similar aromatic profiles are almost always available.

 

I want to make my own flavors from scratch. I'll just put (whatever I have lying around) in PG and use that as a flavoring.

 

This is known as a solvent extraction. While it is possible to get most food items to dissolve at least partially in PG, the resulting liquid will necessarily contain more than simply volatiles and PG. PG will act as a solvent for sugars, amino acids, fatty acids, other lipids, and general plant matter. Because it is impossible to know what is in solution, this practice is highly discouraged. Straining with filter paper can remove solids suspended in solution, but will not remove dissolved components.

 

What about NETs?

 

Naturally extracted tobaccos, or NETs are a solvent extraction of tobacco leaf. Some people feel they provide the closest approximation of the taste of smoking. People who choose to use NETs should be aware that they likely contain sugars, plant matter and lipids. However, this is still likely to be safer than combusted tobacco due to the lower number of organic compounds present and the lack of carbon particles.

 

People sometimes refer to e-juice as oil. Does this mean I can use vegetable oil or olive oil as a solvent?.

#No! Never intentionally vape oils!

 

This can lead to lipid pneumonia.

 

What is lipid pneumonia?

 

Lipid pneumonia is a form of inflammatory disease caused by the presence of lipids in the bronchial tree. Lipids are are a broad category of molecules generally associated with energy storage, although some are used for signaling or as structural components of cell membranes. Fats, vegetable oil, and mineral oil are all types of lipids associated with past cases of lipid pneumonia. Alveolar macrophages treat lipids like any other foreign object in the lungs. They attempt to absorb it. Unfortunately, alveolar macrophages do not have the proper chemical surfactants to break lipids down completely. Instead they become engorged and die. New macrophages attempt to absorb the now dead cells (still engorged), and instead repeat the cycle. Eventually, small airways may be blocked by the mass of dead cells, or in alveoli it may interfere directly with gas exchange. While rare, historical cases of lipid pneumonia have been due to aspiration of regurgitated oils by infants or the elderly. Vaping oils intentionally is essentially identical to aspirating liquid oil accidentally, except more evenly spread throughout the lungs. For this reason you should never vape oil.

Edit: Fixed broken link. Added language comparing volatile levels in ejuice to cigarettes as suggested by /u/T_Mace/