One of the missing safety concerns in vaping is coil and wick health.
The wick, typically made of cotton or a similar synthetic, is used to draw the e-liquid from the storage well into the coil as it is heated. Coils are commonly made of Kanthal, stainless steel composites, virtually pure titanium, nickel composites or a variety of composites of resistance and non-resistance metals.
As the coil heats up, it aerosolizes the diluents (primarily propylene glycol, vegetable glycerin,ethyl alcohol and water) into a colloidal mist that is inhaled. The vegetable glycerin content tends to produce a visible vapor or cloud that can be exhaled into the air. This is not smoke, but merely a mist of very small liquid particles similar to fog or clouds.
As seen in the image above, this is a generic metal heating coil with a generic cotton wick material threaded through it. The e-liquid is added to the wicking material or into the e-liquid channel that the bottom of the wick is soaked in. Once the wick pulls the e-liquid throughout and is fully saturated, the vaping process can begin.
The common diluents listed above are all excellent solvents for aroma volatile compounds (AVCs, or flavoring). A typical e-liquid might be anywhere from 0.1% to 2% composed of AVCs. Most flavoring compounds have boiling points lower than the diluents, but once they are homogenized into the diluents, they’re carried off during the aerosolizing process. With a fresh wick and recently installed new coil, most of the AVCs in the saturated wick will be aerosolized into vapor with the diluents. Common temperatures of these heat coils reach anywhere from 320F to 500F, adjusted by the vaping device user.
Throughout the vaping process, some of the AVCs may not be fully aerosolized as they come into direct contact with the heat coil. Because their boiling point may be significantly lower than the heat coil temperature, there may be a conversion of the compound structure. This may occur when the compound actually burns, or it may happen at a given temperature. The AVCs that are carried with the diluents into vapor aren’t burned and may not go through a conversion step, but the AVCs that make direct contact with the heated coil will start forming a residue in the wick and on the coils. In the second image above, you can see the wick is starting to take on the color of the e-liquid (generally a light brown to amber to yellow color).
As the coil generates this gunking residue, the flavor or vapor production may be slightly diminished, causing the e-cigarette user to take deeper draws off the e-cigarette device. The speed at which the gunking residue is created is a complicated number to calculate as it is directly correlated to the temperature of the coils, how well the wicking material is designed, how thick the e-liquid is, how many AVCs are in the e-liquid, which AVCs are in the e-liquid, how much airflow enters the atomizing chamber, etc. Some e-liquids may not create gunking residue for days or even weeks, whereas other e-liquids can create gunking residue in mere hours or less.
As the process continues over time and repeated draws of the e-cigarette, the gunking residue starts to increase at a faster rate. This is because the residue reduces the amount of e-liquid converted to vapor, and the gunking residue attracts more of the AVCs within the e-liquid solution. The residue compounds are now in direct contact with extremely hot metal coils, greatly increasing the likelihood of burning or conversion to other compounds.
In some cases, this leads to a foul taste during the vaping process. It can also lead to the cotton wick burning as well. In general, when the vapor flavor is foul, the e-cigarette user will replace the cotton and coil with a new one, but not all gunk residues will taste foul enough to warrant a change. Many e-liquid solutions contain a significantly higher amount of AVCs than similar flavors used in food and drink products, and this higher amount can mask foul tastes of gunk residue or conversion of the AVCs into new compounds.
In addition to the flavor AVCs, some e-liquid vendors use artificial or natural sweeteners to increase the palatability of the flavor. These sweeteners can enhance flavor and mouth feel, but they can also create an increase in the gunking residue. Sweeteners are not as easily aerosolized into vapor, and can be sticky, causing an increased attraction of other AVCs to the coil.
Should the e-cigarette user not change their wick and coil, the residue gunking process will continue at faster and faster rates. Eventually, the metal of the coil will no longer be visible, as the gunking residue has completely encased it. This leads to significantly less vapor production and flavor, although not always a foul taste.
The gunk residue in most cases will turn black, or a very dark brown. The cotton will have signs of appearing burned but it may just be extreme gunking residue in the fibers. At this point in the vaping process, the residue continues to be in contact with the hot coils, and the more of the composition of the residue may have gone through changes from the heat process.
Without knowing what the AVC ingredients are in the e-liquid compound, there is no way to know what conversions may be happening and what might be included in the vapor when the cotton and coil have accumulated excess residue.
Increasing safety by monitoring your coil and wick
If your goal is the safest vaping possible, it makes sense to monitor your coil and wick and immediately change them when you start seeing gunk residue.
Also, changing your e-liquid flavor or brand can have a big return. Many vendors use sweeteners and other AVCs that can increase the likelihood of gunk residue forming.
Because there are a variety of compounds that can mask the bitter taste of noxious and potentially toxic compounds created during the heating of AVCs and diluents, it is always safest to monitor your wick and coil health and replace them both upon seeing increased residue.