By Dr Farsalinos and Pedro Carvalho (material sciences expert)
There has been a lot of discussion about my statement during the RY4 radio interview on Friday May 22 concerning the dry burning of coils. This is a process in which vapers prepare the coil and then apply a lot of power to the bare coil (with no wick or liquid), heating it to the level of glowing red. The main purposes for doing this is to: a) check for homogenous distribution of temperature over the coil length; b) avoid hot spots; c) clean the metal from residues due to manufacturing or due to previous use.
During my interview I mentioned that it is not a good idea to dry burn the coil, not even once. Since then I have received a lot of responses, emails and requests from vapers to further expand on this, provide evidence and explain the issues related to this process. I have also received data sheets and specs of the metals used for coils, showing that they are stable at extreme temperatures (usually 1000oC or more).
First of all, I should say that the reactions from the vaping community are a bit exaggerated. I never said that by dry-burning the coil you are making vaping more harmful than smoking. Obviously some vapers who are used to dry-burning their coils for a long time did not like my statement, mostly because they are surprized with what i said. But please, consider that my role is not to say what everyone expects but to say how things are. To further explain my statement, I invited Pedro Carvalho a material sciences expert with good background on metal structure, composition and degradation. Pedro also has deep knowledge on e-cigarettes and is well-known in the Portugese vaping community and abroad. This comment has been prepared jointly by me and Pedro Carvalho.
Vapers should realize that metals used in coils have not been made to be in continuous direct contact with liquid, to evaporate liquid on their surface and to have the consumer inhale the vapor directly from the coil. This is a completely different issue from what the metal specifications suggest. It is well-known that metals have been detected in the aerosol of e-cigarettes. Williams et al. found chromium and nickel which was coming from the coil itself, although they did not dry-burn the coil. Although we explained in our risk-assessment analysis that the levels found were not of significant health concern, this does not mean that we should accept unnecessary exposure.
Dry-burning the coils means heating them to temperatures well above 700oC (we have both measured temperatures under these conditions). This is expected to have significant effects on the structure of the metal and the bonds between metal atoms. This heat treatment in the present of oxygen will promote coil oxidation, change grain size of the metals/alloy, promote new bonds between metal atoms, etc. To this, we should add the continuous contact of the coil with liquid. The liquids may have corrosive properties on the metals, which could further affect the molecular structure and integrity of the metal. Finally, the vaper is inhaling the aerosol directly from the coil itself. All these factors may contribute to the presence of metals to the aerosol. Most materials that are used in e-cigarette products were not developed for this application. In this specific case, the resistance wires were develop and used as heating elements at high temperatures IF NO VECTOR can transport the metal / oxide particles to the human body. This not means that can be used in vaping in the same way.
Several studies have shown that oxidation of chromium can occur at similar temperature of a dry burn process [a, b, c]. Although these studies show the formation of a less harmful chromium oxide, Cr2O3, we cannot exclude the formation of hexavalent chromium. Hexavalent chromium compounds have varied uses in industry and are often used for their anti-corrosive properties in metal coatings, protective paints, dyes and pigments. Hexavalent chromium can also be formed when performing “hot work” such as welding on stainless steel [d, e], melting chromium metal or heating refractory bricks in kilns. In these situations, the chromium is not originally in the hexavalent form. Obviously, we do not expect such conditions to be replicated at the same level in e-cigarettes, but the evidence shows that metal structure can change and we are finding metals in the e-cigarette aerosol. Taking all these facts in consideration, we believe that this procedure should be avoided if possible.
How much is metal exposure elevated by dry-burning the coils? Probably not very much. That is why we think the vapers have over-reacted to my statement on RY4radio. However, we do not see a reason why the exposure to metals should be elevated by doing something which can be avoided. There may be other ways of dealing with the issues related to coils. We think it would be preferable to spend some time making a new coil rather than cleaning a used coil using dry-burns. If you want to remove residues from the manufacturing process of the wires, you can use alcohols and water to clean the wire before preparing the coil. If you feel the setup could result in hot spots, it will make little difference if you decrease your power levels by few watts, or spend more time preparing the coil appropriately. Obviously if you want to exploit and use the last watt that the device can give you, then you might find it impossible to do it without dry-burning the coil. But then, do not expect to be exposed to the same levels of harmful substances as vapers not doing it. The situation is similar to another case: if you want to consume 15 or 20 mL per day by doing subohm-direct lung inhalation vaping, do not expect to be exposed to similar amounts of harmful chemicals as those vaping conventionally (or even by direct lung inhalation) consuming 4mL per day. This is just common sense. We need and we will perform research to quantify the exposure (which we think is not very high), but until then it is not a bad idea to use common sense.
We will repeat our honest opinion that dry-burning the coils will not make vaping similar or worse than smoking. This is clear and there is no need for over-reactions. However, we should reach to a point that e-cigarettes should not just be compared to smoking (which is an extremely bad comparator) but should be evaluated on absolute terms. If something can be avoided, vapers should be aware of it so that they can avoid it if they want to.
[a] I. Murris, Y. P. Jacob, V. A. C. Haanappel, M. F. Stroosnijder, Oxidation of Metals. 2001;55:307-331.
[b] K. P. Lillerud and P. Kofstad, J. Electrochem. Soc1980;127: 2397-2410.
[c] Kaori Taneichi, Takayuki Narushima, Yasutaka Iguchi and Chiaki Ouchi, Materials Transactions 2006;47:2540 to 2546.
[d] Christofer N Gray, Annmarie Goldstone, Philip RM Dare, Peter Hewit. The Evolution of Hexavalent Chromium in Metallic Aerosols. American Industrial Hygiene Association Journal 1983;44:384-388.
[e] John H Dennis, Michael J Frenchi, Peter J Hewitt, Seyed B Mortazavi, Christopher AJ Redding. Control of exposure to hexavalent chromium and ozone in gas metal arc welding of stainless steels by use of a secondary shield gas. Ann Occup Hyg 2002;46:43-48.