When vapers discuss safety issues, they usually take into account the components of e-liquid and the composition of the aerosol inhaled. Of course this is the most important subject, but we should also be aware of metals of which the coils are made.
In general, heating elements are devices that convert electric current into heat. The amount of heat that is released in such a process depends on the current (squared!) and electrical resistance of the conductor.
Heating elements in e-cigarettes are usually made of resistance wires of various types. These include alloys such as kanthal, stainless steel, nichrome and, lately, pure metals: nickel or titanium.
Chemical composition of coils
Kanthal consists of iron (Fe – approx. 70%), chromium (Cr – 20-30%), and aluminium (Al – 4-7%). Upon heating, aluminium metal oxidises and the resulting aluminium oxide forms a protective layer on the surface.
Nichrome is an alloy of Cr (80%) and nickel (20%). When heated to so called red hot temperature an outer layer of chromium(III) oxide develops, which protects the heating element from further oxidation.
Recently stainless steel (SS) has been introduced to the vapers market. Steel itself is an alloy of iron and carbon (C). When at least 13% of chromium (Cr) is added to normal steel it becomes stainless, which means that it does not rust. The amount of Cr in that alloy is sufficient to form a protective layer of chromium oxide, just as in case of nichrome. Other metals present in SS depend on its type, eg. SS 316 contains also small amounts of molybdenum (Mo - 2.5%) and manganese (Mn – 2%).
Particulate matter (PM)
Microscopic solid matter suspended in the air (or e-liquid aerosol) is known as particulate matter. These particles are very tiny – most of them are in the range of 1-10 micrometers. Therefore, upon inhaling, some of them can go deep into the respiratory system, reaching even the alveoli. Tiny specks of metals or their oxides can also be pulled out from the coil surface, forming such particulate matter. The impact of PM on vapers health is still unknown, but we should remember that all humans are exposed (24 hrs/day) to various particulates due to air pollution present everywhere.
Nickel – allergen of 2009
From 2000 American Contact Dermatitis Society awards the title of Allergen of the Year. In 2009 this award went to nickel (Ni). Probably many of you know at least one person allergic to nickel. They cannot wear jewelry made of nickel, some show symptoms of allergy even while using bathroom faucets plated with nickel. Therefore it's not surprising that some vapers also have problems using certain atomizers containing this metal. So if you show typical symptoms of allergy: rash on the skin, itching, redness – stop using e-cig and consult your doctor if the problem persist.
Chromium, molybdenum and manganese
These three transition metals can be found eg. in stainless steel. Chromium (Cr) undergoes passivation by oxygen present in the air. During this process a very thin layer of chromium(III) oxide is formed. It is widely known that chromium(III) compounds are recognised as safe. Fortunately, formation of toxic and carcinogenic chromium(VI) compounds in the case of coils used in e-cigarettes is highly improbable. In spite of the data from 50s and 60s, scientists were not able to prove the role of chromium(III) compounds as vital trace element.
Molybdenum (Mo) and its compounds are not toxic, although high Mo intake may result in so called secondary copper (Cu) deficiency. The amount of Mo in stainless steel is low, so such a problem for a vaper is highly improbable. We should also know that molybdenum is an essential trace element present in many enzymes important for our metabolic pathways.
Manganese (Mn) should not be confused with magnesium (Mg). This metal is very important for human health as it is an element present in a broad class of enzymes. On the other hand, manganese dust is toxic; maximum level of Mn in the air is 5 mg/m3. The presence of such an amount of Mn in the aerosol generated by e-cigarettes is improbable.
Dry burning of the coil
Many vapers used to dry burn their coils in order to get rid of the brown tar-like substance that builds up on the coil after some time.
Those who ask Hamlet's question: to dry-burn or not to dry-burn should read an important text by Farsalinos and Carvalho: Dry-burning metal coils: is it a good thing? In a nutshell: dry-burning definitely changes the inner structure of the metal/alloy. This, in turn, can affect the integrity of the coil, allowing tiny metal particles to be leached by e-liquid. When the liquid forms the aerosol, some of these particulate matter may be present in it, so then it can also enter vaper's respiratory system.
The question, then, arises: does that affect vaper's health to a significant extent? Well, we still don't know. As far as I know there weren't any medical analyses of this problem yet. One thing is sure: if we can avoid any factors which might be unhealthy, we should avoid them.
So my chemist's advice is: do NOT dry-burn the coils. It's much better to replace a dirty coil with a new one than expose your organism to a mixture of unknown chemicals. Vape safe!