The carbonyl group (sometimes called simply carbonyl), chemically speaking, is a functional group composed of one carbon atom and one oxygen atom. There is a double bond between those two atoms, so we can denote carbonyl as C=O

There are several classes of organic compounds containing carbonyl group. The simplest are aldehydes (eg. formaldehyde, acetaldehyde, benzaldehyde, acrolein) and ketones (eg. acetone, diacetyl, acetylpropionyl). More complex are carboxylic acids (eg. acetic acid), esters, amides, imides etc. All of them are sometimes called carbonyls.

  • formaldehyde:
  • diacetyl:
  • butyl acetate
    (ester – apple flavour):

(images from

Many normal metabolic pathways in our cells involve chemicals containing carbonyl groups. Even the exhaled breath of a healthy non-smoking and non-vaping person contains some amount of several aldehydes, such as formaldehyde and acetaldehyde.

Vapers all over the world are frequently alarmed when they read about formaldehyde, acetaldehyde or acrolein found in the aerosol inhaled from e-cigarettes. Well – that's right – chemical analysis confirms the presence of these compounds. They are products of thermal decomposition of two main components of e-liquid – propylene glycol (formaldehyde and acetaldehyde) and glycerin (acrolein).

Here I have to emphasize that thermal decomposition has nothing in common with typical oxidation (burning) which is the main chemical process during smoking. Many chemical compounds undergo changes when they are heated. For example – glycerin heated to 280 0C produces acrolein. This carbonyl compound is produced for example in the kitchen when fat or oil is burnt. It can be easily detected, because even small amounts of acrolein usually makes our eyes and nose itchy.

Other chemical reactions turn propylene glycol into formaldehyde. If the aerosol generated by e-cigarette contains a substantial amount of formaldehyde and/or acrolein, that means something is wrong with the equipment. Usually the vaper applies too much power to the e-cigarette or there is a problem with liquid transport through the wick to the coil. Either way, during normal vaping this is very quickly detected by the user – I am sure almost every vaper has experienced an unpleasant phenomenon called ‘dry puff’.

Scientists use special automatic equipment (so called smoking machines) to turn e-liquid into aerosol. When the aerosol analysis is performed automatically there is no possibility of detecting dry puffs. Therefore some scientists proclaim the finding high levels of carbonyls – and this is later reported by the media. Everybody has seen those alarming headlines: „carcinogenic formaldehyde found in e-cigarettes!”. Well – let me put it into perspective. Many carcinogenic compounds are produced during grilling meat. Have you seen a headline „Your t-bone steak can cause cancer!” lately? Probably not. For years scientists have investigated nitrosamine formation during meat roasting, so this is not a very new information. E-cigarettes are a relatively new product – that's why teh media love those alarming news.

Let's calm down a bit – when an e-cigarette is used in the normal way, ie the coil is not overheated and the transport of the liquid is efficient, the level of carbonyls would be really low. We should remember that compounds like formaldehyde, acetaldehyde and similar are ubiquitous – they can be found even in such healthy products like apples, pears, potatoes, and spinach . Moreover, formaldehyde and acetaldehyde are natural by-products of several important chemical reactions in the human body. Therefore we should not be very worried when some micrograms of carbonyls are incorporated into our cells.

As a chemist I would like to remind you of a very old – and still valid – toxicological rule, credited to Paracelsus: only the dose makes the poison (Latin: sola dosis facit venenum). Analytical techniques of the 21st century allow us to detect almost everything in everything. So we would easily detect various carbonyls (even those carcinogenic) not only in an aerosol from e-liquid, but also in potable water, food, and air. Yes, they are ubiquitous. But, again – one should not just ask whether some chemical is present but also about the concentration or dose.

Many papers have been published on the composition of the aerosol generated in e-cigarettes. The general conclusion is simple: if a vaper uses standard settings (battery voltage, coil resistance), the amount of carbonyls inhaled is, in fact, negligible. So that applies to most of e-cigarette users. On the other hand – among vapers there are also cloud chasers. Generating huge clouds requires much higher power. This results in much higher temperature which, in turn, causes significant increase of the level of carbonyls. Cloud chasers should then be aware of this phenomenon.

To sum up – carbonyls are everywhere. There is no need to panic while reading those news about aldehydes found in vapour. If you follow simple rules - use less power; use wicks and coils providing good transport for liquid; and avoid „dry puffs” - then everything will be OK.

Ceterum censeo Directiva Tobaccorum delendam esse!