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Tobacco use is the leading cause of preventable death around the world.$\ce{^1}$

enter image description here

The stain is considered as a "tell-tale" sign for identifying smokers. Multiple materials stated that it's due to tar (nicotine) disposition. The colouring fades away after the smoker quits. There are also number of search results that point to "removing" these smoking stains in the skin. If it can be removed using chemicals then I wonder what sort of a chemical it utilized there as well.

Is it only nicotine that causes the yellow stains on the skin? What is the role of oxygen deprivation in this scenario? Can this be categorized under smoker's melanosis? What tissues/cells are being "transformed" in this case? What's the biochemical explanation behind the colouring of skin?

How is this staining differe from a ink stain in the skin which lasts for about about 3-4 days?

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  • $\begingroup$ Please do not change the question significantly, especially after answers have been submitted. Instead, ask a new question. $\endgroup$
    – MattDMo
    Commented Jul 31, 2015 at 14:18
  • $\begingroup$ @MattDMo I am not sure if this is a rule within Biology.SE, because there are ample of times where the original post was edited with closely related info/queries. This one from Chemistry.SE is a big example for that. Someone with moderator reps would be able to view its edit history. The answers were editted without any complain. The colouring of skin by smoking (nicotine, tar) and colouring of skin by inks are two common encounters we see day today life. So, they are related and I see it's fair enough to ask these two questions together. $\endgroup$
    – bonCodigo
    Commented Jul 31, 2015 at 16:41
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    $\begingroup$ This is general practice for all Stack Exchange sites, and is referred to as "moving the goalposts." Yes, of course questions may be edited to be clarified, or to add new information requested in comments, etc. However, it is bad form to add new, only tangentially-related questions to the original question, especially after answers have been submitted. Those answers may be downvoted later by users who see the expanded question, and an answer that apparently only addresses part of it. (...) $\endgroup$
    – MattDMo
    Commented Jul 31, 2015 at 17:19
  • $\begingroup$ (...) Additionally, a question should be just that - a single question. Asking multiple different things in the same question makes it more broad, and makes it less likely that a single, concise answer can address all of the points. $\endgroup$
    – MattDMo
    Commented Jul 31, 2015 at 17:22

2 Answers 2

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Pure nicotine is a yellow colored liquid (although some sources say it's a clear liquid.) "Tar" is a complex sludge that is also yellow-brown. So it's difficult to distinguish 'nicotine stain' from a 'tar stain' based on visual inspection. Let's go to other considerations.

The concept of a 'stain' implies that simple hand washing doesn't remove the coloration. Since nicotine is water miscible, one would expect it's easily removed with soap and water. That implies that either a) a smoker's stain is not primarily nicotine, or b) the nicotine reacts with the skin to create the stain.

Nicotine is sufficiently functionalized that it could conceivably undergo some interesting chemical reactions with proteins, but I don't think that's what's happening. A simple observation is that transdermal nicotine patches do not contain a colored agent, and do not stain the skin. Score one point for the "mostly tar" explanation. Cigarette smoke contains far more tar than nicotine, and tar will deposit on most-any surface (at ambient temperatures), so it's parsimonious to proceed with the "mostly tar" explanation.

The picture shows that the stain occurs on both fingernails and skin. The most abundant protein that's common to those two tissues is keratin. Like other examples of protein staining, the mechanism is complicated and poorly understood, but the basic outline is that parts of keratin are greasy (the non-polar amino acids) that associate with other greasy things. (EDIT: AliceD's answer also depicts the well-known staining of teeth, which are not keratinized, showing that the staining mechanism is likely a nonspecific adsorption of greasy things onto surfaces.)

The stain can be fixed through the involvement of free radicals and other reactive species in cigarette smoke, like formaldehyde. In this sense, reactive oxygen species are active participants. I don't think oxygen deprivation plays a role here. I don't think this can be described as a melanosis, as the skin isn't producing melanin in response to smoking but is simply being coated in tar.

Even if the stain was completely fixed, the coloring would fade away as the exterior skin cells are sloughed off, consistent with normal skin physiology. This is the explanation for the fading of any stained epidermis. The persistence of tar stains on teeth (which do not exfoliate) further evinces that the stain is removed through the replacement of skin cells.

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    $\begingroup$ Parsimonious does not mean what you think it means. $\endgroup$
    – MattDMo
    Commented Jul 31, 2015 at 1:24
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    $\begingroup$ BTW, I think you meant perspicuous. At any rate, I wouldn't use it here (maybe on English Language & Usage) as not many native English speakers know what it means, let alone those for whom English is a non-native language. $\endgroup$
    – MattDMo
    Commented Jul 31, 2015 at 1:40
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    $\begingroup$ @MattDMo He/she did not use the word incorrectly. I'm not going to cite a bunch of dictionaries, but you should look at some other ones. $\endgroup$
    – canadianer
    Commented Jul 31, 2015 at 4:26
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    $\begingroup$ @MattDMo, you certainly don't know what I think 'parsimonious' means, although I do find it interesting that a dictionary of that caliber has such a limited scope. Webster-Merriam apparently needs more logicians on their usage panel! $\endgroup$
    – Ryan
    Commented Jul 31, 2015 at 7:05
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    $\begingroup$ @AliceD, you share my intuitions but not my reasoning, making you a good person to know! Anyway, the quote is "[c]igarette smoke contains far more tar than nicotine..." so you're still misquoting! ;) $\endgroup$
    – Ryan
    Commented Jul 31, 2015 at 14:28
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Short answer
Tar deposition is the cause of the yellow cigarette stains.

Background
Both tar and nicotine cause yellow to brown staining. Although nicotine itself is colorless/white, it turns yellow upon exposure to air.

In a study on cigarette stains, the author speaks of tobacco-tar stains, implicitly acknowledging it is tar and not nicotine (John et al., 2013).

Interestingly, e-cigarettes which contain nicotine, but no tar, are reported to leave no stains, pointing to tar as the culprit, and not nicotine.

As to physiological causes of the stains I can say it is simply a tar stain and it has nothing to do with pigment or anoxic conditions, because the teeth and nails also stain yellow:

enter image description here
Tar stains on skin and nails. Source: John et al., 2013.

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Tar staining of the teeth. Source: Identalhub.

Reference
- John et al., BMJ Open (2013); 3:e003304

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  • $\begingroup$ That added part is a bit vague - there are many inks. Permanent marker ink may stay longer than 3-4 days and washing habits may affect this too. How do you know tar persists longer? Consider asking a new question on this and specify what you wish to know. To me it looks like a different question altogether as the ink part is unrelated to the original question. $\endgroup$
    – AliceD ♦
    Commented Jul 31, 2015 at 3:10
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    $\begingroup$ Nice answer, @AliceD. The final link to John et al seems to point to an incorrect URL, however. $\endgroup$
    – Ryan
    Commented Jul 31, 2015 at 7:09
  • $\begingroup$ @Ryan - thanks for the appreciation and thanks for taking the time to click through the link :) I fixed it. $\endgroup$
    – AliceD ♦
    Commented Jul 31, 2015 at 10:22
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    $\begingroup$ @AliceD see my comment on the other answer regarding the color of nicotine. I checked out a bunch of different chemical resources and vendors, and most seem to indicate that it is generally in the yellow-brown spectrum, as it is most commonly exposed to air (such as in a cigarette). I think it's only clear when freshly prepared and/or maintained in an oxidation-resistant environment, which would be the case in pharmaceutical manufacturing for nicotine replacement therapy patches. (...) $\endgroup$
    – MattDMo
    Commented Jul 31, 2015 at 14:27
  • $\begingroup$ (...) Drugs.com indicates that the nicotine is sandwiched between 2 polyester backings and embedded in a matrix of various polymers, so $O_2$ likely can't interact with it. $\endgroup$
    – MattDMo
    Commented Jul 31, 2015 at 14:30

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