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I was linked to a website from Parenting Stack Exchange called drugfreeworld. I then browsed around a bit, and I found this interesting line on the Marijuana page:

Aside from the discomfort that goes with sore throats and chest colds, it has been found that smoking one joint gives as much exposure to cancer-producing chemicals as smoking four to five cigarettes.

This quote then references "Additional Marijuana Facts," from the University of Southern California, May 2015.

Is it true that smoking one joint gives as much exposure to cancer-producing chemicals as smoking four to five cigarettes?

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    According to cdc.gov/mmwr/preview/mmwrhtml/00049790.htm, filters can be as much as 80+% effective at removing tar, CO, and nicotine, which potentially makes a filtered cigarette 1/5 as carcinogenic as an unfiltered one. Granted, the typical joint is often much smaller than the typical cigarette, so that should skew the result. However, since most joints are "roll your own", they could also have chosen any arbitrary size for the joint. Again, I can't check the referenced article, but these are the things I would check when composing an answer. – cpcodes May 9 at 18:22
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    It is also common for marijuana smokers to hold the smoke in their lungs much longer than is usual for tobacco. – plasticinsect May 9 at 19:37
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    yes, that's accurate according to the NIH who explain that even though the same in-question chemical concentrations in pot are only 50-75% higher, due to intake methodology, the result is that "marijuana smoking leads to four times the deposition of tar compared to cigarette smoking". That's not to say that pot smokers fare worse cancer-wise than smokers, in fact, the opposite holds, and it seems that cigarettes tar is particularly damaging to DNA, compared to pot tar. – dandavis May 10 at 21:13
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    @dandavis Maybe you should post an answer – Grumpy says Reinstate Monica May 12 at 1:48
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    AFAIK people generally smoke ~1 joint a day while cigarette smokers tend smoke ~10 cigarettes per day (and stoners probably don't smoke literally every day, while cigarette smokers tend to smoke ~10 cigarettes at Easter, Christmas etc too). – Bakuriu May 13 at 18:52
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The USC webpage states

MARIJUANA USE CONSEQUENCES

  1. The well-confirmed danger of smoking marijuana is lung damage and lung cancer. As examples:

    • 1 joint = 5 cigarettes in terms of amount of carbon monoxide (CO) intake.

    • 1 joint = 4 cigarettes in terms of amount of tar intake.

    • 2 joints = 20 cigarettes in terms of microscopic damage to cells lining the airways.

These appear to be the only relevant numbers. This also appears to be where drugfreeworld gets their numbers for (emphasis added) "smoking one joint gives as much exposure to cancer-producing chemicals as smoking four to five cigarettes." drugfreeworld is likely generalizing CO and tar intake to cancer-producing chemicals.

While the USC webpage doesn't cite sources, a 2008 study wrote (emphasis added):

These factors are likely to be responsible for the five-fold greater absorption of carbon monoxide from a cannabis joint, compared with a tobacco cigarette of similar size despite similar carbon monoxide concentrations in the smoke inhaled.

Regarding tar, a 1998 study wrote (emphasis added):

To compare the pulmonary hazards of smoking marijuana and tobacco, we quantified the relative burden to the lung of insoluble particulates (tar) and carbon monoxide from the smoke of similar quantities of marijuana and tobacco. ...

As compared with smoking tobacco, smoking marijuana was associated with a nearly fivefold greater increment in the blood carboxyhemoglobin level, an approximately threefold increase in the amount of tar inhaled, and retention in the respiratory tract of one third more inhaled tar (P less than 0.001).

A 2008 paper writes:

These differential risks are greater than the 1:5 dose ratio between cannabis and tobacco for carbon monoxide levels and the 1:3 dose ratio for amount of tar inhaled.

This study cites the 1988 study for the 1:5 and 1:3 figures. This shows that the results of the 1988 study are still accepted in 2008. I couldn't find numbers from a more recent study.

A 2013 study wrote (emphasis added):

This concern is heightened by the finding that the smoke contents of marijuana and a comparable quantity of tobacco (unfiltered Kentucky reference cigarette) include roughly similar amounts of volatile constituents (including ammonia, hydrocyanic acid, and nitrosamines) and qualitatively similar tar components (including phenols, naphthalene, and the procarcinogenic benzopyrene and benzanthracene) with the major exceptions of nicotine (found only in tobacco) and [THC], the major psychoactive ingredient in marijuana, and a number of other THC-like (cannabinoid) compounds that are found only in marijuana.

Thus, the carcinogenic compounds in the tar are approximately the same for both marijuana and cigarettes. This means that exposure to more tar is proportionally related to exposure to more carcinogenic compounds.

Does smoking one joint give as much exposure to cancer-producing chemicals as smoking four to five cigarettes?

Smoking one joint gives as much exposure to carbon monoxide as smoking five cigarettes and as much exposure to tar as smoking three cigarettes. As the carcinogenic compounds in marijuana and cigarette tar are similar, smoking one joint gives as much exposure to cancer-producing chemicals as smoking three cigarettes. In this sense, the claim is slightly exaggerated. However, the claim may be misleading as exposure to cancer-producing chemicals (carcinogens) doesn't always directly relate to cancer risk.


A 2014 paper wrote (emphasis added):

Results from our pooled analyses provide little evidence for an increased risk of lung cancer among habitual or long‐term cannabis smokers, although the possibility of potential adverse effect for heavy consumption cannot be excluded.

A 2006 paper wrote (emphasis added):

Our results may have been affected by selection bias or error in measuring lifetime exposure and confounder histories; but they suggest that the association of [lung and upper aerodigestive tract] cancers with marijuana, even long-term or heavy use, is not strong and may be below practically detectable limits.

A 2005 paper wrote:

However, current knowledge does not suggest that cannabis smoke will have a carcinogenic potential comparable to that resulting from exposure to tobacco smoke.

The paper's conclusion explains why this is the case (partially because hydrocarbons in marijuana inhibit enzymes that convert carcinogen precursor compounds into carcinogenic compounds whereas nicotine activates these enzymes).

Thus, while marijuana may expose users to more tar and carbon monoxide, it does not necessarily increase cancer risk relative to tobacco.

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    Does it explain somewhere what counts as a joint? Because you can smoke it in alot of different ways, also adding tabacco to it which could worsen it aswell. – Lyrion May 13 at 14:18
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    @Lyrion Thanks for pointing out your concern. I have edited the question. The 1988 paper looked at "similar quantities of marijuana and tobacco." – Barry Harrison May 13 at 17:20
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    Nice answer, very detailed! I will be accepting an answer soon. – Grumpy says Reinstate Monica May 13 at 20:42
  • @BarryHarrison Yes, I am familiar with Stack Exchange. – Grumpy says Reinstate Monica May 14 at 12:42
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    I'd be interested to see if they looked at the effects of water filtration compared to say actual filters on cigarettes. – JMac May 14 at 16:51
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The claim is incorrect.

Here are some sites whose findings seem to not really back up this extreme claim that one marijuana blunt exposes you to quintuple or quadruple amounts of cancer-producing chemicals as that of a single cigarette:

The last link seems to be quite clear on the fact that they are against marijuana and think it is harmful, yet they do not mention cancer.

Some research says there is NO causal relationship between the two, whilst other says marijuana actually DECREASES the chance of cancer.

Conclusion: Marijuana smoking can cause cancer, to some degree, but this fact is quite likely untrue. Also, marijuana is harmful in many ways whether or not it is cancer-inducing.

  • Thank you for the answer! I may accept it in a day or two if I don't get more answers – Grumpy says Reinstate Monica May 12 at 0:42
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    Welcome to Skeptics! This answer needs a strong edit to remove the conjectures about the source, and to focus on actual evidence. If the pages you reference have some evidence, please cite some of it to show it addresses the question. – Oddthinking May 12 at 0:43
  • @Oddthinking, you have basically removed my whole answer. It isn't like it's a former shell of itself, it is not itself anymore. I would suggest you rather down vote it and then leave a comment as to why you disagreed with its structure and arguments. Also, I'd love to discuss the problems you had with it. Perhaps this comment section won't be the best place but the chat might work? I am not sure though, does this sight encourage long chains of comments under questions and answers? – A. Kvåle May 12 at 0:56
  • Comments are not for extended discussion; this conversation has been moved to chat. – Oddthinking May 12 at 1:02
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    Note that the claim in the question was very carefully couched, language-wise. It says one is "exposed" to more of the chemicals that, in smoking tobacco, can cause cancer. It does not explicitly say that this increases or will cause cancer in marijuana, because, as you point out, it has not been shown. However, by stating like that, they hope that people will assume that the risk and causality go along with it, automatically. Your answer deals with the implied claim head-on, but not with the actual, exact claim. – PoloHoleSet May 14 at 17:22

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