Smoking, smokeless tobacco and cancer (2)
Then when we look at studies of smokeless tobacco use from the U.S. we see mixed results. One particular study published in the 1980s found very high risks of oral cancer. That study by Professor Deborah Winn and colleagues focused on women in southern United States and found that white never-smoking women (who tend to use oral dry snuff powder) had a relative risk of over 4.2 (2.6-6.7), for developing oral and pharyngeal cancer. Women who had used smokeless tobacco for 50 years had a 50-fold increase in risk for some oral cancers. It should be noted that only a tiny minority of smokeless users use the type of dry snuff tobacco that is commonly used by women in these rural parts of North Carolina. It should also be noted that the type of smokeless tobacco with the extremely high concentration of toxins mentioned in my previous post was also a type of dry snuff. So it begins to look like it’s the smokeless tobacco with very high toxin levels that has evidence of a causal effect on oral cancer. In fact, one of the largest studies on the effects of smokeless tobacco use in the United States (by the American Cancer Society) found no significant increase in the risk for oral cancer among smokeless tobacco users. One of these studies examined a population of 2488 smokeless tobacco users with a median age of 57 at enrolment (i.e. they'd used smokeless for decades) and followed them up 18 years later (i.e. mean age 75). This study found only one death from oral cancer in exclusive smokeless tobacco users, and none in former users, which was a slightly lower rate than occurred in people who had never used any tobacco in their life. These findings from the American Cancer Society suggest that any effect of commonly used brands of smokeless tobacco in the United States on oral cancer, if present, is relatively small.
Finally, we can examine the studies conducted in Sweden where the use of a form of low-toxin moist snuff (called snus) is more common than smoking in men. These studies consistently find no increased risk of oral cancer for snus users. One of the best of these studies was published in the Lancet recently and found that snus users have no increased risk of oral or lung cancer, but they have twice the risk of pancreatic cancer of never tobacco users. That same study found higher risks of all these cancers in smokers (e.g. smokers were about 10 times more likely to get lung cancer than either snus users or never tobacco users, who both had similar risks).
So now lets come back the recent report by Hecht and colleagues. It found similar concentrations of a specific carcinogen in smokeless users and smokers. But we have excellent data showing that smokers have much higher risks of lung cancer than smokeless users, and also higher risks of oral cancer than users of some forms of smokeless tobacco. The lung cancer pattern suggests that deposition of carcinogens directly into the lungs is relevant to the causal mechanism as smokeless tobacco has generally not been found to cause lung cancer in humans. But then we also have a higher risk of oral cancer for smokers. This causes one to consider whether the biomarker being measured in the urine in Hecht and colleague's study is very closely linked to the mechanism wherbye tobacco causes cancer in humans. Perhaps there are other toxins (e.g. benzo(a)pyrene) that are important in triggering cancer in smokers, but are found in lower levels in smokeless users? Whatever, the explanation, it seems that urine NNAL levels do not relate very closely to cancer risks in tobacco users.
But when discussing health risks from tobacco we need to be clear that these involve much more than lung or oral cancer, but also other cancers, and also other serious respiratory and cardiovascular diseases. For most of these, the health risks from smoking greatly outweigh the risks from smokeless tobacco. Chronic respiratory diseases are extremely common among smokers but there is neither good evidence nor a plausible rationale linking use of smokeless tobacco to these respiratory diseases.
So although it is clear that smokeless tobacco contains carcinogens and is not harmless, it is significantly less harmful than smoking cigarettes. I agree with Hecht and colleagues that we should not encourage anyone to use smokeless tobacco, when we have safe and effective medicines to help them quit smoking. However, I think we need to be able to give an honest answer to the question, “Are there forms of smokeless tobacco that are much less likely to kill me than smoking cigarettes?”. The honest answer is,”yes”. Similarly, of we are asked whether some types of smokeless tobacco may be less harmful than others, it seems prudent to suggest that some types of tobacco that have higher concentrations of toxins than others, and that the type of smokeless tobacco used in Sweden (snus) appears to be lower in toxins and health risks than much of the tobacco used in the rest of the world. It is far from risk-free (as are many things that people choose to do on a regular basis, like driving a car, eating donuts, drinking beer, mountain-climbing, or having sex), but experts agree that it is about 90% less harmful than smoking:
Similarly, given the relationship between toxin delivery and health effects from smokeless tobacco, it would seem to be sensible to regulate tobacco products in a manner that requires manufacturers to minimize the concentrations of toxins delivered as much as is technically possible.
In the mean time (and also after regulation is in place), manufacturers who produce and market products containing and delivering unnecessarily high quantities of toxins should be legally liable for the effects of their defective products.