The problem in the United States is that the health effects of Pipe smoking are difficult to measure on a longitudinal basis - mostly due to the
relative scarcity of pipe smokers compared to the rest of the population. Additionally, the studies that do exist, are often outdated - reflecting a time when Pipe smoking was often undertaken with other forms of tobacco use, and when pipe smokers often inhaled (something that is more of an outlier in the 'modern' pipe smoker).
BUT, there is still some interesting studies out there. One of the more interesting ones:
http://jnci.oxfordjournals.org/content/96/11/853.full
"In this cohort, pipe smoking conferred a risk similar to or greater than that for cigar smoking for cancers of the lung, larynx, esophagus, and oropharynx and for coronary heart disease."
But this particular study, I believe, lumped inhaling and non-inhaling pipe users together - odd, considering this difference has the greatest effect on what health chances you take.
In a nutshell, the consensus seems to be that pipe smoking is on par (as far as health risks go) with cigar smoking, but less risk than cigarette smoking. As a pipe smoker (non-inhaler), your risks of certain types of mouth cancers will be higher - increasing with the more bowls you smoke per day. The big increase in health risks comes from inhalation from a pipe - several studies have indicated this can actually be worse than inhaling cigars.
However, Studies that solely look as the instances of cancer in pipe smokers vs non-smokers - rarely look at the effect that pipe smoking can have on the other aspects of an individual's life, such as stress management and stress related disease. In fact several studies of both moderate pipe users (1-3 bowls week I think was the range) and alcohol drinkers (same range?) show that this moderate usage can actually increase lifespans over that of total-abstainers. Presumably owning to the calming effects they impart on the user, reducing overall stress. I believe the relevant pipe study was from the Surgeon General in...1964? (I'm too tired to look it up at the moment). However, what I've jotted down here is hardly comprehensive of the available information - so its best to google around and gather your own conclusions.
The Bottom line is that while the positive attributes of stress reduction likely balance a bit against the increased chances of contracting a mouth cancer...your overall risk of those cancers will still be higher than the general non-smoker.
But hey, you can still get lung cancer if you've never smoked a day in your life. Life is about choices, you make them, and they have consequences. So, is the enjoyment you get from puffing, worth the risk? For many here, it is. :
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