How could it possibly not? Smoking anything deposits an array of ash and tar products into your body, and I have yet to observe a studied combustion byproduct that doesn't cause cancer. Also, smoking anything increases risk of infection, which in turn will cause immune stress and necessarily an increase in cancer risk.
Personally I think the damage done to individual people when aggregated in sum is below the noise when compared with pollution from industrial sources, environmental pollution from consumer products, known-cancerous foods such as bacon, hamburgers, and beer, etc. So it's no reason to outlaw marijuana. But people who partake should not be fooling themselves into thinking it's healthy.
Basically, smoke is composed of two components, gases and particulate solids. The gases leave your lungs when you exhale, while some of the solids remain, and cause cancer and other bad things. The concentration of particulate solids is many orders of magnitude greater in tobacco smoke than marijuana smoke, which is why the smell of tobacco smoke will persist in an enclosed space for years, while the smell of marijuana smoke completely dissipates after only a few hours.
Out of my field, so can't offer an explanation, but it appears smoking MJ is not associated with lung cancer or reduced forced vital capacity (a measure of respiratory capability).
Here's a recent (2016) review: [1] Nature, apparently not paywalled. Lung cancer is the topic in the paragraph immediately before the "Discussion" section.
There is ample evidence that smoking weed is bad for the health of the respiratory system. However, it is far less statistically likely to cause lung cancer than tabacco smoking. There are a few citations in this thread that should be able back up the claim.
Smoking isn't healthy and no-one should be making such claims. Other respiratory health issues are just as deadly as cancer.
One variable not mentioned is the number chemicals used in growing tobacco - herbicides, pesticides, etc. - and the chemicals added the the filters of cigarettes. The latter issue is easy to avoid and most home growers try to minimize the use of the former if they use them at all.
you're oversimplifying. the quantity and quality of the smoke is hugely important. cannabis smoke and tobacco smoke are qualitatively different, and the typical amount consumed by cannabis vs. tobacco smokers is a very different quantity (in that tobacco smokers typically consume tens or even hundreds of times more mass of tobacco per day than cannabis smokers do).
certainly any kind of inhaled smoke can cause issues for your lungs, but conflating the cancer risk of daily habitual tobacco smoking (very high) with the cancer risk of daily habitual cannabis smoking (probably present, but not yet known how much more risk it causes) is a distortion.
Personally I think the damage done to individual people when aggregated in sum is below the noise when compared with pollution from industrial sources, environmental pollution from consumer products, known-cancerous foods such as bacon, hamburgers, and beer, etc. So it's no reason to outlaw marijuana. But people who partake should not be fooling themselves into thinking it's healthy.