Have you brought up the polonium argument yet? I'm lazy, so I'm just copying and pasting this from another post of mine.

Quote Originally Posted by graph
Cancer is a very dangerous thing, and there have been arguments back and forth about what is worse. Keep in mind, while tar has shown to be one of many contributing factors in lung cancer, what has shown to be more prevalent in causing any type of cancer is radioactivity. Tobacco industries use polonium-210, which is a radioactive substance, to grow tobacco plants. While the FDA says that polonium-210 was shown to have no negative side affects while ingesting plants grown using the radioactive substance, it is still inconclusive if smoking such plant can cause lung cancer. Also, keep in mind cannabis users don't smoke leaves, but the bud, which contains 33% less tar, and cannabis does not narrow the air passages in the lungs like tobacco.
Now, smoking anything is harmful to you, but there are other ways to use cannabis. For example, you could eat or vaporize.