Aside from nicotine, which is naturally present in tobacco, and THC which naturally present in weed 99% of the chemicals found in tobacco smoke are also present in marijuana smoke.

Or any other dried plant material for that matter, from the dead leaves in your yard to wood smoke from your campfire. Smoke from any burning plant is not good for you to breathe in.

It's a myth thought up by potheads to make them think they aren't hurting themselves by smoking mj. The more mj you smoke, the greater your risk for cancer.