Yes. Pot is probably as bad for your lungs as tobacco is. It seems that marijuana smoke is less noxious than tobacco smoke. For example, if you smoke marijuana in your living room, the whole room will smell of it. But if you smoke a few cigarettes, the marijuana smell will disappear, and you will only be able to smell the cigarette smoke. Marijuana smoke doesn't linger on your clothes and furniture anywhere near as…
There is some damage to the lungs caused by marijuana smoke inhalation. (However, note that marijuana does not necessarily have to be smoked, it can also be brewed as tea.) Interestingly, marijuana smoke has not been found to be carcinogenic, unlike cigarette smoke. However, marijuana smoke is still an irritant to the lungs and can contribute to the eventual development of emphysema.
No, marijuana second hand smoke is less harmful for you than tobacco smoke. Marijuana smoke does not contain the deadly carcinogens that cigarette smoke does. I heard on a documentary that there aren't any known cases of marijuana smoke alone causing lung cancer or emphysema. If someone is smoking weed around you then you don't really have anything to worry about.
Marijuana is a unique substance with unique effects, therefore nothing you smoke will ever feel like marijuana if it's not marijuana. If you're looking for mild relaxing effects, you can try smoking mugwort, sage or damiana. But these won't feel like marijuana, they have their own properties. No legal high will ever feel like marijuana.
Yes. You can, in theory, be allergic to anything. Marijuana is not a common allergen, but it is possible. If you have asthma, marijuana smoke might trigger an asthma attack simply because it is smoke, but in my experience it is less likely to trigger attacks than tobacco smoke or wood smoke. Finally, marijuana is sometimes treated with pesticides that might be toxic or cause an allergic reaction.