Smoking anything, whether its tobacco, weed, or carpet-fresh, is going to be bad for you (carpet-fresh in particular...). Anything that you burn with fire is going to create smoke, which contains tar, since tar is a natural by-product of burning things. However, tobacco is bad for you for many other reasons than tar. So while weed will put tar in your lungs, its not TOO bad. Besides, its like you said, THC dilates the broncial tubes, so you should be good. Particularly if you only were to do it once a week. You should be fine, as long as you stay away from carpet-fresh. It has claimed many children with its finely-scented pine smelling goodness.