The fact is, smoking weed causes some damage to the lung, but not the kind of deep lung damage that causes emphysema.

Second, weed doesn't have as many cancer causing chemicals, even before all the additives in cigarettes.

Finally, not a single death has been related to weed alone (i.e. no cigs), cancer or otherwise.

With that in mind, a bong hit only makes the hit go down easier... the harm-reducing is negligible in respect to %THC inhaled.