In laboratory tests, bong smoke was found to have almost exactly the same ratio of carcinogens to THC as spliff smoke or pipe smoke. There is no 'safe' way to smoke herb, if it's burning, it's not good for you, bongs only cool the smoke. If you want a hit without carcinogens, the only way to go is a vaporizer, but because that only gives you a THC hit (without the cannabinoids) a lot of folks just don't feel that it gets them stoned. The safest way to get a proper stone is to cook & eat the herb.