There was a study done. While bongs do reduce carcinogens, They also reduce THC. Study showed that besides the vapor method, Straight shots like a joint or blunt give the best THC/Carcinogen ratio. Followed by bowls then bongs(which I thought, was a bubbler).
I believe you can find this one on erowid. Link here(scroll to the bottom about waterpipes)
MAPS/CaNORML vaporizer and waterpipe studies
:jointsmile: