There is a very good study out there.
it states that:
joints give you the most THC to carcinogen ratio(aside from the vapor but I'll get to that in a sec)
Bowls are next,
and bongs are the worst offender.
It basically showed that the more contact the smoke made, the more THC was filtered out and carcinogens only decreased a small margin.
Vapors are in a class there own. giving off next to zero carcinogens.
carcinogens give you a head rush, hence the bong high.
I used to love the bong but now I see the truth. I only smoke jibbers now.