You dont absorb anywhere near all the thc in the smoke the first try, However you do add a lot of bad stuff to the smoke each time you exhale. If you wanted to get stoned off fresh smoke in a bong, one to two hits. So you are taking one to 2 hits of fresh air mixed with what you burned.
To get high off the first time you exhale then re-inhale you would probably need 4 - 8 hits, as thc oxidizes fast when we atomize it. Thats 4 - 8 hits of all the crap that was in the smoke the first time PLUS everything your body chose to get rid of through your lungs when you exhaled.
It would be better for your health, and you would get much less of a headache, to have 3 bongs in a row of fresh smoke than the ~13 hits of secondhand. Remember, each time you exhale you are INCREASING the concentration of bad stuff, and DECREASING the concentration of the good stuff.
That being said we like to wear our motorcycle helmets, and do the same thing, when you start to get woozy open the visor...Just wanted ya to have the facts, not telling you to stop! Smoke it how you wanna smoke it and who gives a fuck if these clowns don't like how YOU choose to get high, you aren't making them do it.
Prot