The hold in tech.

i think you get plenty of oxygen along with the smoke. brain damage doesn't happen until at least a few minutes of no oxygen. i learned this while getting CPR certified.

your brain will most likely NOT be oxygen starved from holding in a hit for 15-20 seconds, or even a full minute. you might just be allowing tar and smoke particles more time to stick to you lungs though, so you probably don't need to hold a hit in for more than 6 or 7 seconds.

i AM a scientist

 

I remember seeing a short article in High Times a few years ago about a study on the medical use of herb, that said after 4 seconds you had asorbed almost all the THC you could and anything longer just damaged your lungs.