Regarding 'joints'.

The scientific studies I've read suggest that around 50 percent of the THC is lost during smoking due to the combustion process which destroys and/or vaporises the THC.

In addition, there is also 'exhalation' to consider.
i.e. when you toke not all the THC in the smoke is absorbed by the bronchioles in the lungs, and therefore is just exhaled and lost, however, accurate percentage quantities are (for obvious reasons) difficult to accurately quantify.

I've been researching ingested quantities recently to compare dosage and effects so I'll post up some info on that as well when I've finished compiling it.

Great question! I'll be interested to see what other information gets posted.
Cheers
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