2-in-1 hits by exhaling into a bag?

I have been vaping using my SSV whip vaporizer for a few years now, and I love everything about it. I honestly haven't smoked by myself once since then, only using combustion when I am at a friend's house (and even then, I got most of my friends on the vaporizing bandwagon). I use my SSV almost every day and it's amazing, but I have been thinking recently about the efficiency of my hits. Obviously, any visible vapor when you exhale is wasted THC that is not making it into your bloodstream, and the SSV gets me some extremely big hits. I tried holding my hits in for longer, but it doesn't seem to make a difference beyond 15 or so seconds. I have tried holding my hits in for up to a minute, and the only difference I feel is lightheadedness due to the lack of oxygen.

A couple times now I have experimented by exhaling my first few, more potent hits into a plastic bag, then taking my next hit from that bag. I got this idea from Harold and Kumar in that scene where Kumar smokes for the first time by having that girl exhale her smoke into his lungs. I still get visible vapors after the second hit, and it's just as potent of a feeling. I imagine you could do that a few more times and still produce visible vapors on the exhale, but I haven't tried that. My question is, has anyone else done this? I realize it's the same concept as a bag vaporizer, but I'm more wondering about the high amount of carbon dioxide from my lungs that is mixed in with the vapors. I don't think it's a big deal or will have any negative side effects since carbon dioxide is only bad when it replaces the oxygen in a room, but I thought I would ask the people who are most likely to know about this. Also, if anyone has any ideas on making this method even more efficient, I would love to hear them.

BTW, I do already revape my bud multiple times after its initial go, in case that was going to be someone's efficiency suggestion.

It is not "obviously" thc you are exhaling. Chances are it is NOT much THC, but that is just speculation on my part.

Realistically you need to determine how fast THC is absorbed into the lungs compared to other cannabinoids to make such a statement.

Anyway, I'd personally rather just vape a bit more bud than inhale my previous exhales again. I don't have much info to offer on it, but I believe many/most of the actives in vapor are absorbed rather quickly, but I don't have any sources, its just what I've been told.

I would rather just vape a bit more in a more efficient vaporizer than inhale "second hand air".

When short of bud, I've used this method with some ordinary plastic shopping bag as a temporary storage for the first few hits and I can confirm that it works for me - seems to improve the amount of THC I get from a given quantity of plant material.
When I do this I still see some vapour coming back out of my lungs even after the second cycle - once tried a third cycle but couldn't see much coming out.

I'd be willing to guess that if you can see vapour in recycled exhalations from early hits, it probably includes at least some THC and friends - I don't think there's any good reason to think that adsorption of vapour to the alveolar membrane and subsequent blood absorption is 100% efficient even after a long hold or that THC is selectively adsorbed/absorbed out of the vapour leaving exhaled clouds THC free - but that doesn't mean it's impossible - just unlikely. I'd recommend you try it for yourself and see if you get more from your usual intake.

Recycling your own exhalation a couple of times this way is safe AFAIK. Sure, it's second hand but it's your second hand. Each cycle increases the fraction of CO2 and decreases the O2 content - just be sure to take a few good deep fills of fresh air between bags - the medullary respiratory center will force you to breathe heavily if your blood CO2 level rises (it probably detects the fall in blood Ph as the dissolved CO2 makes carbonic acid) to protect your brain from damage.

Definitely not 100% absorbed, or close to it, but I don't want to inhale stuff my body naturally exhales to get it out of me.

Just my personal preference of course.

Not a good idea for a bunch of reasons. One of them, fungal or bacterial bag contamination.

Close, usually within 5 or so seconds (varies by person/lung capacity), your lungs absorb all the cannabinoids it would be able to. Shotgunning works for a reason (and is a nice excuse to kiss a pretty lady ) It's not that it absorbs all the cannabinoids, but your lungs hit the limit for that inhale.

Now to add my bit, I read somewhere that vapor takes a bit longer to absorb than smoke. I hold for ~7 seconds, works out well for me

And definitely agreed. I make my ABV into edibles so I don't really waste any of it, but I'd rather just add a little kief or another pinch.

I tried this once with my cano bag. I only did one hit and there was alot more moisture in the second hit. This could lead to many health issues if you get bad stuff growing in the bag. I figure it should be ok if you use a disposable bag and get rid of it after each session. Youre basically shotgunning yourself.