I used activated charcoal a few times back when I was smoking. It was interesting, it really was well suited to filtering smoke and it didn't really have a huge impact on the potency either.
It kept the water absolutely crystal clear for many tokes. I would recommend using water after carbon filtering for absolute peace of mind..
I had it placed in an adapter (Carball mentioned above at one point) and with a cone piece/bowl above, attached to a beaker downstem.
That's actually the first adapter I got, branded RooR. I remember now the instructions that came with it, which make sense as to the concerns above.. basically you needed to wash the activated carbon before using it. Larger granule styles might be better suited. What I was using was granulated, roughly 1-2mm^3 chunks. With proper preparation, the filter does not allow for any visible dust to be drawn out of it. The pieces of carbon are solid crystalline, and after washing the dust away, it will only form more if it is ground or manipulated.
This was the more annoying aspect of using the granules, because it was difficult to store long term without it breaking up and forming plenty of dust. Still, I would get a packet and process a bunch and place it in a jar for refills.
Now as to the heat, well this is what google has to say about the production necessary for making it..
"Activation/Oxidation: Raw material or carbonized material is exposed to oxidizing atmospheres (oxygen or steam) at
temperatures above 250 °C, usually in the
temperature range of 600–1200 °C"
So, the stuff is really not sensitive to heat at the levels reached with smoking, let alone vaporising.
I would pull hot embers directly on top of it and generally it wasn't an issue (the only issue being that eventually it would clog and it would restrict airflow after a few uses). The adapter itself would get hot to touch. And the oils from the plant and the tar from the smoke would stick all the carbon together like a cake after some use, causing it to require a good bit of scraping to remove (it retains its granularity throughout, it really isn't that risky a material in the slightest, one of the friendliest and most abundant).
I see nothing wrong with it, it's awesome stuff. 1g has the surface area of a football field.
I did look into particulate inhalation before deciding on using it all those years ago, and all I can say is that the amount of bong hits I would need to get black lung, I would die of old age trying. Miners get black lung, it really takes exposure on the levels of swimming in a visible cloud of carbon for it to be troublesome.
I would think if you placed it between the heater and the herb it would impact the convection efficiency and might not allow the vape to work properly.
I like the idea of filtering the air path before the heater, however if the first thing you do after a toke is breath in the air around you unfiltered, it's probably not worth the hassle.
Placing it in the downstem of a bong makes the most sense given that it's already a solid step from direct inhalation.
I think it makes a lot of sense in terms of vaping, I vape now and have been solely for over 5 years, partly as risk reduction, partly as a lifestyle choice, and mostly because my lungs were showing signs that made me feel like quitting or eradicating the tar input hence the carbon filtration and today's reality.
I don't think it makes sense to vape using one of the many available noxious and poorly designed products out there. But it definitely makes sense to see no reason that simply vaping with a unit that isn't awful is the limit of how you can better cope with getting high. Carbon filtering is going to be a step too far for most people, it's going to absorb a lot of flavour, be an unnecessary hassle and change the experience by a fair amount. But if you like it, more power to you, you will certainly be inhaling less bullshit, there's always a trace amount and you would simply get a smaller slice of that pie with filtering. It would be a measurable difference.
