Nowadays when people smoke cannabis they don't actually combust the flowers instead what they do is heat them gently until the point where the water which is contained in the plant material turns into vapor so that the medicine can be inhaled. This way you don't get any of the trace toxins that are created when you burn carbon and you don't waste medicine by incinerating it.
Just ask anyone who lives in a medical cannabis state and they will attest that this is how it's done, the same applies for concentrates.
