I agree with garden knowm 100%.

The smoke has the obvious in it, TAR, and this will cling to the leaves of your beloved plant, and if you do it enough it'll clog the plant and then the plant wont be able to rid its self of toxins and it'll die.

At first i wasnt sure how it worked, but i did some research, and a long time ago there was a thread on it, so now i know.