Small amounts indeed, but what I'm trying to say is 100 metric tonnes of marijuana being burnt wouldn't have a detectable impact on the environment. Naturally grown tobacco wouldn't either.
Personally I think that if people wants to smoke weed or regular cigs.
They should go to some special places to do that and not just anywhere.
Like even if weed isn´t that harmful, I don´t want to get high from others that smoke...