Its worse to have your window open for 10miles of rush hour traffic than it is to have a couple of siggies in the garden (i dont even smoke) but its worth thinking about how a little indulgence doesn't really effect anything in the grand scheme of things. Its only in excess that you will have issues 
Well I don't smoke, but I need a good old drink every now and then.
Especially after being on the phone to sky customer services ARGH!