That's not actually how it works. If I smell distant car fumes it doesn't mean I eventually going to be harmed from any kind of carsinogens that are in the air. Concentration matters in everything we inhale- the air is full of toxins in general but they're in such small doses as not to matter.
That's how most things we inhale work- pollen counts come from this, drugs, anything you care to name.
And this is what were are talking about. If the ban goes through there will be no 'going outside for a cigarette' because it will be banned there as well. If they ban it in one open space they will eventually ban it in another, and then another, with some sort of health justification. It will be 'go home and smoke your poison', I don't like smoking but I do want a free society thanks.