Passive smoking is one of those things that really get people fired up, on both sides of the fence, so here's my 2p worth:
While it's undeniable that tobacco smoke (subjectively) smells unpleasant (although it can be said to mask some even more unpleasant odours), and indeed irritates a lot of people, I remain deeply sceptical of any purported health effects. Take a look
here, and do a search for "passive smoking". Also, see what people like Dr James LeFanu have to say on the subject.
From my personal point of view, there are several unanswered questions:
1) What's the difference between tobacco smoke and other smoke (eg smoke from wood fires, barbecues, cooking etc, not to mention diesel fumes) What is it in tobacco smoke that makes it particularly dangerous?
2) Humans have evolved over tens or hundreds of thousands of years and, for all but the last few decades, have lived, worked and socialised in smoke-filled caves and rooms. Why should the merest whiff of tobacco smoke all of a sudden become so deadly? It's even been suggested that we, having breathed in smoke throughout our evolution, actually require doses of it for our well-being, just as, as is now being discovered, being 'too clean' is bad for our health.
It's also undeniable that nicotine has some effect on the brain, but so again do a huge number of other things. Any drug which has an effect on the brain only has that effect because it either mimics the effect of natural neurotransmitters, hormones and peptides, or triggers their release. If brain cells have no built-in receptors for a particular substance then that substance cannot have any effect. Cocaine and amphetamines, as well as nicotine to a much smaller extent, work by increasing dopamine levels in the brain. But falling in love has the same (and arguably more powerful) effect, as do, to a lesser extent, pleasurable activities such as laughter and listening to music. If we're to ban smoking while driving because of it's neurological effect, we should then also ban things like listening to music or comedy. And, because of it's very much larger effect, we should also unequivocally ban people from driving for at least two to four years after starting a new relationship. (or, indeed, ending one)
I'm also highly sceptical of the view that smoking while driving is dangerous because of the distracting effect. The act of lighting a fag is normally automatic, requiring no thought, and has little or no distracting effect. Also, while it's possible for a driver to, for example, drop a cigarette, I would suggest that such incidences are sufficiently rare as to be completely insignificant in the scheme of things.
Conversely, I can only guess at the effect of thousands of drivers being denied a fag.