As the poster above says- you are best off thinking "suburban" or if you can afford it, and you want to be quite central-ish, heading upwards and eastwards e.g. Stubbs Road, Bowen Road, the Peak etc.
I would at all costs avoid Central Mid Levels, West Mid Levels and Soho.
I can tell you (as someone who seems to be particularly sensitive to the pollution here) that I can feel ill walking around somedays- headache, sore throat, generally malaise.
I can also tell you, as a keen runner living in central Mid Levels, that I can't just head out of my door and go for a run. I physically feel sick- I can't breathe, and when I stop I retch. I have to walk/bus/taxi it somewhere (e.g. Bowen Road as it's easier on a week night). I always try to run back but I have to stop at the start of Caine Road and retch (sorry for the image), so I generally just walk it.
I have never experienced that anywhere - even on the most polluted roads in London- and I think it's because the high rises trap all the car fumes/make it harder for them to dissipate etc.
That said, my other half does not experience any ill effects from the pollution, so, maybe it's just me!