Differences in pollution levels in London?

Basically I'm getting hacked off (or rather hacking coughs) with the combination of motor pollution and pollen when I go through the City from London Bridge, through Bank, Morgate and Old Street roundabout. Can anyone recommend a less polluted route through the City from South to North that ends up in Islington? I come in from SE London so I can use London , Southwark or Blackfriars bridges.
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Could you use below as a starting point?
http://maps.google.co.uk/maps?hl=en&tab=wl
I did think about using a pollution mask, but they aren't cheap and I've read loads of comments here about the masks causing runny noses and restricting breathing, which to me kind of defeats the object of having a mask in the first place.
I used to live in Islington and walk to work in the City and I now live in Lewisham Borough and cycle to Chancery Lane so I know the area and I struggle to think of a traffic free route...
There's always the Greenwich Tunnel - > Regent's Canal route, but that would be the very last option for me as I don't like cycling on canal tow paths.
That would add quite a hefty bit of time to the commute, you have to walk through the tunnel, you may just about be able to get away with biking it out of rush hour and then yuo'd have to go a fair way north to hit the Regent's Canal and progress along that is fairly slow - bikes are supposed to keep below 5mph or something so they don't hit peds (not that many of them actually do)
http://www.walkit.com/
Thanks! I'll give that a go.
and that walkit site looks useful too.