In reply to Tommy81:
I'm not sure if it's been decided yet if it's worse to cycle than not if it's through busy and stop start traffic. The deep breathing could be bad in making cyclists inhale the particulates very deeply, and it's thought they mak enter the blood stream and get lodged in the brain IIRC, something like that.
The best thing might be to find quieter side roads and not be in too much of a hurry?
Pedestrians walking along side cars inhale less fumes than the drivers, but exercise which makes you breath deeply might be doing more harm than is being avoided, it's not been figured out yet I think.
Tim