Apparently cycling is bad for you...

If you believe the Sunday Times. Diesel fumes, and particulates in particular are said to cause damage to the heart.
The research puts this down to cycle lanes sharing with buses, which are the heaviest users of diesel, and the increase in cars that use this dirty fuel. The advice is that new cycle lanes should be off road/traffic free.
Fortunately this is the direction that Birmingham has been moving in. Nearly all new routes are traffic free (although never quite in their entirety). So much use has been made of canal towpaths, river routes, back roads, and paths that run through council estates.
Of course, if there is a bus route, cycles can use as well, but these are largely used by buses, rather than bikes, as there is usually a safer alternative to mixing it with the big boys.
Manchester is much worse off as there are few alternatives to the arterial routes, so cyclists are forced to share with buses.
I would imagine that following the congestion charge in London that cycling is also cleaner and more pleasant.