Bike Commuting

In my own personal experience of doing it in London for years (nowadays I'm in a different country and just walk to work) as well as a conference I attended way back by a researcher studying exposure to polution in London, if you're doing it in a big city like that, try and find a path that minimizes your exposure to polution, since whilst you actually get a proper daily fill of exercise cycling to work, you're subject to the same risks as people who jog near roads with lots of traffic, which include such unexpected things as a higher risk of heart attack (due to soot microparticles from ICE exhaust transversing the lungs into the blood and ending up accumulating around the heart) as a well as (more expected) problems in the respiratory system because the sulfur oxides emitted by cars (especially diesel) mix with the water in your airways and lungs and turn into acid.

Mind you, it doesn't need to be that much of a detour: from models I've seen for London polution, merely being one street over from a main road massivelly decreases the polution levels one is exposed to.