I've thought about this many times, I still can't decide on who breathes in more ****.

Bikes
+There's more room for exhaust to disperse itself away from you if you're on the bike.
-Obviously I take bigger breaths riding my bike than if I were just sitting in a car.

Car
+Only so much exhaust can get in since you're in a mostly solid capsule.
-If your windows are up and you smell exhaust coming in (although exhaust comes in whether you smell it or not), you're fishbowling exhaust...open the window?

...can't decide. Quantifying either scenario versus the other has so many possibilities. This is why I hate health and science and also why I'm in both fields, I guess.