Originally Posted by
DX-MAN
3. "breathing exhaust" is virtually the same on the road, no matter where you are; there are no filters in most cars, it seeps in (or blows in when the fan is running) the interior all the time. There is the unresolved debate about a faster-breathing cyclist either: a.)breathing more in by faster respiration; or b.)flushing the lungs out through the faster action of air exchange -- pick a side. (I personally prefer NOT to be enclosed with the fumes.) But sitting behind a car just focuses the STENCH, that's really all.
Particulate exhaust operates under the laws of diffusion from a point source (think heavy smoke vs dry ice). Its diffusion is also regulated by ejection velocity and gravity. Moral of the story: do not make a habit of spending time
behind a vehicle (esp diesel) unless you are interested in developing asthma, COPD, and/or cancer.