Someone once posted here about half year ago about a driver who seemed to be deliberately releasing fumes while passing him. Today I experienced the same. Sudden, very thick, blueish fume directed right at me when the vehicle (seems to be an off-duty ambulance) made a close pass by me. So it's another "weapon" for drivers to deal with cyclists whom they don't like to see around them.
I found this old thread and it seems to be a valid concern for those of us who often ride on city streets.