in our hostel in quito, the capital of ecuador:

me:  hey, when you walk around or take the bus in a big city, do your boogers get black?
megan:  yeah, they always do.
me:  it happens to me in london.
megan:  it’s kind of like evidence of all the pollution.
me:  there should be a booger index, don’t you think?
megan:  a what?
me:  like how bad the pollution is, as evidenced by the color of your boogers.
megan: ewww.
me:  yeah.  if your boogers are grey, you’ll be okay.  if your boogers are black, you better turn back!