I love to run outside and ran all summer with no problems. In the winter I'll work out indoors on the treadmill not because of the cold but because of the increased pollution.
A couple of weeks ago I decided to run outside even though it was "misty" out as I couldn't get motivated to go on the treadmill.
Big mistake! I ended up with my first ever sinus infection and the doc said that it was mostly likely caused or spurred on by the pollution.
Good news is that I'm home in Canada for the next 10 days and although it's a bit cool I'm loving running outside again.