In reply to GudrunEnsslin:
When you run, as with any physical activity, you will sweat.
Your head sweats just like the rest of your body.
If your head is covered up it will sweat more.
If you do not wash after running (or other activity), then sweat will build up and start to pong.
Hence you will need to wash your hair as well as the rest of you after running.
Therefore covering up your hair is a waste of time.
This is one reason why I kept my hair relatively short when I was running every day.