A really neat idea!

A co-worker of mine mentioned something she'd heard and tried out...

When you have your windows open and have a fan in them (pulling air in), put a heating/cooling filter between the fan and the window (after marking on the filter which side faces inside). It'll catch all of the pollen and dirt that would otherwise be pulling into your house. It's not that expensive (and you may also be able to find reusable filters) and should be a great help for allergy sufferers and people who live on dirt roads alike.

I'll have to give that a shot next spring.