Hey Jim,
Just a couple of articles [over at the Alpha Rubicon site] that you might find interesting:
Safe Room Fan [1]
and,
HEPA Filter [2]
Regards, – S.C.
JWR Replies: For any SurvivalBlog that might have overlooked it at my Links page [3], I highly recommend the wide variety of practical free references available at the Alpha Rubicon web site [4].