There have been various reports coming out about high levels of radiation in air filters (car and room filters). Here's a letter about someone offering free testing of your filter:
"At KI4U we will for no charge test any submitted used air filters from SurvivalBlogger's vehicles or homes. Here at the lab we are utilizing state of the art isotope identification spectrum analysis with dose rate determination and will e-mail lab analysis results back to any that send in their used air filters."
Full details are here: http://www.ki4u.com/freeairfiltertest.htm
I have no affiliation with these folks, so I can't say one way or another if they are real.