From Eileen O'Connor:
Read message below from Magda.

I'm attaching a paper we presented at the WHO conference on EHS and a study we did at a school in Toronto. We have since repeated the study at two schools in Minnesota with similar results so I have confidence that we are measuring a beneficial effect of the filters, especially since we used dummy filters in the two Minnesota schools.

http://www.buergerwelle.de/pdf/04_havas_ww.pdf
http://www.buergerwelle.de/pdf/04_havas_london_rev.pdf
http://www.buergerwelle.de/pdf/who_havas_stetzer.pdf