Matt Myers attended our workshop in September, and he used it straight through December. On December 7th, he sent me a picture of the blackest HEPA (and pre-filter) I’ve ever seen. Yikes!
Time for a new filter! I’d be happy that black gunk is in the filter and not in my lungs.
Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.