The study was a double-blind, randomized crossover trial in which each participant served as their own control, thereby greatly reducing the role of time-invariable confounders. Up to two homes were enrolled and randomized at a time, with one allocated to receive HEPA filtration and the other sham filtration first. At three weeks, the homes were switched from HEPA filtration to sham or vice versa. There was no washout period between sham and HEPA filtration. While field staff were aware of the type of filter in use, the participants and the lab that analyzed blood samples were not. The approach and methods were largely similar to another HEPA intervention we conducted in public housing in the City of Somerville, which was still in progress at the start of this study.