The short-term effect of residential home energy retrofits on indoor air quality and microbial exposure: a case-control study

Weatherization of residential homes is a popular retrofit procedure to improve the energy efficiency of older homes by reducing building leakage. It is a vital tool in the fight against climate change. Several studies have evaluated the effect of weatherization on indoor pollutants such as formaldehyde and radon, but few studies have evaluated the effect of weatherization on indoor particulates and microbial exposure. In this study, we compared the effect of change in building leakage on indoor pollutants and bacterial communities in weatherized compared to non-weatherized single-family residential homes in New York State. Nine weatherized and eleven non-weatherized single-family homes in Tompkins County, New York were sampled twice: before and after the weatherization procedures for case homes, and at least 3 months apart for control homes that were not weatherized. We found a significant increase in both indoor-outdoor temperature ratio and living-area- and basement-radon levels of weatherized homes compared to control homes. The indoor-outdoor relative humidity ratio significantly decreased in weatherized compared to control homes. The indoor microbiome also became less similar to the outdoor community after weatherization. Compared to the changes in ventilation rate, temperature, relative humidity, and occupancy, the change in season was a more predictive measure of indoor bacterial concentration. Ventilation rate reduction from weatherization procedures led to an increase in indoor radon levels, as well as a warmer and less humid indoor environment. However, it did not affect indoor particulate mass concentration or indoor airborne bacteria load, and did only marginally affect the microbiome composition of residential homes. Finally, we found that changes in airborne bacterial load are more sensitive to shifts in season, whereas radon levels are more sensitive to ventilation rate.