Environmentally persistent free radicals in indoor particulate matter, dust, and on surfaces

Environmentally persistent free radicals (EPFR) are an emerging class of constituents in particulate matter (PM). They are relatively stable with lifetimes of days to years in the condensed phase, influence PM toxicity, and are involved in reactive oxygen species (ROS) generation and multiphase chemistry. The abundance of EPFR has been investigated in laboratory studies and outdoor locations globally, but their prevalence indoors was previously unstudied. In a case study home, EPFR were present at similar concentrations in indoor and outdoor PM, at 2.3 ± 2.8 × 10 12 and 3.6 ± 3.1 × 10 12 spins m −3 , respectively. Indoor EPFR were observed at varying concentrations in airborne PM, dust, and surface samples, with the most concentrated surface loadings observed on windows. Overall, dust and indoor surfaces were estimated to be larger contributors to total in-home EPFR concentrations than PM. For comparison, combined estimates of total EPFR concentrations in indoor PM, dust, and on surfaces are similar to the range of typical indoor ozone concentrations. Due to the reactive nature of EPFR, they potentially undergo rapid cycling and may represent reservoirs of reactivity in these multiphase systems that influence the surface and multiphase chemistry of indoor environments. Here, the exposure of EPFR collected from some surfaces to nitrogen dioxide or ozone was shown to alter EPFR abundances and composition. In all, these results support further investigations of EPFR abundance, composition, and reactivity on indoor surfaces, including in accumulated organic coatings and dust. Long-lived, yet reactive, environmentally persistent free radicals with known human health effects have the potential to influence indoor air quality and chemistry.