Characteristics and potential inhalation exposure risks of PM2.5-bound environmental persistent free radicals in Nanjing, a mega-city in China

PM2.5-bound toxic elements and organic pollutants have been extensively investigated, while limited information is available for environmental persistent free radicals (EPFRs) associated with PM2.5, which may lead to oxidative stress in the human lung when exposed to PM2.5. In this study, the levels and types of PM2.5-bound EPFRs present in Nanjing, a mega-city in China, were analyzed. PM2.5-bound EPFRs were found to mainly be a mixture of carbon- and oxygen-centered radicals. The concentration of PM2.5-bound EPFRs ranged from 2.78. 10(12) to 1.72 x 10(13) spins m(-3), with an average value of 7.61 x 10(12) spins m(-3). The half-life of the PM2.5-bound EPFRs was calculated to be an average of 83.5 days when stored at room temperature, with only weak correlations observed between EPFRs and conventional air pollutants (NO2, O-3, CO and PM2.5)/PM2.5-bound transition metals (Cu, Zn, Cr, Mn, V, Cd, and Ni) and significant correlations between EPFRs and SO2/PM2.5-bound Fe. PM2.5-bound EPFRs can induce the formation of reactive oxygen species (ROS) in both water and a H2O2 solution, which are used to simulate lung solution of a healthy person and patient, respectively. Therefore, PM2.5-bound EPFRs can lead to potential oxidative stress in humans. Overall, PM2.5-bound EPFRs show an obvious temporal variation and can pose potential health risks to humans via the induction of ROS in the lung solution.