Characteristics and potential inhalation exposure risks of PM_2.5–bound environmental persistent free radicals in Nanjing, a mega–city in China

PM_2.5–bound toxic elements and organic pollutants have been extensively investigated, while limited information is available for environmental persistent free radicals (EPFRs) associated with PM_2.5, which may lead to oxidative stress in the human lung when exposed to PM_2.5. In this study, the levels and types of PM_2.5–bound EPFRs present in Nanjing, a mega–city in China, were analyzed. PM_2.5–bound EPFRs were found to mainly be a mixture of carbon– and oxygen–centered radicals. The concentration of PM_2.5–bound EPFRs ranged from 2.78 × 10¹² to 1.72 × 10¹³ spins m⁻³, with an average value of 7.61 × 10¹² spins m⁻³. The half–life of the PM_2.5–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, O₃, CO and PM_2.5)/PM_2.5–bound transition metals (Cu, Zn, Cr, Mn, V, Cd, and Ni) and significant correlations between EPFRs and SO₂/PM_2.5–bound Fe. PM_2.5–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, PM_2.5–bound EPFRs can lead to potential oxidative stress in humans. Overall, PM_2.5–bound EPFRs show an obvious temporal variation and can pose potential health risks to humans via the induction of ROS in the lung solution.