Causes, emission characteristics, and health risks of persistent free radicals and reactive oxygen species in PM2.5 produced by smoldering incense

This study investigated persistent free radicals (PFRs) and reactive oxygen species (ROS) using electron paramagnetic resonance in PM2.5 produced by smoldering incense. The Raman spectroscopy was used to define the structure of carbon on PM2.5. The predominant type of PFRs detected was carbon-oxygen radicals, and their 1/e lifetimes range from 1,164 to 4,932 days. One identified source of PFRs was graphite structural defects, which contribute to catalytic reactivity and the subsequent production of hydroxyl radicals. Incense-generated PM2.5 produces ROS, including hydroxyl and alkyl radicals. Among the eight types of incense PM2.5 examined, PFR emission rates were found to range from 3.2E16 ± 9.0E15 to 8.8E16 ± 5.0E16 spins/g, and ROS emission rates ranged from 1.6E18 to 7.6E18 spins/g. The high chemical reactivity of both PFRs and ROS may pose significant health risks. The health risk of PFRs content in PM2.5 is often assessed using a cigarette tar-based method. It has been estimated that the amount of PM-bound PFRs generated from burning a single incense stick is approximately equivalent to that produced by the cigarette tar amount from one to two cigarettes. Non-thermal plasma intervention reduced the amount of PM2.5 but increased the concentration of PFRs and ROS. The results of real samples indicate that the concentration of PFRs in PM2.5 from indoor incense burning in suburban areas is higher than that in busy urban traffic areas. The intervention will increase the concentration of PFRs. Given the lifestyle of spending extended periods indoors, studying the incense PM2.5-bound radicals is important.