The role of combustion-generated radicals in the toxicity of PM2.5

Electron paramagnetic resonance analyses of samples of ambient, fine particulate matter (PM2.5), as well as Av-ashes and soots from a wood-burning fireplace, a boiler cofired with chlorinated organics, a pilot scale hazardous waste incinerator, and laboratory combustion of various halocarbons reveal that they contain stable free radicals. All of the radical systems were indefinitely stable ill air. Extracts of PM2.5, and the incinerator ash caused damage to DNA, both in vitro and in cell systems. Cellular damage to DNA was prevented by addition of the enzymes superoxide dismutase and/or catalase that destroy the superoxide radical and hydrogen peroxide, respectively. This type of DNA damage and protection is observed for the semiquinone radicals in cigarette tar. Therefore, we have tentatively assigned the structure of the observed radicals as semiquinones. In solution, semiquinone radicals undergo redox cycling, reducing oxygen and forming superoxide radical and hydrogen peroxide. and ultimately the hydroxyl radical that can initiate DNA damage. Since ambient PM2.5 is primarily composed of combustion-generated particulate matter, our results imply that combustion sources produce semiquinone-type radicals that persist on fine particles in the atmosphere. The fine particles provide a carrier for deposit of the radicals deep in the human respiratory tract. These radicals then initiate immune system responses that can trigger the production of further radicals as well as other species that can damage DNA or induce damage to the respiratory tract.