Diesel Exhaust Prime Source for Persistent Free Radicals

The expression PFR, Peristant Free Radicals or EPFRs, Environmentally Persistent Free Radicals explained:

Environmentally Persistent Free Radicals (EPFRs) are formed in combustion and thermal processes including hazardous waste incineration and diesel combustion. These processes create tiny particles called Particulate Matter (PM). PM is classified as coarse (10-2.5 μm), fine (2.5-0.1 μm), and ultrafine (0.1 μm and smaller). (To put this size into perspective, a human hair is approximately 70 μm in diameter, so the largest fine particle is still 30 times smaller than that.) 90% of ultrafine particles are formed from combustion sources, and these particles have the potential to penetrate in to the deepest portion of the lungs (the alveolar region). These particles provide the perfect breeding ground for EPFR formation. The free radical first forms a “loose” bond with the surface of the particle. Next, the EPFR is formed when the loosely bonded free radical forms a chemical bond with metals present in the particle in order to stabilize its own unpaired electron. This process reduces the metal allowing the attached EPFR to have a half life of up to several days rather than the fractions of a second of a normal free radical. EPFRs are also present in some contaminated soils as well as in ambient air. A recent Baton Rouge air sample revealed that airborne fine PM contained EPFRs with concentrations of 1017- 1018 radicals/g of air.

http://www.srp.lsu.edu/files/item24088.pdf