Quantitative measurements of the generation of hydroxyl radicals by soot particles in a surrogate lung fluid

Epidemiological and toxicological studies have shown a relation between the inhalation of atmospheric particles and adverse cardiopulmonary health effects. The generation of reactive oxygen species (ROS) by particles is one current hypothesis for their toxic effects. Thus a quantitative measurement of ROS is important since that will be an index to assess the oxidative stress that particles may cause in the lung. We have developed a technique to quantitatively and specifically measure OH (the strongest biological ROS) in an aqueous, buffered extract solution as a surrogate lung fluid (SLF). Using this technique we quantitatively measured OH formation in SLF containing hydrogen peroxide (HOOH) for samples of flame soot particles, carbon black, and ambient fine particles (PM2.5). We have found that OH is formed by flame soot, independent of transition metals, with a dose-dependent linear response that depends upon HOOH concentration. Experiments with carbon black revealed that its mass-normalized OH generation was ∼10 times lower than that of flame soot, suggesting that carbon black is not a good surrogate for soot particles in health effect studies, at least in terms of oxidative stress. Mass-normalized OH generation by ambient PM2.5 was 6–30 times larger than that of flame soot. While much of the PM2.5 reactivity was suppressed by pretreating samples with a transition metal chelator, there was a significant fraction of reactivity which was not affected. Our results suggest that the in vivo generation of free radicals, specifically OH, by inhalation of PM2.5 is partially due to carbonaceous soot as well as transition metals.