The two PM(2.5) (fine) and PM(2.5-10) (coarse) fractions: evidence of different biological activity.

Recent studies have shown that an increased concentration of environmental particulate matter (PM(10)) is related to many respiratory diseases. One major issue is whether the toxicity of the particles resides in some particular fraction as defined by chemical composition and size. The overall purpose of this study was to compare the in vitro toxicity of coarse (PM(2.5-10)) and fine (PM(2.5)) particulate matter, collected in an urban area of Rome, in relation to their physicochemical composition as assessed by analytic electron microscopy and atomic absorption spectroscopy. In particular, our aim was to evaluate the importance of particle physicochemical components in the induced toxicity. The in vitro toxicity assays used included human red blood cell hemolysis, cell viability, and nitric oxide (NO) release in the RAW 264.7 macrophage cell line. The hemolytic potential has been widely used as an in vitro toxicity screen and as a useful indicator of oxidative damage to biomembranes. We found that human erythrocytes underwent dose-dependent hemolysis when they were incubated with varying concentrations of fine and coarse particles. The hemolytic potential was greater for the fine particles than for the coarse particles in equal mass concentration. However, when data were expressed in terms of PM surface per volume unit of suspension, the two fractions did not show any significant hemolytic differences. This result suggested that the oxidative stress induced by PM on the cell membranes could be due mainly to the interaction between the particle surfaces and the cell membranes. RAW 264.7 macrophage cells challenged with particles showed decreased viability and an increased release of NO, a key inflammatory mediator, and both effects were not dose dependent in the tested concentration range. The fine particles were the most effective and the differences between the two size fractions in inducing these biological effects remained unchanged when the basis of comparison was changed from weight to surface measures. It seemed therefore that these differences relied on the different physicochemical nature of the particles. The main chemical difference between the two fractions resided in a greater abundance of C-rich particles with S traces in the fine fraction. Therefore, we cautiously suggest a role for these particles in the induction of toxicity.