The distribution of ambient air particles varies according to climate, industries, and other sources. In this study, ambient air particles (less than 12.1 ${mu}m$) were monitored from February to August, 2007 as 12 different fractions sorted by a cascade impactor. Particles in the size range from 0.33 ${mu}m$ to 0.76 ${mu}m$ comprised the main fraction of ambient air particles in Seoul, Korea. On the day of an Asian dust event, the particle fraction size increased to 1.25~2.5 ${mu}m$. The different sized particle fractions were also monitored for metals and were found to contain toxic heavy metals including Pb, Cd, Hg, Cr and As. Particle preparations were significantly cytotoxic when exposed to cultured BEAS-2B cells. Microarray analysis of the treated cells indicated a significant up-regulation of a number of genes associated with oxidative stress, including metallothionein, heme oxygenase-1, heat shock protein 70, and NAD(P)H dehydrogenase-1.