Some recent epidemiologic investigations have shown an association between increased incidence of respiratory symptoms and exposure to low levels of particulate matter (PM*) less than 10 microm or less than 2.5 microm in aerodynamic diameter (PM10 and PM2.5, respectively). If particulates are causally involved with respiratory symptoms, it is important to understand which components may be responsible. However, increasing evidence suggests that transition metals present in particles, especially iron, generate reactive oxygen species (ROS) that may be involved in producing some of the observed respiratory symptoms. The hypothesis for this study is twofold: bioavailable transition metals from inhaled airborne particulates catalyze redox reactions in human lung epithelial cells, leading to oxidative stress and increased production of mediators of pulmonary inflammation: and the size, transition metal content, and mineral speciation of particulates affect their ability to cause these effects. This work focused on the relation between physical characteristics of particles (eg, size, bioavailable transition metal content, and mineral speciation) and their ability to generate hydroxyl radicals in cell-free systems and to cause oxidative stress, which results in the synthesis of mediators of pulmonary inflammation in cultured human lung epithelial cells. These relations were studied by comparing size-fractionated, chemically characterized coal fly ash (CFA) produced by combustion of three different coals to obtain milligram quantities of ash. One transition metal, iron, was studied specifically because it is by far the predominant transition metal in CFA. In addition, smaller quantities of particles from gasoline engines, diesel engines, and ambient air were studied. Phosphate buffer soluble fractions from particles from all sources were capable of generating ROS, as measured by production of malondialdehyde (MDA) from 2-deoxyribose. This activity was inhibited over 90% for all particles by the metal chelator N-[5-[3-[(5-aminopentyl)hydroxycarbamoyl]propionamidol-pentyl]-3-[[5-(N-hydroxyacetamido)pentyl]carbamoyl]propionohydroxamic acid (desferrioxamine B, or DF), strongly suggesting that transition metal(s), probably iron, were responsible. Particles from coal or gasoline combustion had greater ability to produce ROS than particles from diesel combustion. Iron was mobilized by citrate (at pH 7.5) from particles of all sources tested; gasoline combustion particles were the only particles not analyzed for iron mobilization because there were not enough particles for the iron mobilization assay. CFA particles were size-fractioned; the amount of iron mobilized by citrate was inversely related to the size of particles and also depended on the source of coal. Iron from the CFA particles was responsible for inducing the iron-storage protein ferritin in cultured human lung epithelial cells (A549 cells). The amount of iron mobilized by citrate was directly proportional to the amount of ferritin induced in the A549 cells. Iron from the CFA was also responsible for inducing the inflammatory mediator interleukin (IL) 8 in A549 cells. Iron existed in several species in the fly ash, but the bioavailable iron was associated with the glassy aluminosilicate fraction, which caused ferritin and IL-8 to be induced in the A549 cells. In crustal dust, another component of urban particulates, iron was associated with oxides and clay but not with aluminosilicates. The crustal dust contained almost no iron that could be mobilized by citrate. Iron could be mobilized from diesel combustion particulates, but at a much lower level than for all other combustion particles. Samples of ambient PM2.5 collected in Salt Lake City over 5-day periods during one month varied widely in the amount of iron that could be mobilized. If bioavailable transition metals (eg, iron) are related to the specific biological responses outlined here, then the potential exists to develop in vitro assays to determine whether particulates of unknown composition and origin can cause effects similar to those observed in this study.