We assessed the extent to which constituents of PM2.5 (transition metals, sodium, chloride) contribute to the ability to generate hydroxyl radicals ((OH)-O-center dot) in vitro in PM2.5 sampled at 20 locations in 19 European centres participating in the European Community Respiratory Health Survey. PM2.5 samples (n = 716) were collected on filters over one year and the oxidative activity of particle suspensions obtained from these filters was then assessed by measuring their ability to generate (OH)-O-center dot in the presence of hydrogen peroxide. Associations between (OH)-O-center dot formation and the studied PM constituents were heterogeneous. The total explained variance ranged from 85% in Norwich to only 6% in Albacete. Among the 20 centres, 15 showed positive correlations between one or more of the measured transition metals (copper, iron, manganese, lead, vanadium and titanium) and (OH)-O-center dot formation. In 9 of 20 centres (OH)-O-center dot formation was negatively associated with chloride, and in 3 centres with sodium. Across 19 European cities, elements explained the largest variations in (OH)-O-center dot formation were chloride, iron and sodium. (C) 2009 Elsevier Ltd. All rights reserved.