The electrostatic charge properties of drug-free metered dose inhaler (MDI) aerosols containing the propellants HFA-134a and 227ea were studied using a modified electrical low pressure impactor (ELPI) with aerodynamic cutoff diameters ranging from 0.028 to 10.07 mu m. The MDIs were spiked with various amounts of water and crimped with Hostaform and PBT (polyester) valves. Polypropylene actuators were dried or wetted by storage in a desiccator or a box saturated with water vapour, respectively. The air humidity was controlled at 5%, 50%, and 92% RH to maintain the actuator surface dryness or moisture during the experiments. The droplet size distributions in the ELPI were deter-mined by chemical assays of another set of inhalers containing crystal violet as a marker. The charge profiles measured were highly variable but on average both HFA-134a and 227ea charged negatively, which was ascribed to the electronegative fluorine atoms in the HFA molecules withdrawing anions from the MDI components. The drug-free MDIs produced lower charges than the commercial medicated ones. The charges of both HFAs shifted towards neutrality or positive polarity as the water content increased. The spiked water would increase the electrical conductivity and/or decreased the electronegativity of the liquid propellant surface. The mean number of elementary charges per droplet decreased with decreasing droplet size. (c) 2007 Elsevier Ltd. All rights reserved.