The aerosol sampling system incorporates a variable inlet, a dilution unit and a pump to ensure aerosol sampling at 15 L/min (Fig. 1).The dilution unit in such a system is necessary to avoid coincidence in the single particle measurements.A variable air flow of 0.2–1.0 L/min was transmitted isokinetically into the dilution unit, the other 14.0–14.8 L/min were drawn by the extra pump. The variable air flow was diluted to 5.0 L/min in the dilution unit and measured by the welas® 2070 detector.Welas® was operated for the particular nebulisation time plus an extra 10 s to ensure the measurement of all the particles that left the nebuliser.