The emission of α and β radiation in the decay of long lived (22.3 yr) 210Pb to 206Pb gives rise to background events in the fiducial volume of the Borexino solar neutrino detector that fall in the energy window of the 7Be neutrino signal. Consequently, 222Rn as the predecessor of 210Pb must be controlled in the central part (inner vessel) of the Borexino detector and in the air of all rooms where parts of the inner vessel are prepared or handled. The Rn monitor designed for Borexino uses electrostatic collection of the 222Rn daughter 218Po on an α detector. The detector (418 l active volume) operates at 30 kV. The detection limit for 222Rn in air is below 1 mBq/m 3. The method of electrostatic collection has the principal disadvantage that the collection efficiency of the daughter ions depends on the degree of contamination of the air sample with water, hydrocarbons or other components. This causes an additional systematic error if the degree of pollution is unknown. A method to correct for this is outlined.