<p>Results of investigations of the charging of micron-size dust particles in the plasma of the humid air ionized at different rates by an external ionization source, such as a beam of fast electrons with an energy higher than 100 keV, are presented. The charging of dust particles was defined on the basis of the kinetic model of ion-molecular processes including more than 600 reactions for electrons as well as negative and positive ions, among them hydrated ions containing up to 12 water molecules inclusive. It was established that, despite the negligible concentration of electrons at a large distance from a dust particle in an undisturbed plasma, the charging of dust particles in the plasma-forming gas ionized at a rate larger than the critical one is determined precisely by the electron flow. Because of this, the charges of the dust particles are fairly high. The influence of the rate of ionization, the composition, and the humidity of the air on the charging of dust particles in it was investigated.</p>