The majority of the Po-218 atoms (the first progeny of radon) are attached to natural aerosols in the air. After their alpha decay, the newly formed nuclei of Pb-214 may detach from the carrier aerosol due to recoil. The average fraction of the nuclei detached in this manner is called the "recoil factor". It is an important parameter in the calculation of the radon-progeny concentration and the related radiation dose in lungs. We determine this parameter for eight common aerosol types and show that it cannot be treated as a fixed constant since it varies strongly with the aerosol particle size, shape, material, and surface/volume type of contamination. The range of values that the recoil factor can take broadens with the hygroscopic growth of the aerosol particles, to the maximum range between approximately 0.1 and 0.8. (C) 2004 Elsevier Ltd. All rights reserved.