acute exposure, whereas radon exposure in homes occurs at low dose rates.

• A factor that represents the change in the nature of the radiation, because the Japanese were exposed largely to sparsely ionizing gamma () rays, whereas radon progeny emit densely ionizing particles.

The dosimetric approach also is subject to major uncertainties, and neither approach is a priori better.

This section on the radiation biology of particles summarizes basic concepts in the field, focusing on how radiation biology can contribute to the assessment of radon risks, namely by using doserate corrections and the radiation quality factor for radon-progeny particles. It also presents selected examples where recent information on mechanisms, oncogenes and tumor-suppressor genes, and possible biologic markers of -particle exposures will be reviewed in depth in a Phase II study.

The decay series for radon involves the emission of two principal particles with energies of 6 and 7.7 MeV. The passage of particles through tissue produces essentially linear tracks that are dense columns of ionizations, which give rise to the locally high radiation dose deposited. The ranges of the two principal particles in tissue are 48 and 71 m. There is some debate over which respiratory epithelial cells are affected in the radiation induction of lung cancer by radon, but it is generally agreed that the target cells (whether basal or serous) are within the range of particles depos-