The pre-eminent role of the lineal energy density of ionising radiations in eliciting biological effects has been recognised for many years. The pre-eminent role of chromosomal changes in the manifestation of human cancers, of mutational and lethal changes has attained equivalent recognition in recent years. Chromosomal alterations can be observed at the single cell level, and in in vitro systems the effects of defined fluences of charged particles can be related to cellular nuclear cross sectional area and hence changes can be assessed on a per particle per cell nucleus basis. This approach transcends reliance on the concept of absorbed dose. Charged particles generated in the Van de Graaff accelerator of the Radiological Research Accelerator Facility (RARAF) have been used to irradiate mammalian cells attached to Mylar substrates. The linear energy transfer (LET) ranged from 10 to 130 keV.µm-1. All classes of chromosomal changes were assessed as a function of fluence and of LET. When the frequency of 'two-break' aberrations per charged particle per cell nucleus is plotted against LET a quadratic relationship results from 10 to 80 keV.µm-1 with indications of saturation at 130 keV.µm-1. Analysis of the frequencies and types of 'two-break' aberrations in conjunction with spatial chromosomal allocation in cell nuclei enables assessment of proximities for lesion interaction and emphasises the dynamism associated with charged particle effects in mammalian cells. Hazards associated with low fluences, i.e. single or low numbers of high LET particle traversals, as might be expected from radon daughter irradiations, will probably not be uniform over the 80-200 keV.µm-1 LET range and will also probably vary based on cell nucleus presentation to incoming particles.