The whole-body dose from inhaled radionuclides is a complex function of factors that describe exposure and physiology. Exposure-related factors may be estimated by considering likely emission rates combined with measured time-varying meteorological (transport) parameters and estimated exercise patterns. Physiological factors, which strongly influence radionuclide intake and the consequent radiation dose, have been catalogued as functions of age and activity level. Both exercise patterns and physiological factors are related to the age of the exposed individual. Combining these factors permits estimates to be made of the span of exposures that may be expected in a typical population. The ratio of highest to lowest radionuclide intake is predicted to be of the order of 40, suggesting that specification of an 'average' value may be misleading if this value approaches regulatory limits. Individuals exposed to maximal risk may be identified, and these individuals are likely to be active children. Activity pattern (lifestyle) is a primary determinant of risk.