The central dose quantities used in radiological protection are absorbed dose, equivalent dose, and effective dose. The concept of effective dose was developed by the International Commission on Radiological Protection (ICRP) as a risk-adjusted dosimetric quantity for the management of protection against stochastic effects, principally cancer, enabling comparison of estimated doses with dose limits, dose constraints, and reference levels expressed in the same quantity. Its use allows all radiation exposures from external and internal sources to be considered together and summed, relying on the assumptions of a linear non-threshold dose–response relationship, equivalence of acute and chronic exposures at low doses or low dose rates, and equivalence of external and internal exposures. ICRP Publication 103 provides detailed explanation of the purpose and use of effective dose and equivalent dose to individual organs and tissues. This publication provides further guidance on the scientific basis for the control of radiation risks using dose quantities, and discusses occupational, public, and medical applications. It is recognised that best estimates of risk to individuals will use organ/tissue doses and specific dose risk models. Although doses incurred at low levels of exposure may be measured or assessed with reasonable accuracy, the associated risks are increasingly uncertain at lower doses. Bearing in mind the uncertainties associated with risk projection to low doses or low dose rates, it is concluded that effective dose may be considered as an approximate indicator of possible risk, recognising also that lifetime cancer risks vary with age at exposure, sex, and population group. A further conclusion is that equivalent dose is not required as a protection quantity. It will be more appropriate for limits for the avoidance of tissue reactions for the skin, hands and feet, and lens of the eye to be set in terms of absorbed dose rather than equivalent dose.
