The Life Span Study of Japanese atomic bomb survivors provides risk estimates that are used widely as the basis for judgements about cancer risks following exposure to penetrating forms of ionising radiation, such as x rays and gamma rays. Interestingly, it is common practice in radiation protection to divide the risk estimates for cancers other than leukaemia by 2 when applying them to occupational and environmental settings of protracted, low-dose-rate exposures.1 2

The International Commission on Radiological Protection formalised this practice with the introduction of a dose and dose-rate effectiveness factor (DDREF).1 The DDREF is the factor by which they suggest dividing linear dose–response estimates from the Life Span Study when applying these estimates for radiation protection purposes to populations exposed to low-dose or low-dose-rate exposures. A DDREF with a value >1 implies that low-dose and low-dose-rate exposures are less effective at causing solid cancers than the radiation exposures received by the Japanese atomic bomb survivors. The use of a DDREF has been widely adopted by regulatory organisations. Nonetheless, the DDREF encompasses some of the most contentious issues in radiation epidemiology including: (1) the shape of the dose–response function at low doses; and (2) the consequences of protracted low-dose-rate exposures when compared with acute, high-dose-rate exposures.

In this issue of OEM, Jacob et al (see page 789) consider recent epidemiological evidence relevant to this assumption.3 Their paper evaluates the …