The authors studied breast cancer incidence in female radio and telegraph operators working on Norwegian merchant ships. The women had potential exposure to light at night, radio frequency radiation (405 kHz -25 MHz), and to some extent, extremely low frequency radiation (50 Hz). The breast cancer incidence in the cohort of 2,619 women who worked between 1920 and 1980 was compared with the Norwegian female population. Personal RF exposure measurements were not available. Instead RF exposure was estimated from job histories. Breast cancer risk in the cohort was 1.5 times that of the general population. In women over 50 years of age, there was a significant trend for exposure to shift work, indicating exposure to light at night and to electromagnetic fields. There was a significant excess of uterine tumours (1.9 times that of the general population) but not of other cancers, although the numbers were low. Elwood in his review considers that the "increased risks for both breast and uterine cancer, with no excesses in leukaemia or similar cancers, are more suggestive of a relationship to reproductive or other lifestyle factors than to an association with RF emissions. An analysis adjusted for age at first birth (a major risk factor for breast cancer), however, still gave a risk factor of 1.5 for the association between breast cancer and work as a radio operator."