Women have been worried recently by press accounts of six comparatively small epidemiological studies suggesting that certain organochlorines in the environment might increase the risk of breast cancer.*RF 1-6* The studies were concerned with 1, 1-dichloro-2, 2-bis(p-chlorophenyl) ethylene (DDE), the main metabolite of the insecticide DDT and also with the polychlorinated biphenyls (PCBs), a group of compounds used in industry - for example, as electrical insulators. How strong are the grounds for concern?

The results are summarised in the figure. The individual estimates plotted are ratios of the mean concentrations of DDE or of polychlorinated biphenyls in fat or serum in women with breast cancer divided by the mean concentrations in control women without breast cancer. Summary ratios were derived from the weighted averages of the log ratios. For DDE the summary ratio was 1.11 (99% confidence interval 0.97 to 1.26). In other words, the women with breast cancer had slightly higher concentrations of DDE than controls but the difference was not statistically significant. For the …