For 30 years cancers have been the most feared diseases linked to environmental chemicals. Recently developmental effects, especially those of the central nervous system, appear to have taken their place at least in part. Both the chemicals involved and the ways of exposure are manifold. Methyl mercury, polychlorinated biphenyls (PCBs), and dioxins derive from fish. Lead comes from multiple sources, historically from leaded gasoline and paints, in some countries also from plumbing. Polybrominated diphenylethers are added to synthetic materials as flame retardants. We are exposed to pesticide residues in our food as well as in our home environment.

Most of these potential new dangers are plagued with controversy. In many cases it is a question of dose. There is no doubt about the developmental neurotoxicity of lead or methyl mercury, but are the present environmental exposures high enough to cause risk? In some cases there are disputes on plausible mechanisms. Such is the case with effects based on endocrine disruption. Is sufficient disturbance of oestrogen or the thyroid hormone system likely, if we are exposed to environmental chemicals which are orders of magnitude weaker than natural hormones, or to chemicals having weak effects on hormone synthesis or metabolism? One of the tricky questions is: When? An otherwise innocuous exposure might be important at a certain developmental stage, within a time window perhaps only lasting a few days during embryonic or fetal development. All of these …