There is particular and growing concern about the threats that chemicals pose to children's health. The main problems include both acute exposure leading to poisoning and chronic, low-level exposures causing functional and organic damage during periods of special vulnerability, when neurological, enzymatic, metabolic and other systems are still developing. Exposure of unborn children to toxic chemicals may produce irreversible effects. For example, low levels of mercury have severe effects on the foetuses of pregnant mothers who ingest contaminated food. Recent research suggests that these chemicals may affect the ability of children to learn, integrate socially, fend off disease and reproduce (Colborn 1997).