Of the 75,000 synthetic chemicals developed after the second world war to make our life easier, only a handful have been screened for long-term health risks.

Safe levels of exposure to chemicals were based on dosage - on the idea that there was a safe threshold below which exposure was not harmful. Since then, scientists have discovered that it's not so much the amount of exposure, but the timing of exposure that creates the disease risk. As we go though life, we enter and leave windows of time in which we are vulnerable to the damaging effect of chemicals, while at other times we are more immune.

· The womb is not the perfect protective bubble we assumed, says American biologist Sandra Steingraber, an internationally recognised expert on the environmental links to human health. The placenta does prevent harmful pathogens such as bacteria from reaching the foetus, but it is useless against synthetic chemicals. What's more, the placenta sometimes actively pumps high levels of dangerous substances from the mother's body into the foetus, tricked into believing they are nutrients. A 1996 US study showed that children whose mothers ate fish from the Great Lakes had high PCB (an organochlorine) levels in the umbilical cord. When they grew up, they failed, or took longer, to reach developmental milestones. Several studies have also shown that chemicals can harm the placenta itself, sabotaging its ability to deliver nutrients to the baby.

· Because their immune system is not developed yet, babies are enormously vulnerable to dangerous chemicals, especially before they reach six months, when they develop the ability to keep harmful chemicals out of their brain. Babies are also more exposed to harmful substances in general, as, pound for pound, they eat more food, drink more water and breathe more air. Mother's milk is now routinely laden with pesticides, flame-retardants, termite poisons and dry-cleaning fluids.

· During adolescence, a small amount of hormones bring about huge changes in the body. Recent studies in Belgium have already demonstrated that synthetic chemicals can alter the timing and duration of puberty.

· As we enter old age, we are becoming more vulnerable to harmful chemicals again because our immune system and liver enzymes don't work as well, and our ability to screen out harmful chemicals starts breaking down. New evidence also suggests that exposure to toxic chemicals can cause, or at least increase, the risk of late-life diseases, such as Parkinson's and Alzheimer's.

· Pesticide Action Network UK is a non-profit organisation working to eliminate the hazards of pesticides. Tel: 020-7274 8895. www.pan-uk.org. Sandra Steingraber is the author of Having Faith, an ecologist's journey to motherhood. www.steingraber.com.