Scientists hunting for gender-bending chemicals thought to be responsible for feminising fish, rising breast cancer incidence and falling sperm counts may have missed the wood for the trees.
Research at the University of London's School of Pharmacy suggests that mixtures of individually harmless chemicals - some man-made, some natural - that mimic the sex hormone oestradiol can have a significant cumulative impact.
The researchers say statutory safety testing of these substances, which are used as additives in plastics, pesticides and household products, are flawed because they ignore this "mixture effect".
Regulators have accepted single-compound tests, in part because it has been assumed that understanding combinations is too complex.
But Andreas Kortenkamp, senior lecturer in toxicology, says that in vitro research he has carried out with colleagues Elisabete Silva and Nissanka Rajapakse indicates that in some cases their effects may add up in a predictable fashion.
Concern about hormone-mimicking environmental contaminants has grown with reports of potentially related health problems.
Industry has defended individual substances by demonstrating that they have a far weaker effect than natural hormones and are found at low concentrations in the environment and human tissues.
Dr Kortenkamp's team studied mixtures of up to 11 different weak sex hormone-mimicking compounds at concentrations that have no detectable effects on their own. In research published in a conference poster, they report that a combination of eight such chemicals had a dramatic impact.
In a separate study, published in the journal Toxicological Sciences, weak oestrogenic compounds were found to have a measurable impact on the much stronger natural hormone oestradiol.