A group of chemicals used widely in food packaging, toys and cosmetics has been linked to early menopause in women.
The findings of the United States study indicated women exposed to the highest levels of the chemicals, called phthalates, went through menopause on average 2.3 years earlier than women who were not exposed.
Phthalates are in a wider group of chemicals believed to disrupt the human endocrine system by interfering with the production and effects of hormones responsible for reproduction and development.
The researchers, who presented their findings to the conference of the American Society for Reproductive Medicine, said their study was the first to demonstrate the effect of endocrine-disrupting chemicals on menopause.
Canterbury University professor of toxicology Ian Shaw said everyone was exposed to phthalates daily because of their widespread use.
"I think it is safe to say that ... phthalates, as part of a complex cocktail of ingested chemicals that interfere with sex hormone responses, are having an effect on human growth and development. I don't think that phthalates will cause such effects on their own, unless the dose is very high, but only as part of a cocktail of endocrine disrupters."
Professor Shaw said that although phthalates were used, for example, in lipstick, the dose from using lipstick was unlikely to be high enough to cause early menopause, although a cocktail of endocrine disrupting chemicals together might be enough to do so.
Associate Professor Andrew Shelling, an Auckland University expert in early menopause, said he had reservations about the study, including that its phthalate finding only just reached the level of statistical significance. He contrasted that to the fact that there was no evidence to show a reduction in the usual age of menopause, which was 50 to 52.
He said early menopause was preceded by an earlier-than-usual reduction in a woman's fertility. It was also associated with an increased risk of osteoporosis. There was only weak evidence of a link to increased risk of heart disease and stroke.
Health campaigner Sue Kedgley, a former Green MP, said consumers could avoid phthalates and other endocrine-disrupting chemicals by choosing products from manufacturers which declared they did not use them. But the problem was complicated by weak labelling requirements.
Ms Kedgley supported the Breast Cancer Network's 10,700-signature petition, presented to Parliament in 2006, urging a breast cancer prevention strategy that tackled the role of synthetic environmental chemicals.
What are phthalates?
They are a group of chemicals.
What are they used for?
Mainly to increase the flexibility of plastics. They have a wide range of uses including in medical devices, pills, toys, adhesives, lipstick, hairspray, perfumes, moisturiser, eye shadow, shampoos, and numerous forms of food packaging including cling film, seals for food jars and drink bottles, and inserts for bottle lids.
How widely are humans exposed to phthalates?
Very widely - by ingestion, inhalation and absorption through the skin.
What are the risks?
Some phthalates have been shown to cause developmental and reproductive problems in laboratory animals, but the human health effects of exposure are less clear.
Many studies have found associations such as genital malformation in newborn boys of exposed men, breast cancer in daughters, and low sperm counts in adult men - but the studies can't prove cause and effect.
Have some countries banned phthalates?
Some have banned certain ones. The EU has banned several kinds in children's toys and limited their use in food packaging and closures and medical devices.
The United States has limited the use of three kinds in children's toys.
Australia has banned plastic toys young children are likely to chew or suck that contain more than 1 per cent diethylhexyl phthalate.