Chemicals in some household products including shampoo, perfumes, deodorants and soaps could cause girls to start puberty early, a study has found.
The study, published in the journal Human Reproduction, examined chemical levels including phthalates, parabens and phenols.
Certain phthalates are used in some scented products such as perfumes, deodorants, soaps, shampoo, nail polish and cosmetics.
Parabens are often used as preservatives in cosmetics and other personal care products and phenols can be used in soap, toothpaste, lipsticks, hairsprays, shampoos and skin lotions.
Researchers observed 179 girls and 159 boys in California born to mothers pregnant between 1999 and 2000.
The team studied the levels of chemicals in the mothers during pregnancy and in the children at nine years old.
They then tracked onset of puberty and found exposure to certain chemicals was linked to the early onset of puberty in girls but no evidence that chemical exposure affected boys.
Lead researcher Dr Kim Harley, associate professor in public health at the University of California, said they found two chemicals were particularly significant.
"Mothers who had higher levels of two chemicals in their bodies during pregnancy – diethyl phthalate, which is used in fragrance, and triclosan, which is an antibacterial agent in certain soaps and toothpaste – had daughters who entered puberty earlier," she said.
"We also found that girls with higher levels of parabens in their bodies at the age of nine entered puberty earlier," she added.
Dr Harley said the study may help explain why the age at which puberty starts in girls has been getting earlier in the last few decades, which has harmful side effects.
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"One hypothesis is that chemicals in the environment might be playing a role, and our findings support this idea," she said.
"Earlier puberty in girls increases their risk of mental health problems and risk-taking behaviour as teenagers and increases their risk of breast and ovarian cancer over the long-term, so this is an important issue to address," she added.