Exposure to pesticides commonly used both indoors and outdoors to kill mosquitoes and other insects on crops may result in boys reaching sexual maturity earlier, researchers have found.

The class of pesticides studied, pyrethroids, accounts for more than 30 percent of global insecticide use, said lead investigator Jing Liu, Associate Professor at Zhejiang University in Hangzhou, China. These chemicals are known endocrine-disrupting chemicals that interfere with the body's hormones.

"We recognise pyrethroids as a new environmental contributor to the observed trend toward earlier sexual maturity in boys," said Professor Liu.  Previous research shows that early puberty increases the risk of diseases in adulthood, for example, testicular cancer in men and breast cancer in women. 

Early puberty also can stunt growth and cause behavioural problems. For the study, presented at the Endocrine Society's 99th annual meeting in Orlando, the team examined 463 Chinese boys aged 9 to 16 years. They found the evidence of recent exposure to pyrethroids in human urine as a metabolite, or molecule, called 3-phenoxybenzoic acid (3-PBA).

The results showed that a 10 per cent increase in 3-PBA was associated with a four per cent increase in the boys' levels of luteinising hormone (LH) and follicle-stimulating hormone (FSH) -- hormones that spur production of testosterone in males.

Having an increased urinary level of 3-PBA raised the odds of a boy being at an advanced stage of genital development by 73 to 110 per cent, Liu said.

Further, when the researchers exposed male mice to cypermethrin -- a widely used pyrethroid insecticide -- they observed an accelerated onset of puberty in the mice. "Given the growing use of pyrethroid insecticides, we must prudently assess these chemicals for their risks to children's health," Liu said.