A common garden pesticide may be harming male fertility by suppressing levels of the sex hormone testosterone, a study has found. Researchers measured by-products of a pesticide, chlorpyrifos, in males undergoing fertility treatment and found that those with the lowest testosterone levels had the most pesticide by-product in their systems.
Chlorpyrifos is permitted under EU regulations for use in agriculture and garden insecticides and is used widely in Britain, but has been subject to stringent control in the US since 2000 because of concerns over its effects on brain function.

John Meeker at the University of Michigan and co-workers at Harvard University and the US Centres for Disease Control and Prevention in Atlanta took urine samples from 268 male partners of couples undergoing treatment at a Massachusetts fertility clinic between 2000 and 2003. Samples were analysed for traces of a substance called TCPY, a breakdown product of chlorpyrifos. The results showed that levels of TCPY varied, but men's testosterone levels decreased steadily the more TCPY they had in their urine. Men with the most TCPY in their systems typically had 10% less testosterone than men with the least TCPY.

The study does not prove a link between the pesticide and reduced fertility, but Dr Meeker believes further tests on the pesticide are urgently needed. "Because it's so common and so many males are exposed to this pesticide, it could be having some negative effects," he said. "A decline in testosterone throughout a population could potentially lead to adverse reproductive health outcomes." The study appears in the January issue of the journal Epidemiology.