An explosion at a chemical factory near Seveso, Italy, in 1976 exposed factory workers and local residents to the pollutant dioxin, presenting an opportunity to track how exposure at different ages affects sperm quality.

Now, results from a new study point to a window of vulnerability in reproductive system development when such chemical disruptions may leave a permanent mark. "The timing is very important," says study co-author Larry Needham of the Centers for Disease Control and Prevention (CDC) in Atlanta.

Led by Paolo Mocarelli of the University Milano–Bicocca in Milan, Italy, researchers analyzed levels of dioxin in blood samples taken in 1976 from 135 exposed males. They collected new samples of blood and sperm from these same men, and compared those data to samples from a control group of 184 men.

The researchers found that the sperm in men exposed before age 10 was feeble and depleted. At the same time, the pollutant seemed to stimulate sperm production and motility in men exposed when they were 10 to 17 years old. Both groups had altered levels of reproductive hormones that can affect developing testes. However, sperm appeared normal in men who were exposed as adults.

The results suggest that exposure to dioxin as a child or teenager can permanently alter the reproductive system. The chemical may meddle with the cells that nourish developing sperm, the scientists speculate in the January Environmental Health Perspectives.

The findings also hint that chemical exposure may be a factor in the decline of sperm quality recently seen in Western nations, Mocarelli says.

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