Teenage boys exposed to organic pollutants are less likely to father boys, warn scientists.

A study in Taiwan on the sex of children born to individuals exposed to the Yucheng mass dioxin poisoning scare in the 1970's has shown they are less likely to have male children if they are exposed to the chemical before the age of 20.

The particular chemical involved in the 1979 Taiwanese disaster, polychlorinated byphenyls (PCBs) has now been banned, but researchers are worried that lower levels of dioxins in the environment could have similar effects.

Researcher Dr Iliana del Rio Gomez, from the London School of Hygiene and Tropical Medicine, said it was vital the number of pollutants were cut.

We should try to moderate the use of chemicals and our daily exposure

Dr Iliana del Rio Gomez

"We should try to moderate the use of chemicals and our daily exposure.

This chemical has now been banned, but it is one of a group of chemicals called dioxins, which are accumulated in the fat and once they are in the body they are there for a long time."

The study, by the National Cheng Kung University Medical College, in Taiwan and the London School of Hygiene and Tropical Medicine, looked at over 2,000 children whose families had eaten food cooked in contaminated oil.

Previous studies have already shown that the men who were babies in the womb during the scare had a reduced sperm mobility and that children born to mothers exposed to the chemical had reduced growth in the area of the brain used for reason and thinking.

The number of boys was reduced

The latest research, published in The Lancet, showed that men exposed to PCBs before the age of 20 had a 35% lower chance of having a baby boy than those who were not exposed to the pollutants.

If they were exposed after the age of 20 they had a one in 10 risk.

Development

Researcher Yueliang Leon Guo said the pollutants resulted in the failure of the sperm which would go on to create male embryos.

"The factors that determine sex are still under investigation, and the mechanisms of action that explain sex-ratio alterations are not yet understood.

"Sexual differentiation in human beings is controlled by genetic and hormonal factors.

"We propose that dioxin-like compounds might have a negative effect on the viability of the Y-chromosome carrying sperm or on the development of XY fertilised eggs."