Men who smoke increase the risk that their children will have asthma – even if they give up long before conception, a study has found.

The study, carried out at the University of Bergen in Norway, found that the earlier a man takes up smoking, the higher the risk that his offspring will suffer from non-allergic asthma.

Young men who start smoking before the age of 15 triple the risk for their offspring, according to Cecilie Svanes, who led the study that examined the smoking habits of 13,000 men and women.

The researchers suggest that sperm are particularly susceptible to the influence of harmful substances in a certain stage of development. But even those who start smoking later could be causing harm to their future children, especially if they smoke over a long period.

If a man smokes for more than 10 years before conception, the risk of his children having asthma increases by 50 per cent, the study found.

“Given these results, we can presume that exposure to any type of air pollution, from occupational exposures to chemical exposures, could also have an effect,” said Svanes.

The study found no link between a mother’s smoking before conception and a child’s asthma: here only tobacco consumption during pregnancy seems to affect the unborn child.

The research was presented at the recent European Respiratory Society (ERS) International Congress in Munich, Germany.