Researchers in the United States have found evidence to suggest that lead can damage sperm.

Many men, such as plumbers, painters and printers, are exposed to lead at work.

Higher lead levels interfere both with the ability of the sperm to bind to the egg and with its ability to fertilise the egg Dr Susan Benoff

High levels of lead are also associated with people who smoke, drink alcohol or fail to exercise.

The findings come as a second study suggests men should not put off becoming a father.

Researchers at the University of California Berkeley have found yet more evidence that men become less fertile with each passing year.

Lead levels

In the first study, researchers examined the semen from the partners of 140 women undergoing their first IVF cycle.

They found that high lead levels in semen were associated with low fertilisation rates.

In further tests, they discovered that lead can prevent sperm from fertilising the egg.

"We have evidence that higher lead levels interfere both with the ability of the sperm to bind to the egg and with its ability to fertilise the egg," said Dr Susan Benoff, one of the researchers.

Interestingly, none of the men who took part in the study were exposed to lead at work.

Dr Benoff, who is based at the North-Shore Long Island Jewish Research Institute in New York, said other factors, such as smoking or drinking alcohol, were possibly involved.

Almost a quarter of men with high lead levels smoked cigarettes or drank alcohol. However, high levels of lead in other men could not be attributed to these factors.

Simply put, sperm slow down with age Andrew Wyrobek

Dr Benoff suggested a lack of exercise could be an alternative explanation. Levels of lead in the blood increase if people fail to exercise.

She added that the findings could help IVF clinics to help couples who are having problems conceiving a child.

Dr Steve Levett, an embryologist at the Centre for Assisted Reproduction at The Alexandra Hospital in Manchester, agreed.

"This should encourage clinics to identify those patients who may benefit from a more rigorous approach," he said.

"In the future, maybe we could test men for toxins such as lead."

In the second study, researchers found that the quality of a man's semen deteriorates with age.

They also discovered that the male biological clock begins ticking when men are in their 20s.

But unlike women, men lose their fertility gradually and can still be fertile in old age.

The UC Berkeley team examined the sperm from nearly 100 healthy men aged between the ages of 22 and 80.

Unhealthy sperm

They tested sperm motility - its liveliness and direction of movement.

They discovered that motility decreases by 0.7% each year.

This means that the chances of sperm being clinically abnormal or unhealthy is 25% at 22 years of age.

By 30 that figure jumps to 40%, rising to 60% at age 40. At the age of 60, 85% of sperm is clinically abnormal.

The researchers believe these changes may be due to general ageing.

"Simply put, sperm slow down with age," said Andrew Wyrobek of Lawrence Livermore National Laboratory.

Professor Brenda Eskenazi of UC Berkeley said the study backed up previous research suggesting that men needed to think about their own fertility.

"Women tend to be the focus in fertility issues," she said.

"What we are saying is that men are not scot-free in this. Many of us have heard of men in their 70s and older kids who have kids but the probability of that happening may be lower than we thought."