The simple act of growing older robs sperm of their full ability to swim fast and find their way to a female egg, researchers found. In a separate study, scientists discovered that men with high levels of lead in their semen produced fewer healthy sperm.

Fortunately, neither condition dooms a man to infertility. Even with some sperm missing or confused, there's plenty more behind them, researchers say.

"As we say, it just takes one," says Dr. Susan Benoff, director of the Fertility Research Laboratories at the North Shore-Long Island Jewish Research Institute and principal investigator of the study on lead exposure.

As the countless younger wives of senior citizen celebrities have learned, even an old man can still produce one lucky sperm that finds its way to a female egg. However, it's not clear when fertility starts to drop off. Research on sperm production and aging has mainly focused on men who can't have children, says Andrew Wyrobek, head of the Health Effects Genetics Division at the Lawrence Livermore National Laboratory in California.

"We wanted to look at how a healthy working population fared," he explains.

So Wyrobek and his colleagues turned to a convenient supply of men of many ages -- the employee roster of the laboratory itself. They gathered sperm samples from 97 men between the ages of 22 and 80 who worked at the laboratory or were retired from jobs there. "It was broad spectrum, including janitors, cafeteria workers, lab scientists, drivers," Wyrobek says.

The researchers report their findings in the Feb. 6 issue of Human Reproduction.

The sperm samples show that fertility starts dropping when men are in their 20s and continues to diminish for the rest of their lives.

"Every year, every decade that goes by, you're slightly less fertile," Wyrobek says.

While the number of sperm in semen remained fairly constant, the sperm slowed down and lost their sense of direction over time, Wyrobek says. "They're not swimming in a straight line and are less likely to collide with the egg. They're just twitching or swimming in circles."

The probability that sperm didn't swim properly grew from 25 percent by age 22 to 85 percent by age 60.

The reasons for the drop aren't clear, Wyrobek says, "but it's another thing for couples to consider when they plan their families."

Another study in the same issue of Human Reproduction suggests that aging men could make themselves even less fertile through exposure to lead.

Benoff and colleagues examined the semen of 140 men whose partners were undergoing in-vitro fertilization. Men with higher levels of lead had lower sperm counts and were more likely to produce damaged sperm that would have a harder time fertilizing an egg, Benoff says.

It's not clear how the men became exposed to lead because most were not smokers, she says. "Even though you reduce lead emissions by taking lead out of gasoline, lead remains in the water and soil for long periods of time. They may be picking it up from food, especially fish, or from hobbies like recreational target shooting, casting bullets, making stained glass and ceramics," she says.

According to Benoff, lead could be responsible for 12 percent of infertility problems among men, who are as frequently infertile as women.

The good news is that men can reduce the amount of lead in their bodies by eliminating exposure to it, she says. Zinc supplements may also help reduce lead's presence in the body.

More information

For more on the dangers of lead and how to avoid the metal, visit the U.S. Centers for Disease Control and Prevention.

To read about how the sperm finds the egg, check out HowStuffWorks.