Benchmarks
Research shows a correlation between decreases in sperm count and motility and increased ozone levels in the air.
Ozone, that trio of oxygen molecules that forms a UV-protecting blanket high in the stratosphere, seems to be less than protective of male fertility when it wanders down into the lower levels of the atmosphere.
According to Keck School of Medicine researcher Rebecca Z. Sokol, M.D., professor of obstetrics and gynecology and medicine, the high ozone levels produced in the lower atmosphere—the result of the release of pollutants—seems to be linked to lower sperm counts and decreased sperm motility in otherwise healthy, fertile men.
Sokol analyzed more than 8,000 sperm samples donated by 50 men in the Los Angeles area over a three-year period (January 1996 to December 1998), and compared them to more than 3,500 samples donated by 35 men from Northern California over the same time.
The original idea, says Sokol, was to see if there were geographic differences between the groups in terms of sperm quality. “We found only minimal differences between Northern and Southern California, with only marginally higher motile sperm counts in Southern California,” she reports. “And we weren’t even sure if those were real; there may very well have been some confounding factors.”
But when she and her Keck
colleagues—including second-year medical student Ian Fowler, statistician Peter Craft and assistant professor of preventive medicine Kiros Berhane, Ph.D.—compared the sperm data with air quality data, they were startled by the pattern that emerged. “There was a significant correlation,” says Sokol, “between decreases in sperm count and motility and increased ozone levels in the air, especially in Southern California.”
In 2000, Sokol reported that sperm counts in general have remained virtually unchanged over the past 50 years, despite reports that the lifestyles of men in the modern world were leading to a reduction in virility by as much as 50 percent. However, her most recent data suggest that there may be a cohort-related effect associated with decreasing sperm counts among fertile men.
This sort of epidemiology-based work is new to Sokol, who spent most of her career doing both basic scientific research into the effects
of toxicants on male reproduction and infertility and clinical studies in the field of male infertility. Then, in 2001, she was awarded a National Institutes of Health senior investigator fellowship award, supported by the National Institute of Environmental Health Sciences.
“It’s a program geared to senior investigators with a strong track record in NIH grants who want to change the focus of their research,” Sokol explains. “One of the points of the fellowship was that I needed to learn about something I knew nothing about—and to say I knew nothing about epidemiology was to put it mildly.” She enrolled in USC’s Master of Public Health program and began to learn, with the help of faculty mentors Thomas Valente, Ph.D., at the Keck School’s Institute for Preven-tion Research, and John Peters, M.D., M.P.H., director of the USC-based Southern California Environmental Health Sciences Center.
Just how ozone might be affecting sperm quality is somewhat of a conundrum, Sokol admits. “The blood-testis barrier is supposed to prevent the body from launching an immune attack against the sperm, and to protect the sperm from any toxins in the blood,” she says. “We know that oxygen radicals interfere with sperm function in the laboratory. So the question is, what physiologic response does ozone inhalation trigger to affect sperm in the body?”
Sokol hopes to investigate that very question. In addition, she says, she would like to do another study of sperm quality—looking at sperm samples from major cities known to have significant increases and decreases in ozone levels, such as New York and Mexico City. n
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