The reproductive hazards of nitrous oxide.

On-the-job exposure to high levels of laughing gas is no laughing matter. A new study suggests that the gas, known more formally as nitrous oxide, may impair a woman's ability to conceive -- troubling a finding for thousands of female dental assistants, hygienists, and dentists in the United States alone.

This is not the first time nitrous oxide has been linked to fertility problems. A 1989 study showed the gas prevented conception in female rats (SN: 3/25/89, p.182).

Andrew S. Rowland of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., and his colleagues decided to take that research a step further, this time by looking at the effect of nitrous oxide on women working as dental assistants.

The researchers began their retrospective investigation by recruiting 459 female dental assistants, ages 18 to 39, who had become pregnant during the four years prior to the study's start. Next, the team collected detailed information about each dental worker's exposure to nitrous oxide, a euphoria-producing analgesic often used in dental offices.

Those who said they were exposed to the gas were asked how many hours per week they worked in a room where patients received nitrous oxide. The women were also asked whether their dental office used equipment to collect unused or exhaled gas and vent it away from the work area.

The team then calculated how many menstrual cycles each woman had had between the time she stopped using birth control and the time she became pregnant. After controlling for factors known to reduce fertility, such as a history of pelvic inflammatory disease, the researchers discovered that women exposed to high levels of nitrous oxide were "significantly less fertile" than women exposed to lower levels of the gas or not exposed.

Women with five or more hours of exposure to high levels of nitrous oxide per week were only 41 percent as likely as their peers to conceive each month, the researchers report in the Oct. 1 NEW ENGLAND JOURNAL OF MEDICINE. The team found no fertility hazard for women exposed to lower levels of the gas, primarily because they worked in rooms where excess gas was vented.

Nobody knows the mechanism by which nitrous oxide interferes with fertility. It is possible that the gas blocks the brain's secretion of gonadotropin-releasing hormone and thus prevents ovulation, Rowland speculates. On the other hand, the gas may disrupt the development of a fertilized egg and thus cause a very early miscarriage, he adds.

In this study, women who reported heavy exposure to laughing gas took longer to conceive than their peers. However, Rowland and other scientists wonder whether chronic occupational exposure to high levels of nitrous oxide may lead to cases of outright infertility.

Of course, women may not be the only ones with fertility problems. Patricia A. Baird of the University of British Columbia in Vancouver wonders whether male dentists or dental assistants also suffer reproductive hazards because of laughing gas exposure. Baird, who wrote an editorial in the same issue of the journal, notes that nitrous oxide has been shown to produce abnormalities in sperm and reduced fertility in male rats.

From 35 to 50 percent of U.S. dental offices use nitrous oxide to kill pain and calm their patients, Rowland says. The Chicago-based American Dental Association advises its members to rely on equipment that minimizes exposure to the gas, according to an ADA spokesman. However, not all dentists follow that recommendation, Rowland says.

The new study suggests that dentists should begin to steer clear of nitrous oxide as a standard painkiller, Baird contends. "It makes sense not to expose staff and dentists to an agent that poses a concern in this way," she says.