No laughing matter.

Laughing gas (nitrous oxide) may help a patient feel no pain, but health officials have suspected for years that dental workers regularly exposed to the gas may develop serious health problems. Despite the threat, some dental offices contain excessive levels of nitrous oxide leaking from faulty equipment, says a new survey by the Georgia Institute of Technology in Atlanta.

Researchers Paul Middendorf and David Jacobs found waste gases at levels of 600 parts per million (ppm) and higher during their survey of 30 Georgia dental offices. In the late 1970s the National Institute of Occupational Safety and Health suggested 25 ppm as an acceptable level.

Jacobs attributes the excessive levels to poor equipment design and inadequate maintenance programs. "Dental offices need scavenging systems [to remove excess gas] and many dentists don't have them," he says. "They also should have local exhaust systems, which hardly any have."

When claims that nitrous oxide could cause liver damage and other illnesses surfaced several years ago, the American Dental Assciation (ADA) issued a "strongly worded" message cautioning dentists to be careful, says Edgar W. Mitchell of the ADA's Council on Dental Therapeutics. But, he says, the study suggests a "complacency" among dentists. "We may need to get that flag out and wave it again," he says. "We know if there's an exposure problem, there's an effective way to remove the gas."

Participants at a National Institutes of Health conference this spring also produced a consensus statement calling for renewed study of nitrous oxide's effects on dental personnel.