(CNN) -- A cluster of cases of a progressive lung disease occurred among dentists and other dental workers treated at one Virginia care center, according to Thursday's Morbidity and Mortality Weekly Report from the US Centers for Disease Control and Prevention.
Of nine patients, referred to as a cluster, seven died during the reported 16-year period. The disease, called idiopathic pulmonary fibrosis, is a chronic, progressive lung disease with a poor prognosis. The cause is unknown.
A cluster is a term used to define a collection of cases "grouped in place and time that are suspected to be greater than the number expected," Dr. Randall J. Nett, lead author of the study and medical officer with the US Public Health Service, explained in an email.
In this case, among 894 patients treated for idiopathic pulmonary fibrosis at the Virginia hospital, nine patients -- or 1% -- were identified as dentists or dental technicians. This number "was about 23 times higher than expected," Nett said.
All the cluster patients -- eight dentists and one dental technician -- were men, with an average age of 64. Though the reason for illness has not been identified, occupational exposures are possible, Nett and his colleagues wrote.
"Dentists and other dental personnel have unique exposures at work," Nett said. "These exposures include bacteria, viruses, dusts, gases, radiation, and other respiratory hazards."
One of the surviving cluster patients reported polishing dental appliances and preparing impressions without using a mask or other protection, according to the report. During these tasks, a dental worker would have been exposed to silica and other compounds with known or potential respiratory toxicity.
"At this time, we do not know what caused this cluster of idiopathic pulmonary fibrosis cases in dental personnel," said Nett, who recommended that workers "wear certified respiratory protection" if the ability to improve ventilation is not practical or effective.
"More work has to be done before we can make any conclusions about the risk dentists or other dental personnel have," Nett said. "CDC will follow up on this newly recognized cluster."
About 650,000 dental personnel work throughout the nation, he said.
Dr. Paul Casamassimo, chief policy officer of the American Academy of Pediatric Dentistry's Pediatric Oral Health & Research Center, said the new report is "not surprising."
For years, dentists have been exposed to plaster and dust from drilling, in addition to the toxic substances listed in the report. The cluster patients were all older, so they may not have been using the most up-to-date practices for personal protection and safety, he noted.
"We do work with materials and with human bioproducts that are potentially damaging to our bodies if we inhale them," Casamassimo said. Younger dentists are taught differently than in the past, so they know to delegate certain work or procedures to laboratories that meet more safe and stringent ventilation requirements, he said.
Today, dental personnel also have required protections from the Occupational Safety and Health Administration and the National Institute for Occupational Safety and Health, he said.
His own father was an "old-school" dentist.
"My father passed away at 79," Casamassimo said. Though his father did not die of pulmonary fibrosis, he had respiratory problems.
"This probably has been more common than we have known in the past," he said.
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