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By Pat Rizzuto
Oct. 31 — Silicon carbide made through a traditional production process can cause cancer in workers, the International Agency for Research on Cancer (IARC) concluded in a Lancet Oncology report published Oct. 30.
Occupational exposures to silicon carbide (CAS No. 409-21-2 ), which is used to make grinding wheels, cutting tools, refractory linings and other products, can cause lung cancer when the chemical is made in an electric furnace using a method called the Acheson process, IARC said in a paper that summarizes the conclusions an expert panel made during an October meeting.
The panel also concluded that silicon carbide “whiskers”—long fibers that are manufactured as substitutes for asbestos—are probably carcinogenic to people, IARC said.
Silicon carbide fibers, which are unwanted byproducts of silicon carbide production, are possibly carcinogenic to people, IARC said.
Nearly 300 million pounds of silicon carbide were produced in or imported into the U.S. in 2011 by companies including 3M Co., BASF, Dow Chemical Co. and the Saint-Gobain Corp., which, on its website, describes the Acheson process as a commonly used production method.
IARC was established by the World Health Organization to convene expert panels to classify chemicals, viruses and other exposures as to their carcinogenicity.
IARC's most recent panel consisted of 21 scientists from universities and government agencies in 10 countries who met in October to evaluate fluoro-edenite, silicon carbide fibers and whiskers and carbon nanotubes.
Of the three, carbon nanotubes are projected as having the broadest industrial and consumer applications, because they already are being used to strengthen and add other characteristics to fabrics, plastics, rubbers, electronics and composite materials.
Fluoro-edenite, an asbestos-like mineral found in Italy, is carcinogenic to people, IARC found.
The mineral was first identified around the Etna volcano near Biancavilla, Italy, the Lancet paper said. People living in that part of Italy have an excess of mesothelioma that can't be explained by chance, the paper said.
Carbon nanotubes are so diverse that their potential, as a group of chemicals, to cause cancer in people couldn't be determined, IARC said.
IARC did, however, classify one particular carbon nanotube. A compound identified as multi-walled carbon nanotube-7 was possibly carcinogenic to humans, IARC said. The nanotube has a diameter of 4.9 nanometers and is 49 micrometers long, according to information IARC provided Bloomberg BNA.
Male and female rats and male mice injected with that particular carbon nanotube developed peritoneal mesothelioma, a cancer in the lining of the abdomen.
No human studies were available for carbon nanotubes, the panel said.
Adverse effects that rodents experienced after inhaling multi- and single-walled carbon nanotubes included acute or persistent lung inflammation and thickened and scarred lung tissue.
Cellular studies showed carbon nanotubes could mutate DNA and cause other chromosomal aberrations.
Lynn Bergeson, managing partner of Bergeson & Campbell P.C., which has tracked global developments of policies and regulations for nanotechnologies, said IARC's conclusions about carbon nanotubes reflect the state of the science as it exists today and are thus necessarily limited.
“The good news is there is no data at this time to suggest carbon nanotubes cause cancer,” she said.
The information, however, suggests continued vigilance and diligent product stewardship is needed by manufacturers and others, including employers whose workplaces have carbon nanotubes, she said.
“The conclusions are helpful in reminding employers and others that exposure to carbon nanotubes and other workplace substances should be minimized to the greatest extent possible to prevent any adverse human health impact,” Bergeson said.
Franklin Mirer, an environmental and occupational health professor at the City University of New York's School of Public Health at Hunter College, said the data available for IARC to review was limited.
Companies should presume carbon nanotubes are probably carcinogenic and protect workers by using strategies the National Institute of Occupational Safety and Health describes in its Current Intelligence Bulletin 65, said Mirer, who formerly directed the United Auto Workers Health and Safety Department.
In the bulletin, NIOSH recommends exposures to all carbon nanotubes and nanofibers be controlled to less than 1 microgram per cubic meter of respirable elemental carbon as an eight-hour time-weighted average.
The protections NIOSH recommends aren't merely precautions, Mirer said. There is sufficient evidence to conclude that a failure to protect workers would be reckless with the health of exposed people, he said.
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Copies of the IARC paper can be purchased from the Lancet Oncology at http://www.thelancet.com/journals/lanonc/article/PIIS1470-2045%2814%2971109-X/fulltext.
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