Twenty one experts from ten countries met at the WHO's International Agency for Research on Cancer (Iarc) in Lyon in October to assess the carcinogenicity of silicon carbide and carbon nanotubes. Their findings have been published online in the Lancet Oncology, and will later be published as an Iarc monograph.

Compounds or physical factors assessed by the Iarc are classified into four groups, based on the existing scientific evidence for carcinogenicity:

• Group 1: carcinogenic to humans – there is enough evidence to conclude that it can cause cancer in humans;

• Group 2A: probably carcinogenic to humans – there is strong evidence that it can cause cancer in humans, but at present it is not conclusive;

• Group 2B: possibly carcinogenic to humans – there is some evidence that it can cause cancer in humans, but at present it is far from conclusive;

• Group 3: unclassifiable as to carcinogenicity in humans – there is no evidence at present that it causes cancer in humans; and

• Group 4: probably not carcinogenic to humans – there is strong evidence that it does not cause cancer in humans.

Silicon carbide (SiC) particles are manufactured by the Acheson process as industrial abrasives. SiC fibres are a by-product. SiC whiskers are similar, but made by another process for use in composite machine tools. The Iarc working group found that there is substantial evidence that workers operating the Acheson process have an excess of lung cancer mortality. They concluded that occupational exposures from the process should be classified as a Group 1 process. But multiple exposures to other compounds led them to conclude that SiC fibres alone were a Group 2b substance.

There is little evidence on human exposure to SiC whiskers, they say. But experiments in which the products were implanted into rats resulted in development of mesothelioma and were classified as a Group 2A carcinogen.

Carbon nanotubes (CNTs) are manufactured for incorporation into fabrics, electronics, plastics and composites to improve their structural properties. Human exposure occurs during the cleaning of manufacturing reactors, but little human data is available. The working group found that one multiple-walled CNT, known as MWCNT-7, tested on rodents, caused mesotheliomas and adenomas. This was found to be possibly carcinogenic to humans (Group 2B).

Other studies with similar multiple-walled and single-walled CNTs were inconclusive. But there was plenty of evidence from rodents and human lung tissue that CNTs induced genetic lesions and chromosomal aberrations. Single-walled and multiple-walled CNTs, except MWCNT-7, were therefore regarded as unclassifiable (Group 3).

The meeting also considered the carcinogenicity of the mineral fluoro-edenite, which occurs in Italy and Japan. It was classified as carcinogenic to humans (Group 1) on the basis of evidence that exposure causes mesothelioma in humans and animals.