The substance was included in its Report on Carcinogens, which also added chemical formaldehyde and a botanical known as aristolochic acids are listed as known human carcinogens. Six other substances: captafol, certain inhalable glass wool fibres, o-nitrotoluene, riddelliine, styrene and cobalt-tungsten carbide were added as substances “that are reasonably anticipated to be human carcinogens”, the organisation said in a press release.

The Report on Carcinogens is a congressionally mandated document that is prepared for the HSS Secretary by the US National Toxicology Program (NTP). The report identifies agents, substances, mixtures, or exposures in two categories: known to be a human carcinogen and reasonably anticipated to be a human carcinogen. According to the HSS, a listing in the Report on Carcinogens does not by itself mean that a substance will cause cancer. Many factors, including the amount and duration of exposure, and an individual's susceptibility to a substance, affect whether a person will develop cancer.

The NTP reportedly used established criteria to evaluate the scientific evidence on each candidate substance under review, drawing upon the scientific expertise of several US federal agencies, including the National Institutes of Health, Centers for Disease Control and Prevention, Agency for Toxic Substances and Disease Registry, US Food and Drug Administration, U.S. Environmental Protection Agency, U.S. Consumer Product Safety Commission, and Occupational Safety and Health Administration.
 
In tests, cobalt-tungsten carbide (in powder and hard metal form), otherwise known as cemented or sintered carbide, showed limited evidence of lung cancer in workers involved in cobalt-tungsten carbide hard metal manufacturing.
 
The 12th Report on Carcinogens now includes 240 listings. It is available here.