Is tungsten carbide toxic? Solid pieces, dust and cobalt

The hazard the sources describe is airborne dust, fume and mist. The International Tungsten Industry Association (ITIA), the industry's trade body, says the normal and intended use of finished carbide articles is expected to produce little or no exposure. None of the sources says anything either way about handling solid scrap.

The US National Toxicology Program (NTP) says dusts holding both cobalt and tungsten carbide can come from making, finishing or maintaining hard-metal products, such as sharpening or grinding. IARC, the World Health Organization's cancer agency, says cutting with hard-metal tools gives much lower exposure than making them, but sharpening and maintaining them can put cobalt in the air at several hundred micrograms per cubic meter.

Making parts from powder is where exposure peaks: NTP says several studies there reported cobalt near or above 5,000 micrograms per cubic meter. In one study NTP cites (Stefaniak et al. 2007), cobalt-bearing particles were finest from dry grinding, at a mass median aerodynamic diameter of 6 microns, and coarsest from scrap reclamation and pressing, at over 18 microns.

Sources: ITIA: health and environment; NTP Report on Carcinogens, 13th ed.: cobalt-tungsten carbide; IARC Monographs Volume 86 (2006).

In carbide with more than 2% cobalt, the cobalt binder is the bigger hazard. Hard metal is the name NTP uses for cemented carbide: tungsten carbide (WC) grains held together by a metal binder, most commonly cobalt. NTP says commercial cobalt-tungsten carbide generally holds 3% to 16% cobalt.

In 1987 OSHA reported NIOSH's finding that above 2% cobalt, the cobalt adds more to the potential health hazard than the tungsten carbide and all other components. In rat studies cited by NTP, cobalt-tungsten carbide put more cobalt into the urine than cobalt alone, which suggests the carbide makes cobalt more available to the body.

The federal ATSDR says no specific health effects have been tied to tungsten exposure in humans, and that HHS, IARC and EPA have not classified tungsten as a carcinogen. Under the EU CLP Regulation, the industry's own Tungsten Consortium lists tungsten carbide by itself as not classified.

Sources: NTP Report on Carcinogens, 13th ed.: cobalt-tungsten carbide; NTP RoC background document on hard metals (2010); LLNL manuscript (2018); OSHA letter of interpretation, 1987; ATSDR ToxFAQs: tungsten; ITIA: health and environment.

Hard metal lung disease is a rare disease caused by breathing particles of hard-metal alloys. NTP says it is not seen with cobalt alone and is a giant-cell interstitial pneumonia that can develop into lung fibrosis. IARC says there is little doubt that cobalt plays a critical role in causing it.

A 2016 case series in J Bras Pneumol cites studies of 425 and 319 hard-metal workers that found no cases, and one that found 1 case among 290 production workers, adding that the imaging of the time may have missed some.

OSHA reports that among 12 workers who made or ground tungsten carbide tools and developed interstitial lung disease, eight died. That is a case series, not a rate. The 2016 authors say the disease should always be considered when tool sharpeners, toolmakers or operators of grinders, lathes and milling cutters develop breathing problems.

Asthma is the other lung risk IARC names for cobalt. OSHA says that once the airways are sensitized, cobalt exposure causes an obstructive lung disease with wheezing, cough and shortness of breath.

Sources: NTP Report on Carcinogens, 13th ed.: cobalt-tungsten carbide; IARC Monographs Volume 86 (2006); J Bras Pneumol 2016: hard metal lung disease; OSHA: cobalt dust.

The enforceable federal limit for cobalt is OSHA's permissible exposure limit (PEL) of 0.1 mg/m³ as an 8-hour time-weighted average (TWA). For carbide over 2% cobalt, limits are written as cobalt (Co); a NIOSH recommended exposure limit (REL) is advice, not law.

Older papers carry a trap: OSHA's January 1989 rule set 0.05 mg/m³ for cobalt and 5 mg/m³ for insoluble tungsten, but a federal appeals court remanded it and those limits are not in force. OSHA's 5 mg/m³ for insoluble tungsten applies to construction and maritime work only, leaving no OSHA PEL for it in general industry.

Sources: OSHA: cobalt dust; OSHA chemical data: tungsten carbide over 2% cobalt; OSHA chemical data: tungsten, insoluble compounds; NTP Report on Carcinogens, 13th ed.: cobalt-tungsten carbide; ATSDR ToxFAQs: tungsten; NIOSH archive: 1989 OSHA rule, tungsten; NIOSH archive: 1989 OSHA rule, cobalt.

In Monographs Volume 86 (2006), IARC wrote: “Cobalt metal with tungsten carbide is probably carcinogenic to humans (Group 2A).” It judged the human evidence limited, and a number of working-group members supported Group 1, the top category.

Cobalt metal without tungsten carbide sat lower in 2006, in Group 2B (possibly carcinogenic). At its March 2022 meeting for Volume 131, IARC moved cobalt metal (without tungsten carbide or other metal alloys) to Group 2A, on animal and mechanistic evidence. The same volume put weapons-grade tungsten alloy, which is not cemented carbide, in Group 2B.

NTP lists cobalt-tungsten carbide powders and hard metals as “reasonably anticipated to be a human carcinogen”, first listed in its Twelfth Report on Carcinogens (2011). It bases the listing partly on consistent excess lung-cancer deaths among hard-metal manufacturing workers. Two French studies it cites found higher lung-cancer risk from exposure before sintering (the furnace step in how tungsten carbide is made) than after it, though NTP says presintered and sintered material release similar amounts of cobalt ions.

Sources: IARC Monographs Volume 86 (2006); IARC Volume 131 Q&A (2022); IARC Volume 131 news (May 2023); NTP Report on Carcinogens, 13th ed.: cobalt-tungsten carbide.

Cobalt can sensitize the skin as well as the lungs. IARC says cobalt exposure may lead to allergic contact dermatitis, particularly in hard-metal workers, and OSHA says that after sensitization, contact with cobalt causes dermatitis in skin creases and on rubbed surfaces of the arms, legs and neck.

In a 2008 skin-wipe study of three carbide production facilities cited by NTP, the highest geometric mean cobalt levels were on workers at the powder-handling one. OSHA adds that poorly managed metalworking fluids can cause skin irritation and allergic contact dermatitis.

Sources: IARC Monographs Volume 86 (2006); OSHA: cobalt dust; NTP RoC background document on hard metals (2010); OSHA: metalworking fluids.

Catch the dust where it is made. OSHA lists process enclosure, local exhaust ventilation, general dilution ventilation and personal protective equipment for cobalt dust and fume, and ITIA names local exhaust, enclosures and respiratory protective devices for powder handling and metal forming.

Wet grinding is not automatically safe. The Cobalt Institute says wetted or liquid forms of cobalt have lower emission potential, but IARC says coolants used to grind and resharpen hard-metal tools build up dissolved cobalt as they are recycled, which raises the chance of breathing cobalt in the mist they give off.

Leave the dust at work: ATSDR says to shower and change before going home if tungsten dust gets on your clothes. OSHA also warns that cobalt dust can form explosive mixtures in air.

Sources: OSHA: cobalt dust; ITIA: health and environment; Cobalt Institute: human health; IARC Monographs Volume 86 (2006); ATSDR ToxFAQs: tungsten.

Do not grind, cut or break pieces to sort them. NTP counts processing hard-metal scrap for recycling among the ways people are exposed, and the weight check on the density page can flag carbide without making dust.

Sludge, sweepings, powders and turnings are what ITIA and USGS call soft scrap. USPS makes the mailer identify a material's flashpoint and toxicity before mailing it, and a package that holds liquid must be marked as liquid on the outside with orientation arrows, so tell us which coolant or oil your sludge held. For loose powder, see tungsten carbide powder.

Worn inserts, drills and end mills still hold their tungsten and cobalt, so send them whole. Give the form, rough weight and photos on the quote form; ship from any state and we pay the shipping, or we pick up in every city in California, Arizona and Nevada, as selling tungsten carbide explains.

Sources: NTP Report on Carcinogens, 13th ed.: cobalt-tungsten carbide; ITIA newsletter on recycling (2019); USGS Open-File Report 2005-1028; USPS Publication 52, section 323; USPS DMM 601.3: packaging standards.