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Airborne ultrafine particles of titanium dioxide—the world’s most popular coatings pigment—are a potential cause of cancer in the workplace, the National Institute for Occupational Safety and Health has concluded in a new set of recommendations.
The newly discovered health risks are outlined in Occupational Exposure to Titanium Dioxide, a new “Current Intelligence Bulletin” published by NIOSH, which is part of the Centers for Disease Control and Prevention.
NIOSH issues Current Intelligence Bulletins (CIBs) to disseminate new scientific information about occupational hazards. A CIB may draw attention to a formerly unrecognized hazard, report new data on a known hazard, or disseminate information about hazard control.
Ultrafine Particles Cited
“NIOSH has determined that ultrafine TiO2 is a potential occupational carcinogen…,” Dr. John Howard, NIOSH director, wrote in the bulletin’s Foreword.
“NIOSH recommends controlling exposures as low as possible, below the RELs [Recommended Exposure Limits],” the bulletin says.
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Bill Fox, Altairnano Inc.,
Dr. Aleks Stefaniak Dr. Mark Hoover, NIOSH |
| A Scanning Electron Microscopy (SEM) image shows agglomerated ultrafine-sized particles of rutile TiO2. |
Those limits are 2.4 mg/m3 for fine TiO2 and 0.3 mg/m3 for ultrafine (including engineered nanoscale) TiO2, as time-weighted average (TWA) concentrations for up to 10 hours per day during a 40-hour work week, according to Howard.
1.45M Metric Tons
TiO2 is a noncombustible, white, crystalline, solid, odorless powder used extensively in many commercial products. It is the most common pigment currently used in paints, coatings and varnishes. U.S. production was about 1.45 million metric tons per year in 2007. The number of U.S. workers currently exposed to TiO2 dust is not available.
TiO2 is produced and used in the workplace in varying particle size fractions, including fine (defined as all particle sizes collected by respirable particle sampling) and ultrafine or nanoparticles (defined as the fraction of respirable particles with a primary diameter of <0.1 µm [<100 nm]).
The bulletin, which reviews both animal and human data, describes exposure monitoring techniques and exposure control strategies; and discusses avenues of future research.
Particle Size Cited
NIOSH found that the adverse effects of inhaling TiO2 in the workplace may not due to the material itself, but to the tiny size of the poorly soluble, low-toxicity (PSLT) particles in the lungs.
“TiO2 is not a direct-acting carcinogen, but acts through a secondary genotoxicity mechanism that is not specific to TiO2 but primarily related to particle size and surface area,” the bulletin said.
Ultrafine particles are believed to penetrate the epithelial lining and lung interstitial spaces to a greater extent than larger particles, more readily enter cells, and cause greater lung inflammation and oxidative stress, researchers say. PSLF particles have been linked to persistent inflammation, tissue damage, fibrosis, and lung cancer in rats.
Fine Particle Data Inconclusive
The bulletin reviews exposure to ultrafine/nano and fine TiO2 in both manufacturing and end-user facilities.
NIOSH found insufficient data to draw the same conclusion about risks of fine TIO2 particles, but it reported that the “tumor-response data are consistent with that observed for ultrafine TiO2….”
NIOSH recommended in 1988 that TiO2 be classified as a potential occupational carcinogen and that exposures be controlled as low as feasible. In 2006, the International Agency for Research on Cancer (IARC) concluded that there was sufficient evidence of carcinogenicity in animals but inadequate evidence of carcinogenicity in humans.
NIOSH notes that the health effects of the fast-growing nanomaterial industry is still evolving and invites comments about nano-size titanium dioxide for future bulletin updates at nioshdocket@cdc.gov.
Coating-Modified Particles
NIOSH also evaluated the potential for coatings to modify the toxicity of TiO2, as many industrial processes apply coatings to TiO2 particles. TiO2 toxicity has been shown to increase after coating with various substances.
However, NIOSH said, TiO2 toxicity has not been shown to be weakened by application of coatings. NIOSH said the TiO2 risk assessment could be used as a “reasonable floor for potential toxicity, with the notion that toxicity may be substantially increased by particle treatment and process modification.”
The agency added, however: “These findings are based on the studies in the scientific literature and may not apply to other formulations, surface coatings, or treatments of TiO2 for which data were not available. An extensive review of the risks of coated TiO2 particles is beyond the scope of this document.”
Nanocoatings Growth
More than 1,300 products containing some kind of nanomaterial are already on the market. NIOSH has been in the forefront of nano-related policy as related to workplace safety. Late last year, the agency released a draft version of a similar document on occupational exposure and carbon nanotubes; a final version is expected soon.
The explosion of the nanocoatings market would be hard to overstate. In 2009, the global market for nanocoatings and nanoadhesives totaled about $2.3 billion, according to BCC Research. The firm projected that the industry would experience a compound annual growth rate (CAGR) of 39.3% annually, bringing its global value to $3.6 billion by 2015.
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