Carcinogenic hazards from inhaled carbon black, titanium dioxide, and talc not containing asbestos or asbestiform fibers: recent evaluations by an IARC Monographs Working Group

Our plain-English summary of Carcinogenic hazards from inhaled carbon black, titanium dioxide, and talc not containing asbestos or asbestiform fibers: recent evaluations by an IARC Monographs Working Group, by R.A. Baan (Inhalation Toxicology, 2007).

The study

• Author: R.A. Baan
• Journal: Inhalation Toxicology
• Published: 2007;19 Suppl 1:213-28
• Type of article: Review
• Cited by: 113 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

In February 2006, an IARC Monographs Working Group reevaluated the carcinogenic hazards to humans of carbon black, titanium dioxide, and talc, which belong to the group of poorly soluble, low-toxicity particles. The review of the relevant literature and the evaluations by the Working Group will be published in Volume 93 of the IARC Monographs series. This article summarizes the Working Group's conclusions. Epidemiological studies among workers in carbon black production and in the rubber industry provided inadequate evidence of carcinogenicity. The overall data from cancer studies in rodents exposed to carbon black provided sufficient evidence of carcinogenicity. The Working Group evaluated carbon black as possibly carcinogenic to humans, Group 2B. Reviewing the epidemiological studies in the titanium dioxide production industry, the Working Group concluded that there is inadequate evidence of carcinogenicity. Overall, the results from rodent cancer studies with titanium dioxide were considered to provide sufficient evidence. Titanium dioxide was evaluated as possibly carcinogenic to humans, Group 2B. Epidemiological studies on talc miners and millers provided inadequate evidence of carcinogenicity of inhaled talc not containing asbestos or asbestiform fibers. The evidence from rodent cancer studies was considered limited. The Working Group evaluated inhaled talc not containing asbestos or asbestiform fibers as not classifiable as to its carcinogenicity to humans, Group 3. The Working Group noted that prolonged exposure to inhaled particles at sufficiently high concentrations in experimental animals may lead to impairment of normal clearance mechanisms in the alveolar region of the lung, resulting in a continued buildup of particles that eventually leads to excessive lung burdens accompanied by chronic alveolar inflammation. The inflammatory response may give rise to increased generation of reactive oxygen species, cell injury, cell proliferation, fibrosis, induction of mutations, and, ultimately, cancer. Since many of these steps also occur in workers in dusty jobs, such as coal miners, data on cancer in animals obtained under conditions of impaired lung clearance were considered relevant to humans. In addition, impaired lung clearance in rodents exposed to ultrafine particles occurs at much lower mass concentrations than with fine particles, which adds to the human relevance.

In plain English

This article was a review in which the author, from the International Agency for Research on Cancer (IARC), summarized the conclusions of an IARC Monographs Working Group. In February 2006 the group reevaluated cancer hazards from inhaled carbon black, titanium dioxide, and talc not containing asbestos or asbestiform fibers. It reviewed earlier evaluations and newer studies of workers (carbon black and titanium dioxide production workers, rubber workers, talc miners and millers) and of rodents. Carbon black had inadequate evidence in humans and sufficient evidence in animals, and was classified as possibly carcinogenic to humans (Group 2B). Titanium dioxide, previously Group 3, was also placed in Group 2B. Inhaled talc not containing asbestos or asbestiform fibers had inadequate human and limited animal evidence, and remained Group 3, not classifiable. The article states that Group 3 is not a determination of safety. Talc containing asbestiform fibers, rated carcinogenic to humans (Group 1) in 1987, was not re-evaluated. Cosmetic talc was outside the article's scope. The Working Group concluded that the lung cancer mechanisms seen in animals were reasonably well understood, but that it was unclear to what degree they operate in humans.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Review: summary of the evaluations made by an IARC Monographs Working Group, covering epidemiological studies and experimental animal studies
• Who was studied: Three agents (carbon black, titanium dioxide, talc). Evidence reviewed came from published studies of workers in carbon black production and user industries, titanium dioxide production, and talc mining and milling, plus studies in rodents.
• Exposure: Inhalation of carbon black, titanium dioxide, and talc not containing asbestos or asbestiform fibers. Cosmetic talc use was also evaluated by the Working Group but was not discussed in this article.
• Main results:
  • Carbon black: inadequate evidence in humans and sufficient evidence in experimental animals; evaluated as possibly carcinogenic to humans (Group 2B)
  • Titanium dioxide: inadequate evidence in humans and sufficient evidence in animals; evaluated as possibly carcinogenic to humans (Group 2B). The previous evaluation had been limited animal evidence and Group 3.
  • Inhaled talc not containing asbestos or asbestiform fibers: inadequate evidence in humans and limited evidence in animals; evaluated as not classifiable as to its carcinogenicity to humans (Group 3)
  • Per the IARC Preamble quoted in the article, a Group 3 evaluation is not a determination of non-carcinogenicity or overall safety
  • The previous Working Group (1987) had evaluated talc containing asbestiform fibers as carcinogenic to humans (Group 1). This category was not re-evaluated by the 2006 group.
  • Among reviewed evidence, a meta-analysis (Wild, 2006) of miners and millers of nonasbestiform talc reported a lung cancer SMR of 0.92 (95% CI, 0.67–1.25) for talc millers from five countries and 1.20 (95% CI, 0.86–1.63) for talc miners from Italy, Norway, and the United States
  • Mechanistic information did not change any of the overall evaluations
• The authors' conclusion: The Working Group classified carbon black and titanium dioxide as possibly carcinogenic to humans (Group 2B) and inhaled talc not containing asbestos or asbestiform fibers as not classifiable (Group 3). It concluded that the mechanisms of particle-induced lung cancer in experimental animals (impaired clearance, inflammation, cell proliferation, oxidative stress leading to gene mutation) are reasonably well understood and that qualitatively similar mechanisms exist in humans.
• Limitations the authors state:
  • No data were available to characterize or quantify worker exposure to ultrafine primary particles
  • The degree to which all elements of the animal mechanism operate in humans is not clear
  • Studies in talc-user industries gave no details about the type of talc used and were not discussed

Topics

The National Library of Medicine indexes this study under subjects our site covers: Lung cancer, Talc and vermiculite.

Medical terms in this study, explained in our Lexicon: Carcinogenic, Talc, Asbestiform, IARC, Inflammation, Reactive Oxygen Species (ROS), Fibrosis.

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