Assessment of the physicochemical properties of chrysotile-containing brake debris pertaining to toxicity

The Acute Toxicity of Mineral Fibres: A Systematic In Vitro Study Using Different THP-1 Macrophage Phenotypes

Alveolar macrophages are the first line of defence against detrimental inhaled stimuli. To date, no comparative data have been obtained on the inflammatory response induced by different carcinogenic mineral fibres in the three main macrophage phenotypes: M0 (non-activated), M1 (pro-inflammatory) and M2 (alternatively activated). To gain new insights into the different toxicity mechanisms of carcinogenic mineral fibres, the acute effects of fibrous erionite, crocidolite and chrysotile in the three phenotypes obtained by THP-1 monocyte differentiation were investigated. The three mineral fibres apparently act by different toxicity mechanisms. Crocidolite seems to exert its toxic effects mostly as a result of its biodurability, ROS and cytokine production and DNA damage. Chrysotile, due to its low biodurability, displays toxic effects related to the release of toxic metals and the production of ROS and cytokines. Other mechanisms are involved in explaining the toxicity of biodurable fibrous erionite, which induces lower ROS and toxic metal release but exhibits a cation-exchange capacity able to alter the intracellular homeostasis of important cations. Concerning the differences among the three macrophage phenotypes, similar behaviour in the production of pro-inflammatory mediators was observed. The M2 phenotype, although known as a cell type recruited to mitigate the inflammatory state, in the case of asbestos fibres and erionite, serves to support the process by supplying pro-inflammatory mediators.

US EPA's TSCA risk assessment approach: a case study of asbestos in automotive brakes

The United States Environmental Protection Agency (US EPA) is currently refining its approach for risk assessments conducted under the amended Toxic Substances Control Act (TSCA), largely based on recommendations from the National Academies of Sciences, Engineering, and Medicine (NASEM). We identified several issues with the current TSCA risk assessment approach that were not addressed by NASEM in its recommendations. Here, we demonstrate these issues with a case study of the 'Risk Evaluation for Asbestos, Part 1: Chrysotile Asbestos,' which US EPA released in December 2020. In this evaluation, US EPA found that occupational and some consumer uses of automotive brakes and clutches that contain asbestos result in unreasonable risks. These risks were calculated from estimated exposures during brake work and an inhalation unit risk (IUR) developed for chrysotile asbestos. We found that US EPA overestimated risk as a result of unrealistic inputs to both the exposure and toxicity components of the risk equation, and because the Agency did not fully consider relevant epidemiology and toxicity evidence in its systematic review. Our evaluation demonstrates areas in which the TSCA risk assessment approach could be improved to result in risk evaluations that are supported by the available scientific evidence.

A critical review of the 2020 EPA risk assessment for chrysotile and its many shortcomings

From 2018 to 2020, the United States Environmental Protection Agency (EPA) performed a risk evaluation of chrysotile asbestos to evaluate the hazards of asbestos-containing products (e.g. encapsulated products), including brakes and gaskets, allegedly currently sold in the United States. During the public review period, the EPA received more than 100 letters commenting on the proposed risk evaluation. The Science Advisory Committee on Chemicals (SACC), which peer reviewed the document, asked approximately 100 questions of the EPA that they expected to be addressed prior to publication of the final version of the risk assessment on 30 December 2020. After careful analysis, the authors of this manuscript found many significant scientific shortcomings in both the EPA's draft and final versions of the chrysotile risk evaluation. First, the EPA provided insufficient evidence regarding the current number of chrysotile-containing brakes and gaskets being sold in the United States, which influences the need for regulatory oversight. Second, the Agency did not give adequate consideration to the more than 200 air samples detailed in the published literature of auto mechanics who changed brakes in the 1970-1989 era. Third, the Agency did not consider more than 15 epidemiology studies indicating that exposures to encapsulated chrysotile asbestos in brakes and gaskets, which were generally in commerce from approximately 1950-1985, did not increase the incidence of any asbestos-related disease. Fourth, the concern about chrysotile asbestos being a mesothelioma hazard was based on populations in two facilities where mixed exposure to chrysotile and commercial amphibole asbestos (amosite and crocidolite) occurred. All 8 cases of pleural cancer and mesothelioma in the examined populations arose in facilities where amphiboles were present. It was therefore inappropriate to rely on these cohorts to predict the health risks of exposure to short fiber chrysotile, especially of those fibers filled with phenolic resins. Fifth, the suggested inhalation unit risk (IUR) for chrysotile asbestos was far too high since it was not markedly different than for amosite, despite the fact that the amphiboles are a far more potent carcinogen. Sixth, the approach to low dose modeling was not the most appropriate one in several respects, but, without question, it should have accounted for the background rate of mesothelioma in the general population. Just one month after this assessment was published, the National Academies of Science notified the EPA that the Agency's systematic review process was flawed. The result of the EPA's chrysotile asbestos risk evaluation is that society can expect dozens of years of scientifically unwarranted litigation. Due to an aging population and because some fraction of the population is naturally predisposed to mesothelioma given the presence of various genetic mutations in DNA repair mechanisms (e.g. BAP1 and others), the vast majority of mesotheliomas in the post-2035 era are expected to be spontaneous and unrelated in any way to exposure to asbestos. Due to the EPA's analysis, it is our belief that those who handled brakes and gaskets in the post-1985 era may now believe that those exposures were the cause of their mesothelioma, when a risk assessment based on the scientific weight of evidence would indicate otherwise.

Simulated biological fluids - a systematic review of their biological relevance and use in relation to inhalation toxicology of particles and fibres

The use of simulated biological fluids (SBFs) is a promising in vitro technique to better understand the release mechanisms and possible in vivo behaviour of materials, including fibres, metal-containing particles and nanomaterials. Applications of SBFs in dissolution tests allow a measure of material biopersistence or, conversely, bioaccessibility that in turn can provide a useful inference of a materials biodistribution, its acute and long-term toxicity, as well as its pathogenicity. Given the wide range of SBFs reported in the literature, a review was conducted, with a focus on fluids used to replicate environments that may be encountered upon material inhalation, including extracellular and intracellular compartments. The review aims to identify when a fluid design can replicate realistic biological conditions, demonstrate operation validation, and/or provide robustness and reproducibility. The studies examined highlight simulated lung fluids (SLFs) that have been shown to suitably replicate physiological conditions, and identify specific components that play a pivotal role in dissolution mechanisms and biological activity; including organic molecules, redox-active species and chelating agents. Material dissolution was not always driven by pH, and likewise not only driven by SLF composition; specific materials and formulations correspond to specific dissolution mechanisms. It is recommended that SLF developments focus on biological predictivity and if not practical, on better biological mimicry, as such an approach ensures results are more likely to reflect in vivo behaviour regardless of the material under investigation.