The toxicology of chrysotile-containing brake debris: implications for mesothelioma

Abstract

The global use of "asbestos" in various commercial products has led to a wide range and pervasive legacy of disease. One such use of chrysotile asbestos was brake pads and was utilized commonly in automobiles and heavy vehicles. The result of incorporation of chrysotile into brake pads is associated with the exposure of mechanics fitting and servicing vehicles to liberated chrysotile fibers. Despite the proven exposure, the relative risk of malignant mesothelioma (MM) in this occupational population is broadly seen as low. The toxicity of particulates, including fibers such as chrysotile, is driven by a combination of dose and physicochemical properties. As such, it is plausible that chrysotile released from brake pads may have undergone modification, thereby altering the pathogenicity profile. The impact of high sheer stress causing shortening of long fibers, heat modification, binding of resin matrix to the fiber surface on the relative toxicity of brake debris with regards to MM is considered. It is apparent that released chrysotile can undergo significant modification, reducing the long fiber dose although not all modifications may lead to reduced toxicity.

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

“…There is well-documented historical exposure of mechanics to chrysotile following machining and replacement of chrysotile containing brake pads. Despite this exposure to a known carcinogen at varying levels (including those above prescribed limits), case study accounts of mesothelioma in mechanics have not translated into evidence of excess risk as determined by numerous epidemiological studies and meta-analysis (Poland and Duffin 2019). It is important to recognize that such conclusions are not universally shared yet the purpose of this study was to determine if there is a possible physicochemical basis for altered pathogenicity.…”

“…Differences in fiber composition (e.g. relative proportion of biosoluble components such as Mg in the brucitic layer) as well as degree of fibrillation effecting both surface area and accessibility of biosoluble components may all effect dissolution and clearance (Poland and Duffin 2019).…”

“…This concordance in mass loss between BD and chrysotile is likely due to the chrysotile component (up to 50% of the total mass composition; Weir and Meraz 2001). As recently reviewed (Poland and Duffin 2019), the dissolution and clearance of chrysotile from the lung can be viewed as a two-step as described by Gualtieri (Gualtieri 2018). Firstly there is a rapid dissolution of magnesium from the chrysotile which is known to be susceptible to leaching especially in acidic conditions (Virta 2002) as found in the phagolysosome of a macrophage.…”

“…It is important to note however that such epidemiological conclusions are not universally shared (Egilman and Billings 2005) and furthermore, there are case studies describing MM in auto mechanics working with chrysotile containing friction products (Langer and McCaughey 1982;Finkelstein 2015). For further discussion of the literature, refer to Poland and Duffin (2019).…”

“…Yet the debris released from chrysotile containing brake products contains what may be considered adulterated chrysotile owing to the (a) incorporation into a resin matrix, (b) pressure and heat curing, (c) high sheer stress during grinding, and (d) heat due to grinding friction. It is therefore certainly plausible that such conditions could modify the morphological as well as physicochemical characteristics of the chrysotile released from the resin matrix (Poland and Duffin 2019).…”