15. The asbestos era, asbestos substitution and chrysotile

Historical material continuity: asbestos, calcium silicate and the period thereafter

Calcium silicate was not introduced in high-temperature applications only as a subsequent substitute after asbestos was phased out. The material class was already represented in the market for technical thermal and pipe insulation during the asbestos era.

A historical product sheet from Johns-Manville documents this connection directly. It advertises "J-M Thermobestos" for use up to 1,200 °F and explicitly describes the product as "calcium silicate insulation". The sheet is part of a product overview by the long-established asbestos manufacturer Johns-Manville for thermal and pipe insulation. It thus shows that, during the asbestos era, calcium silicate did not form a separate world of materials outside the asbestos market, but was part of the same high-temperature insulation environment. [6]

The later elimination of the asbestos fibre addressed the asbestos-related fibre hazard. However, it did not automatically remove the calcium oxide chemistry relevant to chromate formation from insulation systems. Calcium silicate products continued to be offered as high-temperature insulation; at present, Johns Manville, for example, also markets calcium-silicate high-temperature insulation, now explicitly asbestos-free. The change in the fibre component is therefore not to be equated with the elimination of the basic calcium component.

Asbestos-mineral wool mixtures and composite systems

Historical material development did not proceed exclusively as a switch from pure asbestos to pure mineral wool. The Johns-Manville patent US2389460A, granted on 20 November 1945, describes thermal insulation boards containing stone wool and asbestos fibres in a weight ratio of approximately 2:1, bonded by a plastic binder. It documents an expressly intended fibre mixture. [20]

US2162387A of 13 June 1939 is additionally relevant to high-temperature technology: the high-temperature cement described for boiler and furnace joints contains 10 per cent by weight asbestos fibres, 5 % mineral wool and 8 % sodium silicate, as well as 27 % sand and 50 % fireclay. Asbestos, mineral wool and a sodium component were thus already provided for in the same formulation at that time. The historical assessment must cover such mixed and composite systems including their basic components. [21]

The historical and the present-day risk logic must therefore be separated and at the same time considered together:

Early research approach: the US report of 1969

Ralph J. Sullivan's Air Pollution Aspects of Chromium and its Compounds contains two separate points of reference. Section 3.4.1 Asbestos, printed pages 24-25, reports contemporary considerations on chromium and other trace substances as contributors to asbestos-associated cancers. It states a chromium content of approximately 1,500 µg/g in the asbestos material cited. Section 3.4.3 Cement, page 25, explains chromate formation through heat treatment of Cr₂O₃ under alkaline conditions and asks about its significance for other materials, for example asbestos. [17]

The first reference concerns a possible additional biological damage component; the second a thermochemical research question. These two historical questions must be pursued separately: via fibre-associated metal chemistry on the one hand, and via the composition, interfaces and operating conditions of asbestos-containing insulation systems on the other. The report was prepared in 1969 for the US National Air Pollution Control Administration within the Department of Health. It is held today in the archive of the US Environmental Protection Agency (EPA); the EPA was not its commissioning body at the time. [17]

Chrysotile asbestos as a possible parallel Cr(VI) source: the findings of Walter et al.

The study by Walter et al. (2022) does not examine chromate formation on insulation materials, but the release of chromium from chrysotile asbestos. Precisely for this reason, it decisively complements the historical perspective: it makes visible that asbestos exposure involving chromium-containing chrysotile cannot necessarily be understood exclusively as fibre exposure.

Walter et al. show a chromium content in the low gram-per-kilogram range in the chrysotile examined. In its initial state, the chromium is predominantly present as poorly soluble Cr(III). Under oxidative conditions at physiological pH, in particular in the presence of hydrogen peroxide (H₂O₂), chromium was mobilised from the fibres and detected in solution exclusively as Cr(VI). The mobilised Cr(VI) was predominantly present as the chromate oxyanion.

The previously insufficiently investigated chromium pathway

Walter et al. make clear that an involvement of chromium in the carcinogenicity of chrysotile had already been postulated earlier. What was insufficiently understood, however, was the behaviour and reactivity of fibre-associated chromium in the lung and pleura; to the authors' knowledge, the uptake capacity of the relevant cells had not previously been investigated. The study addresses these knowledge gaps experimentally. [4]

The study further demonstrates that lung epithelial, mesothelial, lung carcinoma and mesothelioma cells express Cr(VI) transporters and take up the Cr(VI) added to the culture medium in the cell uptake experiments. In the experiments, intracellular chromium accumulation reached up to approximately ten times the concentration in the surrounding medium. In the authors' view, this mobilised and cell-permeant Cr(VI) can therefore contribute to tumour initiation and tumour progression in asbestos-associated diseases.

The finding must be read precisely:

• It does not diminish the independent carcinogenicity of the asbestos fibre.

• It does not transfer the known iron/fibre chemistry to chromium. Walter et al. found no relevant contribution of chromium to H₂O₂ decomposition or to hydroxyl radical formation; this pathway remained essentially attributable to iron.

• It adds an additional, Cr(VI)-mediated genotoxic pathway to the damage mechanisms.

The study thus opens a historically relevant question: to what extent were disease risks in chrysotile-containing exposures attributed exclusively to the effect of the fibres, although Cr(VI) may additionally have been mobilised from the fibres under inflammatory-oxidative conditions? Walter et al. do not answer this question quantitatively. They do show, however, that a parallel Cr(VI) involvement must, on sound scientific grounds, be included in the assessment.

Asbestos substitution: elimination of the fibre hazard, not necessarily of the entire reaction system

The historical transition from asbestos-containing to asbestos-free high-temperature insulation was initially, and rightly, a substitution of the asbestos fibre. Because of its independent, serious carcinogenic effect, the asbestos fibre was no longer tenable. Its removal from insulation materials was therefore imperative.

This fibre substitution, however, is not to be equated with a complete material substitution of the entire high-temperature insulation system. Historically, asbestos fibres were combined with inorganic components in insulation systems. For loose asbestos fibre mixtures, the USGS expressly lists, among others, hydrated calcium silicate as an inorganic mixture component for thermal, electrical and acoustic insulation products. [8] As early as 1901, US Patent 668,684 moreover describes a fire-retardant insulation composite consisting of outer layers of asbestos paper and a mineral wool core layer. [7]

With the elimination of asbestos, what changed in many applications was thus primarily the critical fibre component. Insofar as the asbestos-free successor products continued to contain calcium, sodium, magnesium or potassium oxides from fibres, fillers, binders or other raw materials, the basic-oxidic chemistry of the insulation was retained. On chromium-alloyed hot components, the material preconditions for the formation of chromium(VI) compounds therefore also remained in place.

The technically precise assessment is therefore:

Asbestos substitution eliminated the asbestos fibre, which was no longer justifiable, but not necessarily the reaction pathway, independent of it, from a chromium source, basic oxides, oxygen and thermal stress to calcium and/or sodium chromate.

“Asbestos-free” is therefore no statement as to whether a high-temperature insulation material on chromium-alloyed hot components is chemically inert or carries no risk of chromate formation. A preventively complete substitution must cover both hazard dimensions: it must exclude the asbestos-related fibre hazard and, in the intended application, avoid the formation pathway to chromium(VI) compounds. This presupposes that the insulation contains no chromate-forming basic oxides or that their reaction availability under the conditions of use is reliably excluded.