Lichens on asbestos-cement roofs: bioweathering and biocovering effects

Our plain-English summary of Lichens on asbestos-cement roofs: bioweathering and biocovering effects, by S.E. Favero-Longo, D. Castelli, B. Fubini, et al (Journal of Hazardous Materials, 2009).

The study

• Authors: S.E. Favero-Longo, D. Castelli, B. Fubini, R. Piervittori
• Journal: Journal of Hazardous Materials
• Published: 2009;162(2-3):1300-8
• Cited by: 19 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Asbestos-cement roofs, the most widespread sources of airborne, toxic and carcinogenic asbestos fibres, are often colonized by lichens. Since these latter are physical and chemical weathering agents, they have been often considered as significant responsible of disaggregation processes increasing fibre dispersion. Consequently, official guidelines for the management of asbestos often suggest their removal. Weathering and/or covering effects of lichens on asbestos-cement, however, have never been deeply investigated and available procedures to evaluate asbestos-cement aging do not take the biological colonization into account. In this study we show that a 25% lichen cover modifies physical and chemical properties of asbestos-cement sheets containing chrysotile and crocidolite fibres. By innovatively coupling pull up tests and image analysis of linear structures, we show that fibre loss is significantly lower ( approximately 30%) where lichens develop and offer a physical barrier to the fibre detachment. Below the most covering lichens (Acarospora cervina, Candelariella ssp.), chrysotile and crocidolite undergo a partial incongruent dissolution, which in laboratory assays generally determined a reduction of their surface reactivity. Because of their biocovering and bioweathering effects, lichens on asbestos-cement play a role which differs from the current public opinion and the assumptions of some official regulations, acting as effective spontaneous bioattenuation agents.

In plain English

This study examined lichens growing on corrugated asbestos-cement roofs of garages dating from the 1960s in the Balangero asbestos-mine area near Torino, Italy. The roofing, made by Eternit, contained chrysotile and crocidolite fibres. In six 100 cm2 sample areas, the authors measured lichen cover. They then pressed adhesive tape onto the roof, pulled it off, and used image analysis to measure the fibre bundles it removed. Lichens covered over 25% of the surface on average, mostly the species Acarospora cervina. Lichen-covered areas gave up significantly fewer fibre bundles than bare areas: 2.8 versus 7.5 centimetres of bundles per square centimetre, which the authors called a reduction of about 70%. Candelariella lichens gave no significant protection. Under lichens, chrysotile fibres had lost magnesium, and crocidolite fibres under Acarospora had lost sodium. Citing earlier laboratory work, the authors suggested these changes could partly inactivate the fibres. They concluded that removing or chemically killing lichens appears counterproductive where asbestos-cement cannot be completely replaced with other materials. They noted that the bundles measured were thicker than the directly inhalable fraction, and that lichens rarely cover whole roofs.

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: Field and laboratory study of lichen-colonized asbestos-cement roofing. It combined lichen-cover surveys, adhesive-tape pull up tests with image analysis of fibre bundles, microscopy, chemical analysis of fibres (SEM-EDS) and X-ray powder diffraction. It did not study people or health outcomes.
• Who was studied: Six sample areas (relevés) of 100 cm2 each on corrugated asbestos-cement roofs of garages dating from the 1960s, in the mining area of Balangero (Torino, Italy). Chemical analyses covered small numbers of individual fibres: 10, 8 and 10 chrysotile fibres (Candelariella, Acarospora, control) and 7, 9 and 5 crocidolite fibres.
• Exposure: Eternit asbestos-cement roofing containing chrysotile fibres from the Balangero mine, plus crocidolite fibres from Southern Africa. The roofs were colonized by lichens, mainly Acarospora cervina and Candelariella species.
• Main results:
  • Across the six sample areas, lichens covered over 25% of the roof surface on average (the table average is 28.6%). Acarospora cervina had the highest average cover, at 24.3%.
  • In pull up tests, lichen-covered areas released significantly fewer fibre bundles than uncolonized areas. The figures were 2.8 versus 7.5 centimetres of fibre bundles per square centimetre, which the authors describe as a reduction of about 70%.
  • Acarospora thalli reduced fibre loss by up to 70%. Candelariella species, which have scattered thalli, did not offer a significant protective shield.
  • Lichen hyphae penetrated the asbestos-cement. Under Candelariella they reached down to 2 to 3 mm. Under Acarospora they reached only a few hundred micrometres.
  • Chrysotile fibres in contact with lichen hyphae showed a significant loss of MgO compared with uncolonized fibres. MgO fell from 45 wt% to 33% under Candelariella and to 27% under Acarospora, with a relative rise in SiO2 from 48 wt% to 54% and 56%.
  • Crocidolite fibres contacted by Acarospora were significantly depleted in Na2O and showed a relative SiO2 increase from 54 to 61 wt%. Changes in crocidolite under Candelariella were not significant by Tukey's test.
  • Calcium oxalate monohydrate was found at the interface between Acarospora and the roof. No oxalates, but minor iron oxy-hydroxides, were found below Candelariella.
  • This study did not test fibre toxicity. The authors relied on earlier laboratory experiments, which reported that similar chemical changes decreased the Fenton activity of chrysotile and crocidolite.
• The authors' conclusion: The authors concluded that lichen cover acts as a partial but effective spontaneous bioattenuation factor on asbestos-cement roofs. They suggested lichen weathering could contribute to partial inactivation of the fibres. In their view, removing or chemically killing lichens appears counterproductive where asbestos-cement cannot be completely replaced with other materials.
• Limitations the authors state:
  • The fibre bundles measured (diameter above 5 micrometres) do not correspond to the fibre fraction considered directly inhalable.
  • Lichens rarely cover whole roofs, and cover values above 25% are only sporadically reported.
  • Higher lichen cover is associated with older roofs, whose uncolonized areas may be more deteriorated.
  • Where old lichen centres decayed, some loss of roof material during the decay could not be ruled out.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Balangero mine, Balangero, about 20 km north of Turin, Piedmont

All asbestos mines

Topics

Medical terms in this study, explained in our Lexicon: Asbestos-Cement, Carcinogenic, Chrysotile, Crocidolite, Surface Reactivity.

Related studies

• Chrysotile asbestos is progressively converted into a non-fibrous amorphous material by the chelating action of lichen metabolites — S.E. Favero-Longo, F. Turci, et al. (2005)
• A biomimetic approach to the chemical inactivation of chrysotile fibres by lichen metabolites — F. Turci, S.E. Favero-Longo, et al. (2007)
• Risk assessment of the decay of asbestos cement roofs — A. Campopiano, D. Ramires, et al. (2009)
• Releasability of asbestos fibers from weathered roof cement — A.F. Oberta, L. Poye, et al. (2018)
• Interactions of sterile-cultured lichen-forming ascomycetes with asbestos fibres — S.E. Favero-Longo, M. Girlanda, et al. (2007)
• Mobilization of asbestos fibers by weathering of a corrugated asbestos cement roof — T. Ervik, S. Eriksen Hammer, et al. (2021)

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