High aspect ratio materials: role of surface chemistry vs. length in the historical "long and short amosite asbestos fibers"

Our plain-English summary of High aspect ratio materials: role of surface chemistry vs. length in the historical "long and short amosite asbestos fibers", by M. Tomatis, F. Turci, R. Ceschino, et al (Inhalation Toxicology, 2010).

The study

• Authors: M. Tomatis, F. Turci, R. Ceschino, C. Riganti, E. Gazzano, G. Martra, D. Ghigo, B. Fubini
• Journal: Inhalation Toxicology
• Published: 2010;22(12):984-98
• Cited by: 36 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

In nanotoxicology the question arises whether high aspect ratio materials should be regarded as potentially pathogenic like asbestos, merely on the base of their biopersistence and length to diameter ratio. A higher pathogenicity of long asbestos fibers is associated to their slower clearance and frustrated phagocytosis. In the past decades, two amosite fibers were prepared and studied to confirm the role of fiber length in asbestos toxicity. Long fiber amosite (LFA) and short fiber amosite (SFA) have here been revisited, to check differences in their surface properties, known to modulate the biological responses elicited. We report: (i) micromorphology (abundance of exposed cylindrical vs. truncated surfaces; (ii) surface reactivity (oxidation and coordination state of surface iron, free radical generation and oxidizing potential); (iii) activation of nitric oxide (NO) synthase in lung epithelial cells, as representative of an inflammatory cell response. LFA shows a higher free radical yield, stimulates, more than SFA, NO production by cells and reacts with ascorbic acid, thus depriving the lung lining layer of its antioxidant defenses. The higher activity of LFA than SFA is ascribed to the presence of Fe2+ ions poorly coordinated to the surface. SFA shows only a large number of loosely bound Fe3+ ions, pristine Fe2+ ions having been oxidized during the grinding process converting LFA into SFA. Several factors determine a higher toxicity of LFA than SFA, beside length. The lesson from asbestos indicates that other features besides aspect ratio contribute to the pathogenic potential of a fiber type. All these aspects should be considered when predicting the possible hazard associated to any new fibrous material proposed to the market, let alone nanofibers.

In plain English

In laboratory tests, the authors re-examined two historical samples of South African amosite asbestos: long fibers (LFA) and short fibers (SFA) ground from LFA. Earlier work used them to study length; this study asked whether grinding also changed surface chemistry. Grinding preserved crystal structure, but surface area rose from 6 to 11 square meters per gram and the short fibers' surface iron was more oxidized. Per unit of surface, long fibers released more reduced iron (2.5 versus 0.6 micromoles per square meter) and consumed more of the antioxidant ascorbic acid (20 versus 16). In one free-radical test, long fibers generated carbon-centered radicals while short fibers gave nearly undetectable traces until ascorbic acid was added; even then they yielded about half as much per unit of surface. In human lung epithelial cells (A549), long fibers at 35 micrograms per square centimeter increased nitric oxide production; short fibers had no effect at any tested dose. Glass wool control fibers had no significant effect. The authors concluded that differences once attributed only to length might also be due to surface changes from grinding.

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: Laboratory (in vitro) study: physical and chemical tests on fiber samples without cells, plus tests on cultured human lung cells
• Who was studied: Two historical amosite samples: long fiber amosite (LFA) and short fiber amosite (SFA). The Institute of Occupational Medicine (Edinburgh, UK) supplied them; Johns Manville Corporation had prepared them from the same batch of South African amosite. Cell tests used the human lung epithelial cell line A549. MMVF10 glass wool fibers served as a negative control.
• Exposure: Amosite asbestos, long versus short fibers. SFA was made by grinding LFA in a ceramic ball mill. SFA lengths ranged from 1 to 10 μm, with less than 1% longer than 5 μm. LFA lengths ranged from 2 to 100 μm, with more than 30% longer than 5 μm. Cells were exposed for 24 h to 0.35 to 35 μg/cm2 of LFA or SFA, or to 35 μg/cm2 of MMVF10.
• Main results:
  • Grinding did not change the crystal structure: both LFA and SFA showed the typical pattern of fibrous grunerite (amosite) on X-ray diffraction and Raman spectra.
  • Surface area was 6 m2/g for LFA and 11 m2/g for SFA.
  • At 6 Torr, SFA adsorbed about twice as much nitric oxide gas in total (4.7 μmol/m2) as LFA (2.5 μmol/m2). The authors read this as more poorly coordinated surface iron on SFA.
  • X-ray photoelectron spectroscopy showed an iron binding energy of 55.8 eV for LFA and 56.6 eV for SFA. The authors interpreted this as iron in a higher oxidation state on the SFA surface, probably from grinding.
  • Oxidized iron (Fe3+) removed by deferoxamine over 7 days: SFA 22 μmol/m2, LFA 16 μmol/m2.
  • Reduced iron (Fe2+) removed by ferrozine: LFA 2.5 μmol/m2, SFA 0.6 μmol/m2.
  • Both amosite samples released hydroxyl radicals in the presence of hydrogen peroxide. SFA appeared more potent per mass, but the authors reported non-significant differences per unit surface. MMVF10 generated no hydroxyl radicals.
  • In the formate test without a reducing agent, LFA generated carbon-centered radicals while SFA generated nearly undetectable traces. With ascorbic acid added, SFA became active. Per unit surface, LFA still generated about twice what SFA generated with ascorbic acid.
  • Ascorbic acid consumed after 4 days: LFA 20 μmol/m2, SFA 16 μmol/m2, MMVF10 4 μmol/m2.
  • Cysteine consumed by the end of incubation: SFA 0.8 μmol/m2, LFA 0.5 μmol/m2, MMVF10 0.15 μmol/m2.
  • In A549 cells after 24 h, SFA had no effect on nitric oxide synthesis at 0.35 to 35 μg/cm2. LFA at 35 μg/cm2 increased both extracellular nitrite and intracellular NO synthase activity.
  • MMVF10 glass fibers at 35 μg/cm2 did not significantly change nitrite release or NO synthase activity in A549 cells compared with controls.
• The authors' conclusion: The authors concluded that the different biological activities of LFA and SFA, previously attributed only to fiber length, might also be partly due to surface properties acquired when the long fibers were fragmented into short ones. They suggested that parameters beyond aspect ratio should be measured when assessing nanofibers.
• Limitations the authors state:
  • The authors state that the iron-to-silicon ratios measured by X-ray photoelectron spectroscopy are only semi-quantitative.
  • The authors state that the greater nitric oxide activation by LFA could have either of two causes: reduced iron (Fe2+) sites present on LFA but absent on SFA, or the different levels of oxidized iron (Fe3+) on the two samples.
  • The authors state that the biological effect of excess iron at the fiber surface is still unclear.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Amosite.

Medical terms in this study, explained in our Lexicon: Aspect Ratio (Length-to-diameter ratio), Amosite, biopersistence, Frustrated Phagocytosis, Fiber Length, Surface Reactivity.

Related studies

• Long and short fiber amosite asbestos alters at a different extent the redox metabolism in human lung epithelial cells — C. Riganti, E. Aldieri, et al. (2003)
• Distinct Pro-Inflammatory Mechanisms Elicited by Short and Long Amosite Asbestos Fibers in Macrophages — R. Leinardi, J.R. Petriglieri, et al. (2023)
• Effect of chemical composition and state of the surface on the toxic response to high aspect ratio nanomaterials — B. Fubini, I. Fenoglio, et al. (2011)
• Potential toxicity of nonregulated asbestiform minerals: balangeroite from the western Alps. Part 2: Oxidant activity of the fibers — F. Turci, M. Tomatis, et al. (2005)
• Chemical differences between long and short amosite asbestos: differences in oxidation state and coordination sites of iron, detected by infrared spectroscopy — A. Graham, J. Higinbotham, et al. (1999)
• The iron-related molecular toxicity mechanism of synthetic asbestos nanofibres: a model study for high-aspect-ratio nanoparticles — F. Turci, M. Tomatis, et al. (2011)

Browse more research in Inhalation Toxicology, or return to medical research.