Reanalysis of historic elemental speciation filters to investigate the presence of fibrous mineral particles using microscopy techniques

Our plain-English summary of Reanalysis of historic elemental speciation filters to investigate the presence of fibrous mineral particles using microscopy techniques, by N. Talbot, K.N. Dirks, W. Fan, et al (Frontiers in Chemistry, 2022).

The study

• Authors: N. Talbot, K.N. Dirks, W. Fan, H. Patel, S.B. Costello, M. Brook, P. Davy
• Journal: Frontiers in Chemistry
• Published: 2022;10:1032624
• Cited by: 3 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

A case is presented for the value of archiving air quality filters to allow for retrospective analysis of emerging contaminants, that is filter constituents not considered to be harmful (and thus not identified or quantified specifically) at the time of collection but subsequently considered to be of interest. As an example, filters from a 20-year historical archive consisting of 16,000 filters from three sites across Auckland are re-examined for the presence of elongated mineral fibres known to be present in rock across the city. Originally collected for the purpose of the source apportionment of particulate matter, 10 filters from each of the three sites were chosen for reanalysis based on their high silica and aluminium content, and thus considered more likely to contain fibre-like particles (FLP). These filters were analysed using various microscopic methods, including phase contrast microscopy (PCM), scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). The results show that although the commonly used fibrous polytetrafluoroethylene (PTFE) material of the filters may hamper the visual identification of any fibre-like particles under a certain length, their key components are able to be identified using a combination of PCM and SEM when they are of a suitable dimension and have settled in a certain orientation on the filter. In this case, the use of EDS confirmed the silicon content of the fibres and also revealed elemental spectra. Although the exact identification of the mineral fibre is uncertain, the EDS scan is consistent with hazardous zeolites such as erionite, known to be present in the rock found in Auckland. This study highlights the value in maintaining filter archives for the purpose of investigating the historical evolution of emerging atmospheric pollutants.

Copyright © 2022 Talbot, Dirks, Fan, Patel, Costello, Brook and Davy.

In plain English

This laboratory study tested whether archived air-pollution filters from Auckland, New Zealand, could be re-examined for fibre-like mineral particles, especially the fibrous zeolite erionite, which is found in rock around the city. The authors chose 30 filters with high aluminium and silica content, 10 from each of three sites: rural Patumahoe, central Queen Street, and suburban Henderson, which lies within 5 km of known erionite-bearing rock. Phase contrast microscopy found few or no fibres on the Queen Street and Patumahoe filters, and Henderson filters had the most. Eight Henderson filters from May 2011 then underwent electron microscopy with chemical (EDS) analysis. This found long, thin, silicon-rich fibres that the authors described as respirable. One fragment on the 14 May 2011 filter had a chemical spectrum closely matching a known erionite sample from Auckland bedrock. The authors stated that the material and its origin could not be identified with certainty, that the results were indicative only, and that the method was not designed to quantify fibres on the filters. They concluded that further investigation is warranted and that filter archives are valuable for this kind of analysis.

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 methods study re-examining archived air-quality filters with microscopy (phase contrast microscopy, then scanning electron microscopy with energy-dispersive x-ray spectroscopy)
• Who was studied: 30 archived 24-hour air particulate filters, 10 from each of three Auckland sites, chosen for high aluminium and silica content. The sites were the Patumahoe/Patamahoe rural site (sampled 2010 only), Queen Street (a city-centre shopping street) and Henderson (a residential/light industrial suburb); Queen Street and Henderson were sampled from 2006 onwards. Eight Henderson filters from May 2011 went on to electron microscopy.
• Exposure: Airborne fibre-like mineral particles in ambient city air, specifically erionite and other fibrous silicas and zeolites. Asbestos itself was not studied; the authors note that asbestos-type microscopy methods are potentially the most suitable.
• Main results:
  • Almost all of the woven fibres making up the PTFE filter material itself were less than 1 µm long, so fibres of interest under 1 µm could be hard to tell apart from the filter.
  • On phase contrast microscopy, very few fibres were found on the 10 Queen Street filters. The Discussion states that no fibres were found on any Patumahoe or Queen Street filter.
  • The Henderson filters had the most visible fibre-like particles, found on several filters from around the same period in 2011.
  • The Henderson filter of 14 May 2011, which had the highest fibre count over 30 fields of view, was selected for focus. It sampled 23 m3 of air over 24 hours.
  • On the 14 May 2011 Henderson filter, soil made up 23% of the source apportionment based on aluminium and silicon, compared with a long-term average of 7%.
  • Eight Henderson filters from May 2011 (one sample every third day) were analysed by SEM/EDS.
  • EDS analysis showed silicon-rich fibres on the Henderson filter, which the authors described as appearing long, thin and respirable.
  • The EDS spectrum of one crustal fragment (fragment 2) on the 14 May 2011 Henderson filter closely matched that of erionite, compared against a known erionite bedrock sample from Riverhead.
  • Some silicon-rich particles were hexagonal clustered shards rather than elongated fibres. Their chemical composition was very similar to the reference erionite.
• The authors' conclusion: The authors concluded that the material and its origin could not be identified with certainty. They considered that nearby quarrying, together with the similarity of the zeolite EDS scan on the filter to erionite from Auckland bedrock, suggests further investigation is warranted. They also concluded that the study shows the value of keeping air-filter archives for retrospective analysis.
• Limitations the authors state:
  • Identifying individual fibres with SEM/EDS alone has limitations, and the results are indicative only.
  • Because few fibres were present, the material and its origin could not be identified with certainty.
  • Fibres were counted over 30 fields of view to choose a filter, but the method was not designed to quantify fibres on filters. It aimed only to test whether fibres could be located and identified.
  • The 30 fields of view used for the fibre count covered only a small fraction of the filter area.
  • Fibres under 1 µm were more likely to be embedded in the PTFE filter and hard to distinguish from the filter's own fibres.
  • Filter thickness allowed only partial illumination under phase contrast microscopy, which could make smaller fibres hard to identify.
  • Selecting filters by high soil (aluminium and silicon) content did not reliably pick filters with fibres.
  • Some particles may have been fibres that were crushed or broken into smaller pieces.
  • There is limited literature and no apparent standard method for identifying and quantifying erionite on air-sampling filters.

Topics

Medical terms in this study, explained in our Lexicon: phase contrast microscopy (PCM), Scanning Electron Microscopy (SEM), Zeolite, erionite.

Related studies

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• Alveolar cell stretching in the presence of fibrous particles induces interleukin-8 responses — A. Tsuda, B.K. Stringer, et al. (1999)
• Elemental analysis of asbestos fibers by means of electron probe techniques — I.A. Rubin, C.J. Maggiore (1974)
• Clearing and mounting techniques for the evaluation of asbestos fibres by the membrane filter method — J.M. Le Guen, S. Galvin (1981)

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