Under the Microscope

What is asbestos and why do we find it in contaminated soils?

by Dr Claire Stone


Asbestos is a term used to refer to a group of six fibrous silicate minerals. They are known by the common names: Amosite, Chrysotile, Anthophylite, Tremolite, Actinolite and Crocidolite. Asbestos fibres can penetrate into the terminal bronchioles of the lungs and may give rise to asbestosis and mesothelioma.

Asbestos minerals have a range of desirable properties including: sound absorption, tensile strength, its resistance to fire, heat, electrical and chemical damage; and as such these minerals have been mined and used for a wide range of products for thousands of years. During the late 19th and 20th centuries the processing and use of asbestos in construction materials increased in prevalence. Industrial buildings were constructed from asbestos containing tiles and roofing sheets; and even in the domestic home many buildings
have textured coatings which contain asbestos. Often these materials make their way through to soils and made ground through deconstruction processes; although this is just one source of many. Once material is in the ground it can be disturbed by earth moving processes leading to the degradation of the parent material and subsequent release of fibres which may then be bound up in the soil matrix.

Sampling challenges

Obtaining a representative sample for environmental chemical analysis is always a challenge, but there are steps that the lab can employ to homogenise material sent for testing. For asbestos, the contaminant may be quite inhomogeneously distributed in the soil and therefore field sampling is critical to ensure that a sufficient number of representative samples are taken, and that the sample volume taken is suitable for final laboratory analysis.

Laboratory analysis

In the UK over the last 10 years the standard of laboratory testing has been improved by the mandatory requirement for the identification of asbestos in soils to be an accredited parameter. Whilst it is not unusual for each laboratory to have their own in-house accredited procedure, the actual analysis and identification of asbestos in the sample is identical. This is again the case with laboratory quantification methods. Much work has been done to produce a standard method (the Standing Committee of Analysts Blue Book) and this can be downloaded from the SCA website: www.standingcommitteeofanalysts.co.uk

However there are still some key areas of uncertainty:

• The volume of sample submitted for analysis (typically 500ml to 1l gives rise to a sample volume of approximately 1kg)
• The mass of dried sample inspected by stereomicroscopy

To understand the impact of these, the analytical process must be understood. All steps are undertaken in a controlled environment to minimise any cross contamination of samples and any health and safety risks to staff:

  1. The sample submitted to the laboratory for asbestos testing is visually examined for any large pieces of asbestos containing material. If any are found these are tested and identification confirmed.
  2. A representative sub sample is then dried for microscopic screening.
  3. The dried sample is then subjected to microscopic analysis (20-50x magnification) and any suspect material/fibres removed and identified.
  4. Identification is through the traditional polarising light technique, which is detailed extensively in the publication: HSG 248 – The analysts’ guide for sampling, analysis and clearance procedures; and as such this will not be discussed in any detail.

It is clear that if approximately 1kg is submitted for analysis and the final test mass by microscopic technique is less than one 20th of that 1kg, then this may not be as representative as if say a 10th of the sample is examined.

Further commentary on this can be found in a variety of publications including the AGS, otherwise known as the Association of Geotechnical and Geoenvironmental Specialists – www.ags.org.uk/2019/02/ variability-in-asbestos-analysis-in-soil/).

Quantification analysis can be carried out in two complementary ways: Gravimetric Analysis or Fibre Counting. Where gravimetric analysis is carried out, all ACMs and fibre bundles are determined gravimetrically and their asbestos content as a percentage of the whole sample is then determined.

Where fibre counting is used, the fibres identified are measured and using known density factors; the equivalent mass is determined and used to calculate the asbestos content as a percentage of the whole sample determined. These two techniques may be used in isolation or together.

What kinds of data are produce

A laboratory carrying out asbestos identification and subsequent quantification in soil can produce a great deal of analytical data, which can be used to further inform risk assessors of the types of material in the soil and the risk associated.

A great deal of work has been carried out and the CAR-SOIL guidelines issued by the JIWG, or Joint Industry Working Group, (https://www. claire.co.uk/component/phocadownload/category/36-asbestos-in-soil) which have led to the generation of complex models requiring detailed data input from laboratory testing.

When a laboratory carries out asbestos quantification analysis, each type of material found is isolated and weighed. Additionally, individual fibres/bundles of each asbestos type are determined separately. As such, a laboratory will have available the following information:

• Total dry mass of sample
• Asbestos type (from PLM analysis) in each ACM type, if required
• Total mass percentage of asbestos
• Total mass percentage of amphibole asbestos (possible breakdown into amphibole types)
• Total mass percentage of chrysotile asbestos • Total gravimetric (ACM) mass percentage
• Total gravimetric (ACM) percentage of each ACM type
• Total mass percentage of free fibres
• Total mass percentage (or fibres/g) of respirable fibres
• Mass or percentage of material analysed
Asbestos containing materials found include:
• Loose insulation
• Blanket, tape, cloth, rope and string
• Paper, felt* (excludes any non-asbestos composite component)
• Millboard
• Compressed fibre gaskets
• Sprayed coating
• Thermal insulation – composite
• Thermal Insulation – caposil/caposite
• Loose fibrous asbestos debris*
• Brake pads, clutch plates
• Cement
• Insulating board (excludes any non-asbestos composite component)
• Asbestos sheeting/board debris**
• Cement – ‘asbestos wood’
• Thermoplastic floor tiles
• Thermal insulation – sectional
• Reinforced plastic and resin composites
• Bitumen felt, DPC etc.
• PVC floor tiles
• Textured coatings
*Debris not readily identifiable as coatings or insulation
**Debris not readily identifiable as AIB or any other board type

Where a material has been found as debris, (i.e. the original material type can only be assumed and not confirmed); the JIWG tool uses inputs regard the degradation and weathering of the material. This information is often best gleaned on site; however, laboratory photos and comments may assist in the identification of the following status:

  1. Intact – very good condition ACM/ACM fragments
  2. Weathered – slight degradation in ACM; material still retains its basic integrity
  3. Degraded – significant degradation in ACM; material has lost its basic integrity
  4. Disaggregated – dominated by loose fibrous material; extreme degradation in ACM and/or free asbestos fibres/fibre bundles

Alternative analysis methods

The methods discussed so far are fairly routine and common place across most environmental testing laboratories; however, testing methods are constantly evolving, partly with the desire to determine lower and lower concentrations of asbestos fibres in samples and also to evaluate other mineral silicate fibres which may be present and could also be hazardous to health.

“testing methods are constantly evolving, partly with the desire to determine lower and lower concentrations of asbestos fibres in samples”

Alternative methods employed include the use of Scanning Electron Microscopes (SEM). These are used commonly across Europe and through the manner of the analysis they allow for accurate discrimination of the fibres found by chemical fingerprint confirmation. Their use for the analysis of soils, however, is somewhat limited at present and the equipment is quite costly.

In the USA, ASTM methods also promote the use of electron microscopy as Transmission Electron Microscopy (TEM). Both of these are now emerging into use for the analysis of asbestos in air samples and swabs as routine techniques.

Respirable fibres

It is accepted that the primary source of risk from asbestos is the inhalation of respirable fibres. A respirable fibre is one classified as having a length of greater than 5µm, a width of less than 3µm with an aspect ratio (length:width) of greater than three. These fibres are ones capable of penetrating to the terminal bronchioles of the gas exchange region of the lung and represent the most significant hazard to human health as they have the potential to cause asbestosis and mesothelioma with only minimal exposure.

Exposure, control and SGV limits

The UK Control of Asbestos Regulations 2012 (CAR 2012) came into force on 6 April 2012, updating previous asbestos regulations to take account Directive 2009/148/EC. This document defines a control limit for asbestos of 0.1 fibres /cm3 air. This control limit is not a ‘safe’ level and exposure from work activities involving asbestos must be reduced to as far below the control limit as possible.

There is little information available, however, regarding acceptable or known risk levels of asbestos in soils and other environmental matrices. In terms of waste, material containing > 0.1% asbestos is classified as hazardous waste, but human health risk from soils and associated materials is still based on the < 0.001% asbestos in soil. The < 0.001 % value is derived from a research paper published by Addison et al working at the IOM. This figure was obtained by defining the limit at which a soil sample, under certain conditions may release asbestos fibres at the control limit. This figure has never been ratified by any regulator.

Lab testing

It may not always be possible or appropriate to perform site based monitoring, and the laboratory world has responded by looking at differing options for lab based methods which can help provide the same levels of data for risk assessment. Varying approaches have been taken across the UK, US and Australia (amongst others), with the basic principle of a fine soil sample being agitated in some manner under a controlled flow of air with fibres collected via filtration and fibres identified using PCOM. This is, I appreciate, a fairly concise summary of a significantly more complex process and the specifics of each approach vary somewhat in the doing, but the intention is to generate a fibre concentration in air (fibres/ml) of the respirable asbestos fibres at a specific level of dust generation.

At i2, an existing British Standard BS EN 15051-2 (Workplace exposure – Measurement of the dustiness of bulk materials – Rotating drum method) was used as a basis for the technique, adapting the process to focus specifically on respirable asbestos fibres.

The method itself uses a rotating drum to perform the process of sample agitation under a continual flow of air. A series of foam and membrane filters are used to collect the dust fractions generated, allowing us to gravimetrically quantify the amount of total dust generated, and then using PLM/PCM (Polarizing Light Microscopy/Phase Contrast Microscopy) quantify the number of respirable fibres released. Each fibre is measured (both length and width) with the aspect ratio calculated to ensure it meets the criteria for respirable status.

From the empirical data, the concentration of dust generated in mg/m3 and also the fibre concentration in air in fibres/cm3 can be calculated.

Based on dust levels specified by the user, the fibre concentrations can be normalised to any specific level of dust – either to meet a particular standard/exposure limit or to match known concentrations from on-site dust monitoring.

A success story

The method was originally designed to look at a pre-dried sample, as this would give a maximal fibre and dust release, but it is possible to vary the moisture content (either using the as-received material or by re-adding moisture to a required level) to fit the criteria that is relevant and is required for the investigation. Typically, the test would be carried out on the finest fraction of the submitted sample (PCOM fraction) as a worst case scenario, but it is possible to process any part of the sample provided. Ultimately, the questions posed on site are to be answered by varying the test parameters to more closely match the requisite conditions.

Further work

Asbestos in the environmental sector is an emotive subject and is now a key focus area of a number of industry working groups including: the Society of Brownfield Risk Assessors (SoBRA – www.sobra.org.uk), Network for Industrially Co-ordinated Sustainable Land Management in Europe (NICOLE – www.nicole.org), as well as the aforementioned joint industry working group where information and publications are available via the CL:AIRE website (www.claire.co.uk).

A recent SoBRA white paper has indicated that the critical level of interest for amphibole asbestos types may lie comfortably below the standard reporting limit for established analytical methods such as the previously mentioned SCA Blue book, and as such work must be done within the lab community to determine cost effective and robust ways of achieving this.

Change of perspective

What if we look at things slightly differently? It’s all very well knowing the types and concentrations of asbestos in soil, but does this provide adequate information regarding risk? Interest in activity based sampling or laboratory techniques that reliably and robustly replicate soil disturbance to look at fibre release is increasing. A highly contaminated soil that contains non friable pieces of ACM, which are less likely to break down, may present a lower risk than a soil with less ACMs present, where these ACMs are highly friable and as such present a significant risk of fibre release.

Author Details

Popular Articles by Dr Claire Stone