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The prevalence of asbestos in soils is coming to the fore as a key area for investigation of contaminated land sites. Often found in soils through demolition and migration processes, asbestos can be found in a variety of forms – from complete pieces of asbestos containing material (ACMs) through to loose fibres. The nature of the asbestos contamination can present quite different risks to those working on the site and also have a significant influence over remediation strategies, and even future use. Given the potential cost associated with these issues, it’s key to understand laboratory testing that is available to better inform decisions made as part of the land management process.
What is Asbestos and Why do we Find it in Contaminated Soils?
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; 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, however, there are steps that the lab can employ to homogenise material sent for testing. For asbestos, the contaminant may be quite heterogeneously 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 15 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, however, there are still some key areas of difference between laboratories which need to be taken in to account:
- The volume of sample submitted for analysis (typically 500ml –1L – giving 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.
- 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.
- A representative sub sample is then dried for microscopic screening.
- The dried sample is then subjected to microscopic analysis (20-50x magnification) and any suspect material/fibres removed and identified.
- 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.
A note when considering sampling volume and testing volume, it is important to consider taking a sensibly sized representative sub sample for the microscope analysis. For example, if 1kg is submitted 1/10th of this would be approximately 100g; caution should be given to results taken from a much less representative sub sample – say 1/50th (e.g. 20g sub sample from a 1kg submitted sample). It should also be noted that it may not be practical to view more than 100g underneath most stereomicroscopes.
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 % of the whole sample determined. Where Fibre counting is used (using PCOM – phase contrast optical microscopy) the fibres identified are measured and using known density factors; the equivalent mass is determined and used to calculate the asbestos content as a % of the whole sample determined. These two techniques may be used in isolation or together.
What Kinds of Data are Produced
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 laboratory carrying out asbestos identification in soil can produce analytical data which can be used to further inform risk assessors”
A great deal of work has been carried out and the CAR-SOIL guidelines issued by the Joint Industry Working Group (https://www.claire.co.uk/component/phocadownload/category/36-asbestos-in-soil) has led to the generation of complex models requiring detailed data input from laboratory testing.
When a laboratory carried out asbestos quantification analysis, each type of material found is isolated and weighed; additionally individual fibres/bundles of each asbestos type are determined individually. These outputs can be used in the JIWG tool.
So Where is the Risk?
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.
Once the material on site has started to break down, the potential to release fibres increases and the associated risk of asbestos fibre released to the air also increases. These fibres may be “respirable” and therefore present the greatest risk.
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.
“> 0.1% asbestos is classified as hazardous waste, but human health risk from soils is still based on the < 0.001% asbestos in soil”
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.
Measuring the Risk on Site
Often where asbestos is a known risk on site, either airborne fibre monitoring is carried out at intervals (this is retrospective, but may be done on site quickly so can almost give an immediate result) or personal monitors are worn and evaluated at a later time. The risk of doing this is that the assessment of the risk is totally retrospective.
What has previously been ‘missing’ in existing laboratory testing is how we translate the percentage mass of asbestos within a soil into a meaningful assessment of risk. As mentioned, the primary concern for human health is the presence of respirable fibres, and whilst these can be identified as part of the PCOM quantification it gives no indication of the likelihood of release to air under site conditions. It is possible to determine potential air fibre concentrations, but these look at a worst case scenario based on the number of fibres that fit the definition, again without factoring the likelihood of release.
One recent approach to filling in this gap has been to look at Activity Based Sampling (ABS). Put simply, one would simulate normal on site activities under controlled conditions and then measure the actual fibre release using some form of air sampling equipment and then fibre counting techniques using PCOM. The USEPA approached this using personal air sampling kit carried by site workers (suitably attired to protect against inhalation) as they performed various activities either common to the site or its intended function.
In the UK, an approach was taken where an area of the site was enclosed in an air tight tent while the surface was agitated using various pieces of equipment. The dust that was released to the air would be extracted by pump through a filter, and that filter reviewed by PCOM for respirable fibres. Given a known volume of air extracted, a fibres/ml in air concentration can be calculated.
Lab Testing Alternatives
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 being 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. The intention is to generate a fibre concentration in air (fibres/cm3) of the respirable asbestos fibres at a specific level of dust generation.
An approach adopted by a number of UK laboratories is to use a rotating drum method based on an existing British Standard BS EN 15051-2 (Workplace exposure – Measurement of the dustiness of bulk materials – Rotating drum method), 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 the determination of total dust generated, and then using PLM/PCM (Polarising 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.
“an approach adopted by a number of UK laboratories is to use a rotating drum method based on an existing British Standard BS EN 15051-2”
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.
Data obtained from a comparative study assessing activity based sampling analysis with results from the rotating drum method indeed show a good correlation between the two quite different techniques.
“whilst not intended as a replacement for site monitoring, laboratory based methods represent a valuable weapon in the risk assessment armoury”
Whilst not intended as a replacement for site monitoring, laboratory based methods such as this represent a valuable weapon in the risk assessment armoury, and give an extra level of flexibility to the approach that can be taken on site to ensure the risk from asbestos is known and managed.
So How Can this be Used to Improve Sustainability?
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. This can therefore be optimised to control levels minimising cost and waste.
This technique has also been employed successfully within bench studies to try and ascertain the optimal conditions for mitigating risk on a site. Repeat testing of submitted material under varying conditions can allow the identification of the point at which fibre or dust release is kept to within acceptable parameters. This method has been used successfully to assess the required water concentration within the soil to successfully inhibit fibre release and also the appropriate concentration to add various binding materials, again with the purpose of reducing fibre release.
“understanding the potential fibre release from a material may allow a previously unknown risk to be evaluated”
Understanding the potential fibre release from a material may allow a previously unknown risk to be evaluated and more cost effective measures put in place, or even the re-use of asbestos containing soils where it is shown that fibre release is minimal and this is suitable to the intended use of the site.
Additionally, when using the test to characterise the use of binding materials, it could be possible to completely re-purpose “waste” materials on site when the treatment carried out minimises (or even completely inhibits) fibre release.
