The topic of asbestos is emotive, as exposure is directly linked as a causative agent to lung diseases and conditions such as mesothelioma and other specific lung cancers, asbestosis and non-malignant pleural disease.
Asbestos related diseases take many years to develop so the current statistic of around 5,000 deaths in 2015 is linked to widespread historical use of asbestos containing products. The UK’s Health and Safety Executive (HSE) publishes annual statistics regarding deaths caused by asbestos related diseases and this number increased steeply over the last 50 years, largely as a result of asbestos exposure prior to 1980. This figure is currently expected to continue at the same level for the rest of the decade before declining.
Whilst the use of asbestos in construction materials in the UK has been banned for some years, the historical legacy continues to cause problems when assessing contamination of the environment from past use. Asbestos containing materials have permeated into the soils around us and their potential to release airborne fibres is clearly something which requires understanding as ultimately it is these airborne fibres which present a risk to human health.
What is asbestos
Asbestos is a naturally occurring fibrous silicate mineral which may be found in the following forms.
All asbestos contains silicon in the form of a silicate lattice and the various types are characterised by the amount of other metal atoms in the lattice. These metals include magnesium, iron, calcium and sodium (in Crocidolite).
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.
Asbestos in the environment
Asbestos has found a wide range of uses in the past due to its properties for sound absorption, tensile strength and its resistance to fire, heat, electrical and chemical damage. These include lagging, cement, insulation boards, gaskets, ropes and blankets, spray and textured coatings and thermoplastic tiles. Through building works, demolition and other physical processes these may be broken down and the final fate of the material may reside within a soil matrix. Once the material has started to break down, the potential to release fibres increases and the associated risk of asbestos fibre release to the air also increases. These fibres may be “respirable” and therefore present the greatest risk.
“once the material has started to break down, the potential to release fibres increases and the risk of asbestos fibre release to the air also increases”
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.
“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”
A simple way to control asbestos fibre release when working on site is to spray the area with a fine mist. Keeping the soil wetted is well known to minimise the potential for fibre release. There is, however, little information available to quantify this effect or even define how effective it may be. 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.
Methods of analysis
Traditional laboratory analysis techniques are based on a three stage process: identification, Gravimetric quantification, and fibre counting/ PCOM quantification.
Using a combination of direct observation and microscopic review fibres are identified by a suitably qualified analyst and subjected to a series of confirmatory processes to qualitatively identify the fibre (asbestos) type and where applicable the Asbestos Containing Material (ACM) type. These processes include looking at the morphology (shape or form) of the fibre and the behaviour of plane polarised light when passed through the fibre under a microscope. The various asbestos fibre types will each show differing characteristic traits, including a clear colour shift when rotated through 90 degrees if mounted in a specific Refractive Index (RI) solution.
“processes include looking at the morphology of the fibre and the behaviour of plane polarised light when passed through the fibre under a microscope”
Following directly from the identification process we can (again using visual inspection and stereomicroscopy) identify and physically remove any fibres, fibre bundles or ACM fragments. Using very sensitive weighing apparatus, we can measure the mass of all the asbestos removed and report that as a percentage (w/w) of the total sample. Any ACM fragments are included as a maximum possible asbestos content using manufacturing concentrations published within the HSG264 Asbestos : The Survey Guide, published by the HSE.
Fibre counting/PCOM quantification
Should there be free fibres present within the sample, it is often prudent to perform a PCOM (Phase Contrast Optical Microscopy) quantification in addition to the gravimetric. It may be that the sample contains either small fibre fragments that cannot be removed and accurately weighed, either due to physical size or mass, or the original review showed no asbestos but the sample proved positive on a pinch test. In these instances we take a fraction of the fine soil fraction and generate a water suspension where the fibre can then be removed by filtration and counted using higher magnification microscopy (PCOM). These results can then be combined with the gravimetric data to provide a quantified mass concentration of asbestos in the sample. As part of the process, each fibre identified is measured (length and width) allowing us to make an assessment of the number of respirable fibres present.
Activity Based Sampling
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. We can, using existing formulae, 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.
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. 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.
“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.”
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.
“this method has been used successfully to assess the required water concentration within the soil to successfully inhibit fibre release”
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.
Within the laboratory at i2 Analytical, this approach has been extensively validated using a range of sample materials spiked with Amosite and Chrysotile fibres, ultimately culminating with the award of UKAS (ISO 17025) accreditation in June 2017. 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 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.