Improvements in methods
There have been efforts to improve the analysis for asbestos in the last ten years:
Analysis methods changed significantly around 2011-2014 with more scrutiny from UKAS. Prior to this, laboratories tended to offer a standalone visual screen to determine if asbestos containing material was present in the sample, but this would only cover pieces of asbestos containing material (ACM) and fibre bundles, and would not include small fragments or free fibres.

• A Blue Book method has been developed and while still in draft the basic methodology has generally been adopted by the majority of laboratories across the UK.

The Blue Book method includes identification of asbestos fibres under a microscope. This analysis is a time intensive process using analysts with a high level of skill and training to identify, count and measure fibres on the filters examined under the microscope.

Remaining Variability
Despite these changes there have been a number of comments on the variability of soil analysis for asbestos including an article in Geoenvironmental Matters which states that “Certainly, it has been rumoured that remediation contractors have come to know which laboratories quote “find asbestos” and which ones don’t.”[1]

[1] Is the quantification of Asbestos in Soils still a lottery? http://geoenvmatters.com/is-the-quantification-of-asbestos-in-soils-still-a-lottery/

In the light of these comments we carried out a review to try to determine the sources of variability across a number of laboratories. As part of this, we spoke to six laboratories and asked about their processes.

Asbestos Screen
A key issue we identified was inconsistency in the asbestos screen (Stage 1 of the blue book method). This is a critical step in all asbestos analysis in soil. If no asbestos is detected in the screen, then typically no further quantification is carried out.

The Stage 1 screening process involves three steps:
1. visual screen of the whole sample.
2. inspection of a dried sub-sample under a stereo microscope (x20 – x40).
3. small representative ‘pinch’ samples mounted on microscope slide at a higher magnification (x80 – x500) using Polarized Light Microscopy (PLM)/ Phase Contrast Microscopy (PCM) techniques.

If asbestos is found at any stage the screen is halted and asbestos is reported as being present.

We had heard rumours that not all laboratories were drying the samples prior to the second stage of the screen, thereby potentially making asbestos harder to detect, however at the time of our review in Spring 2018 the requirement to dry had been introduced into the draft Blue Book method and all the laboratories we spoke to were drying samples prior to the inspection under a stereo microscope.

For this second step of the screen we did encounter wide variation in mass of the sub-sample ranging from 20g to 100g and the amount to be used is not specified in the standard. There is no detection limit on the asbestos screen. It, however, seems obvious that a laboratory screening a larger sub-sample is likely to have a lower detection limit than a laboratory screening a smaller sub-sample but also may have higher costs as the process is more time intensive.

Further Quantification
Stage 2 in the Blue Book method is gravimetric analysis. This involves identification and removal of visible ACMs for gravimetric analysis and subsequent detailed Stage 2 analysis. The detailed Stage 2 analysis comprises inspection of a representative sub-sample under a stereo microscope and the removal of smaller ACM fragments and fibre bundles for identification and gravimetric analysis to determine asbestos percentage by weight.

We note that typically the laboratories tended to use a similar mass of sub-sample for Stage 2 gravimetric analysis to that used in the Stage 1 screening. Interestingly the latest draft of Blue Book sets out that a 20g to 50g sub-sample should be taken forward for Stage 2. Those who have previously taken a larger sub-sample could be at a disadvantage in terms of technical compliance with the Blue Book method and analysis cost, even though their method has a greater chance of finding asbestos. The latest draft of the Blue Book may thus push some laboratories towards a lower sensitivity screening and gravimetric quantification method.

From our discussions it is evident that most laboratories currently carrying out asbestos quantification analysis report the concentration of asbestos from the Stage 2 gravimetric analysis as a single value combining different types of ACM with the mass of fibre bundles. Some of the laboratories have indicated they are able to provide a breakdown of these fractions on request which could be very valuable for those carrying out risk assessment.

Summary
As the CIRIA guidance C733 on Asbestos in soil and made ground states, it is important that asbestos analysis is done well. In the light of the above, we recommend that those procuring laboratory analysis for screening of asbestos in soils using accredited laboratories should discuss the sample preparation and sub-sampling with their laboratory to gain a greater understanding of the quality of the analysis being carried out and its sensitivity. The information on the method should be included alongside the analysis results to enable those using the data to understand its potential limitations. Discussion with the laboratory may also help further increase the understanding of the results of subsequent gravimetric quantification.

Written by Barry Mitcheson, Principal Consultant at Wood Environment and Infrastructure Solutions UK Limited