1 MDHS. Methods for the Determination of Hazardous Substances 87. Fibres in air Health and Safety Laboratory Guidance on the discrimination between fibre types in samples of airborne dust on filters using microscopy November 1998. introduction determination of results above the clearance indicator demonstrates that the enclosure is not clean, even if 1 At present there are two approved methods for some of the dust is non-asbestos. However, in determining airborne fibre concentration; one for asbestos exceptional circumstances it can be applied to clearance (MDHS 39/41) and one for man-made mineral Fibres testing. For example in asbestos removal operation, after (MMMF) (MDHS 592). Both methods are based on cleaning or where non-asbestos dust is suspected of collecting particulates by drawing air through a membrane being drawn into the enclosure from operations outside filter which is analysed at 500x light microscopy. As the and increasing the fibre' concentration (eg gypsum analysis relies on manual fibre counting of a relatively particles from plaster board, or MMMF Fibres from glass small number of Fibres , the method is recognised as one of fibre insulation).
2 This discrimination method must not be the least precise analytical techniques used in the used for the assessment of compliance to the control occupational environment. The fibre count is subject to a limits for asbestos in MDHS 39/41 or to the maximum number of systematic and random errors as well as exposure limit for MMMF in MDHS individual counter bias. This bias is particularly important if an attempt is being made to discriminate between fibre Note: The European Reference Method and the types and may have a large influence on the accuracy of Approved Methods described in MDHS 39/41 and the results. This MDHS gives a strategy and method for MDHS 592 could be superseded in 1999 by a World fibre discrimination based on the optical, crystallographic Health Organisation method which may be adopted by the and chemical properties of the Fibres . It should be European emphasised that discrimination using a light microscope is, in general, not easily carried out, and depends on the skills 3 To assess compliance with regulations, airborne fibre of the analyst and the techniques available.
3 Concentrations are measured by sampling a known volume of air through a membrane filter and counting the numbers 2 For regulatory purposes in the UK, the Control of of Fibres (>5 m in length, <3 m in width and with an aspect Asbestos at Work Regulations (CAWR)3 define asbestos ratio >3:1) in a number of graticule areas using 500x phase as any of the following fibrous materials (or any mixture contrast light microscopy (PCM). However, the use of PCM. containing them): chrysotile, amosite, crocidolite, fibrous alone does not give sufficient information to positively anthophyllite, fibrous actinolite and fibrous tremolite; the discriminate between respirable fibre types. No single diseases asbestosis, lung cancer and mesothelioma have technique is capable of identifying all Fibres : different been associated with inhalation of asbestos. MMMF is the techniques must be used for different Fibres . Specific term used for various inorganic materials which have been methods are available for most fibre types: for example, made into Fibres : they have names such as rock wool', analytical transmission electron microscopy (TEM) has the slag wool' and refractory (or ceramic) MMMF' (sometimes potential to identify all airborne asbestos Fibres and ultra.)
4 Referred to as MMVF), and are subject to a maximum violet (UV) fluorescence microscopy can identify para- exposure limit (MEL) under the Control of Substances aramid Fibres . Considerable care must be taken when Hazardous to Health (COSHH) Regulations, COSHH discriminating between fibre types. ACOPs and EH40 (revised annually).4 In some situations it is important to distinguish between regulated and non- regulated fibre types when fibre evaluations are carried GENERAL METHOD. out. This may be applicable to situations described in MDHS39/41 as background sampling, leak (enclosure Principle check) sampling for assessment of respirator performance and reassurance. It is not recommended that this be 4 This method provides Guidance on various techniques generally applied to clearance testing, since the which may be used to discriminate between fibre types. 1. It should only be used after the routine evaluation by above) at least two characteristic properties of each fibre PCM has been completed.
5 The recommended should be examined to permit discrimination. techniques covered by this MDHS are: 7 The analyst should choose the most appropriate Polarised Light Microscopy (PLM); technique for the strategy selected (see Table 1). However, UV Fluorescence Microscopy; if the types of fibre are not known, a decision hierarchy Scanning Electron Microscopy (SEM), with Energy may be adopted (see Appendix 1). The results and their Dispersive X-ray Analysis (EDXA); implications should be evaluated after each analysis. Transmission Electron Microscopy (TEM), with EDXA. and Selected Area Electron Diffraction (SAED). Table 1: Guidance in selection of the appropriate method This table helps the analyst to select the appropriate method and strategies The last two of these techniques rely either on duplicate for different fibre types based on the capabilities and limitations of the samples having been taken or on half of the original filter methods. being available.
6 (Re-sampling might be adopted but the airborne Fibres , both type and concentrations, may be Methods different to the initial sample.) PCM/PLM PCM/UV SEM-EDXA TEM-EDXA- fluorescence SAED. Scope Primary fibre Strategies analysed 5 Following evaluation of fibre concentration by PCM, this method employs various microscope techniques to Asbestos Exclude other Not Include Include all examine crystallographic and optical properties, and Fibres >1 m recommended asbestos Fibres element compositions, which may permit discrimination diameter Fibres > m between certain fibre types. Discriminating decisions (limited use diameter if some types should be taken with caution and should not be based on of other the observation of a single characteristic (such as mineral morphology or UV fluorescence) unless additional Fibres are information on the environment from which the sample was present). taken is available. The extent to which this method can be MMMF Include Not Include Can be useful implemented will depend on the equipment available and MMMF recommended MMMF Fibres for Fibres from upon the training of, and care taken by, the microscopist.
7 Fibres > 1 m > m known source diameter diameter, can Strategy which are be useful if isotropic source of fibre is known 6 Various strategies for fibre discrimination may be considered depending on the purpose of the analysis Other Exclude Not Include Fibres Include all and the degree of identification required: Table 1 lists the mineral Fibres other Fibres recommended > m Fibres ; can >1 m diameter, if identify or microscopical methods which are the most useful, and diameter, eg source is discriminate notes their applicability to various fibre types. rutile needles known: can between Discrimination can be applied in two ways, either (cannot be used to fibre types POSITIVELY' when a fibre is IDENTIFIED and then always be discriminate INCLUDED in the count, or NEGATIVELY' when a fibre used to between discriminate some types can be shown to differ from the type being evaluated and between of mineral therefore is EXCLUDED from the count. Thus it is different types fibre essential first to define the purpose of the fibre count, of mineral and second to decide which strategy is appropriate.)
8 Fibres ). Synthetic Not Inlcude Exclude Exclude other Examples of strategies are: organic recommended, certain types inorganic inorganic Fibres Fibres except for a of organic Fibres > m of all widths positive identification of asbestos Fibres present by known source fibre from a diameter which give an TEM-EDXA; which may known source EDX spectrum have a which positive identification of para-Aramid Fibres by UV characteristic fluoresce at fluorescence; property specific exclusion of MMMF from a PCM/PLM fibre count; wavelengths exclusion of para-Aramid from a PCM/UV fibre count. Each technique (PCM/PLM, SEM-EDXA or TEM-EDX- Reference material SAED) has limitations and may give different answers. Therefore, it is necessary to understand the techniques 8 It is essential that the laboratory has a range of and how they can be used. Table 1 provides a guide for reference materials appropriate to the fibre types being the main classes of Fibres encountered.
9 Even when discriminated. Bulk samples should be collected, where populations of Fibres exhibit certain characteristics, these possible, as a reference material, preferably at the same may not be shown by all individual Fibres : thus, unless site as the air sample. Laboratories can prepare their own further information is available (as noted in paragraph 5 sets of reference samples containing Fibres of interest. 2. INITIAL PCM FIBRE COUNT or when other changes (for example, introduction of a high resolution objective) are made, that the same Fibres are Sample preparation analysed. Analysts should be aware that the discrimination between Fibres <1 m diameter may not be possible using 9 The membrane filter must be prepared using the 40x, light microscope objectives. In these relevant procedure (MDHS 39/41 or MDHS 592). If circumstances, discrimination may be possible only by using additional analysis (for example, TEM) is anticipated, additional equipment with higher resolution objectives.)
10 Samples and blank filters should be cut in half with a scalpel using a rolling action with the filter carefully held at The characteristics which can be observed by the the edge. Half of the filter then can be mounted, and the respective techniques are summarised in Table 2 and other half can be kept for the subsequent investigation. detailed in Appendix 4. Table 2: Properties, and the techniques by which Fibres may be Fibre counting observed 10 Equipment and fibre counting procedures must accord Techniques and identification mode with those specified in MDHS 39/41 or MDHS There should be no discrimination at this stage. PCM PLM UV SEM TEM. Results Property Plane Crossed First Fluores- EDXA EDXA . polarised polars order red cence SAED. (or other 11 The results of the initial PCM fibre count will determine light compen . if there is a need for further analysis. For example, if sator). reassurance sampling is being carried out and the result Morphology.