Document G9eYKoqx6GpJ7BbwxjGQya7v

4m Mu. Vd J9, No. 1. pp JS? JkJ. WJ Pttitled mi Greet BroatA Wilt 4*7* *$ JVOO-OOO Ptf|4fm Preu Ud IVN5 Hridvh OciV|WIHNUl II^KM Society ASBESTOS AND OTHER FIBRE LEVELS IN BUILDINGS S. ALTREE-WiLLiAMS't and J. S. Preston} `Occupational Hygiene Consultants. Canberra. Australia ^Scanning Electron Microscopy Unit, Australian National University. Canberra, Australia Abstract--A Nuclcpore filler/SEM method was used to determine the airborne concentration of asbestos and other fibres in office and plant buildings. The monitoring/SEM/XRM A conditions were chosen to match the collection efficiency, fibre detectability and convenience of the membrane filtcr/optical microscope method. Additionally, the SEM/XRMA technique provides identification of counted fibres and a detection limit of I fibre I.'1 under the routine conditions of occupational hygiene sampling. Application of the method to the collection of 193 air samples in the occupied areas of22 asbestoscontaining office and plant buildings gave the following asbestos counts: 22 fibre l."1 II sample): 3 fibre I."1 (3 samples); 2 fibre I.'1 (S samples); I fibre I.'1 (22 samples);0 fibre I.'1 (162 samples). Organic fibre levels ranged from 63-0 fibre I.'1 (median < 4 fibre I." ') Mineral (non-asbestos) fibre levels ranged from 13 to 0 fibre I."1 (median I fibre I.'1)- INTRODUCTION The recognition of the carcinogenic potential of respirable asbestos dust under occupational and para-occupational exposure has led to concern regarding the cancer hazard from asbestos in the environment. In Australia, this concern has focused on the question of airborne asbestos exposure in buildings containing asbestos fire-proofing, thermal insulation, etc. The determination of asbestos in environmental samples is generally recognised as requiring the application of electron microscopy (Beckett, 1973; Middleton, 1982). The membrane filter/optical microscope [MF/OM] method (NH & MRC, 1976; Walton, 1982) used for the evaluation of asbestos at occupational levels is unsatisfactory for this task because (i) the method gives non-zero and variable fibre counts for the blank collection filter (NH & MRC, 1976; Rajhans and Sullivan, 1981) and (ii) the method cannot identify the counted fibres. Point (i) limits the detection limit of the method for a 4-h sample to 10-20 fibres l._l. Point (ii) is important because, at environmental levels, the majority of respirable fibres found are generally not asbestos. The Nuclepore polycarbonate filter/scanning electron microscope method INPF/SEM] can match the collection efficiency, fibre-width detectability, and convenience of the MF/OM method but, additionally, provides a significant improvement in fibre number detection limit and a means of individual fibre identification (Beckett, 1973;Spurny et a/., 1970; Middleton, 1982). It was therefore selected for use in the present study of airborne concentrations of asbestos and other t Ol7 Present address: Division of Chemical and Physical Sciences. Deakin University. Victoria. Australia 357 10003146 tSK S Aitkil-Williams and J S. Pueston fibres in office and plant buildings. The monitoring/SEM/XRMA conditions were designed to match the fibre size (width) detection limit of the MF/OM method that is commonly used for the measurement of asbestos fibres in occupational situations. THE NPF SEM METHOD USED 1. Sampling Head. Open face. 25 mm dia. (Gelman catalogue No. 1107). Brass cowl, length = 55 mm. internal diameter = 32 mm. Filter. Nuclepore polycarbonate, 0.8 /tm pore-size. 25 mm dia. (Nuclepore stock No. 110609). No filter pre-treatment was made. Samples were collected on the shiny side of the filter. Collection area = 370 mm2. Technique. Fixed-point samples were collected. The sampling head was located at chest height, with the collection filler facing downwards. The samples were collected under conditions of dynamic ambient air-flow during office hours. 2. Analysis Instrumentation. Cambridge Stereoscan 180 scanning electron microscope with Link 290 energy-dispersive X-ray microanalysis unit. Preparation. A sector was cut from the Nuclepore collection filter by scalpel, directly mounted on a 12.5 mm carbon stub by use of a natural organic liquid adhesive, coated with a 10 nm conducting layer of carbon using a liquid nitrogen trapped evaporative coating unit, and then counted and analysed. The specimen, as prepared, was capable of practical inspection to a resolution of 20 nm. SEM conditions. Accelerating voltage = 30 kV Beam diameter at specimen = 12.5 nm Signal mode = secondary electrons Stage = greatest practical working distance of 35 mm Specimen lilt = 25' Nominal magnification = 1200 x Average magnification at tilt = 1250 + 50x Field area at tilt = 0.0073 + 0.0004 mm2 Counting. Quasi-rectangular fields counted along radial traverses. Fibre selection criteria. Length>5 ftm. Width<3 pm. Aspect ratio 3:1 The practical limit on the size of a fibre that was consistently seen under the routine sampling and counting conditions was0.3 pm fibre width at 5 pm fibre length. Fibres of width = 0.2 pm were not uncommon, while fibres of width =0.1 pm were occasionally found. Such fibres were counted when found. 10003147 ns were d that is ations. s cowl. re stock ie shiny cated at ollected >pe with scalpel, dhesive, trapped repared. - routine 7ibres of isionally 'Asbestos and other fibre levels in buildings .W X-rax microanalysis. Each fibre found was excited by a static electron beam and the X-rays emitted from the fibre were sampled with an energy-dispersive X-ray detector of 149 eV resolution, under conditions of take-off angle and proximity to give a count rate of 500 counts s'1 for a 3 /rmdia. amosite fibre. Sufficient counting time was allowed to give a total count of 15,000 counts over a range ofO-10 keV, thus providing good peakto-background ratios for all elements of interest. The X-rays emitted from the fibre thereby provide a measure of the major and minor chemical elements (z> 10) in the individual fibre and so allow its identity to be determined (Champness et al.. 1976). 3. Factors considered in developing the method Nuclepore polycarbonatefilter (NPF). The use of SEM strongly favoured the use of the Nuclepore polycarbonate filter for dust sampling. The NPF is made of solid, flat plastic (about 10 pm thick) through which nominal pore-size holes have been etched by collimated atomic particle bombardment and chemical dissolution. The flat surface makes the NPF an excellent choice for SEM work (Beckett, 1973; Spurny et al.. 1979). The choice of pore-size for the NPF was determined by the detectability requirement for fibre width. The fibre-width detectability of the MF/OM method is 0.3 pm (for 5 pm fibre length). This limit allowed an 0.8 /mi pore-size filter to be used. Spurny et at., (1979) have shown that for face velocities of 85 mm s"1 and above the collection efficiency of the 0.8 pm NPF for 0.3 pm amosite fibres is 80% or higher. Specimen preparation and mounting. To reduce the possibility of ambiguous EDXRM A results (and hence false identification of fibres), it was important that stub, collection filter, adhesive, and coating be limited to materials composed of light elements (e.g. z < 10) whose characteristic X-rays are not detected by EDXRMA. It is this concern which contributes to the reason for avoiding coating the collection filter with gold. Gold, when excited with 30 kV electrons, will produce X-rays of the following energies (in keV): Ma = 2.I23, M/3=2.205, Lct=9.711, Lj?= 11.5, L/ = 13.4. Three problems arise because of the presence of such background X-rays: (i) some element peaks from fibres are obscured by overlapping gold peaks (e.g. overlap of Au La, P with S Ka), (ii) additional preferential excitation of some elements in the fibre will occur (e.g. Fe Ka by Au L lines; Si Ka by Au M lines), (iii) the excitation of elements in a nearby or attached fibre or particle may occur and be interpreted as emanating from the target fibre. SEM magnification. The fibre-width detectability also determined the SEM magnification that needed to be used and. hence, the SEM field size. A nominal magnification of 1200 x was found to be adequate to ensure routine detectability of all fibres 0.3 /*m wide at 5 /mi length. Airborne fibre concentration detection limit. AN Airborne fibre 1."1 ^ anrt (U 10003148 360 S. Altkee-Williams and J S. Preston where A = collection area of filter. mm2 a = area of SEM counting field, mm2 N = number of fibres found n=number of fields counted r=sampled air flow rate. I. min'1 resampling time. min. Equation (1) shows that, if a typical occupational hygiene air sampling rate and time (21. min'4 h) .s used and if 11. is chosen as the volume of sampled air actually examined under the SEM at a magnification of 1200 x, then about 100 SEM fields must be counted. This number of fields can be counted in about 1 h. The significance of the fibre count number, N, can be directly assessed from a knowledge of the volume of air sampled that was actually examined in the SEM and from the known confidence intervals for the Poisson mean (Hall and Selinger, 1985; Crow and Gardner, 1959). For a singlesample taken under thegiven conditions, with 0 asbestos fibres counted, the 80% confidence interval for the fibre count collected on the filter is 0-1.82 fibre l.~'. For two samplings and a 0 asbestos fibre total count, the upper confidence limit falls below 1 fibre 1."'. Thus, the combination of circa 100 SEM fields with routine hygiene sampling conditions gives a practical detection limit for duplicate samples of 1 fibre l.~1 of air (i.e. 0.001 fibre ml.~1 of air). This is an improvement of at least 10 times on the detection limit obtainable by the MF/OM method. COMPARISON WITH THE RTM2 METHOD The direct use of the Asbestos International Association's RTM2 method (1984) for this work was investigated. It was not found to be the optimum method for the task at hand. The RTM2 method is also a NPF and SEM-based method. However, it uses a gold-coated Nuclepore filter for dust collection and ashes the collected sample before SEM/EDXRMA analysis. RTM2 has been developed for application to the task of determining mineral fibres in samples of high organic-fibre content (e.g. air samples in an asbestos-cement factory that is in the process of change to an organic-fibre-cement factory). The gold-coating protects the collection filter while ashing eliminates organic fibres in the sample. For the task of determining the respirable asbestos fibre content in office and plant buildings, however, the RTM2 method has the following disadvan tages in relation to the original Beckett method as developed and reported here: (i) The ashing step eliminates organic fibres in the sample and, in so doing, negates the use of the NPF/SEM method as a supplementary technique to the MF/OM method for samples of low and/or mixed fibre content. (ii) The gold-coating on thecollection filter forms a highly emissive background in the secondary electron mode used in SEM counting. This significantly reduces the contrast between the collected fibre and the background, thus necessitating the use of higher magnifications. The magnification of 2000 x needed in the RTM2 method, triples the SEM counting time relative to this method. (iii) The gold-coating produces gold X-ray lines that interfere with the EDXRMA. as discussed above. 1000314? ale and ictuaily 1 fields from a M and .1985; is, with cted on int, the .Ttpling ' of air on the (1984) le task uses a before ask of oles in ement game tent in dvanre: doing, to the indin duces ating n the 4MA, Asbestos and other fibre levels in buildings 361 (iv) Specialized pre-treatment of the collection filter is required by the SEM laboratory before air sampling can be undertaken. (v) Specialized ashing of the collection filter is required after sample collection and before scanning electron microscopy. R ESULTS The NPF/SEM Method has been applied to monitoring the air in twenty-two office and plant buildings. The majority of these buildings contained asbestos sources (three were monitored after the removal of asbestos sources, two had no known asbestos source) and these sources had been installed at the time ofconstruction of the buildings. All sources had been in situ for at least 10 yr. The details of the asbestos type and source within each building and of the results found in the monitorings are given in Table 1. The monitoring results refer to samples taken in the occupied parts of each building and generally in those areas most likely to be contaminated with airborne asbestos from the nominated sources. A summary of the monitoring results follows; Buildings: Total = 22 (Office = 19, Plant = 3). Asbestos present; Chrysotile = 9, Amosite=9, Asbestos removed = 3. No asbestos = 2. Total number of air samples collected = 193. Asbestos fibre concentrations found: 1 sample 22 fibre I.' 3 samples 3 fibre I." 5 samples 2 fibre 1." 22 samples 1 fibre I.' 162 samples 0 fibre I." Crocidolite = 2. The 22 fibre I.'1 result was obtained in a room containing a man-hole access to a roof space in which loose floe amosite insulation was spread on top of the ceilings. Organic fibre concentrations found: 1 sample 63 fibre I." 1 sample 59 fibre I/ 1 sample 25 fibre l.` 13 samples 10-20 fibre 1/ 58 samples 5-9 fibre l.` 119 samples 0-4 fibre I.' Mineral (non-asbestos) fibre concentrations found: 3 samples 10-13 fibre I.'1 10 samples 5 9 fibre I.'1 36 samples 2 4 fibre I.'1 144 samples 0 1 fibre I.'1 10003130 162 S Ai rRH'-Uii hams jih) J S Phkston T\lll I I AlKIHIKM KISI'lKWtll WIIIMm (Ml nllllK MIIKIUS KIIM WII I'l \U III IIDIM.V Building number Asbestos source type and location Number of samples Asbestos fibre I.'1 Organic Mineral fibre I.'1 fibre I."1 (Number of samples! 1 Amosile. fire doors, umber 3 faced, unsealed al lop and bottom s Chrysolite: asbestos-cement K board surrounding re-heaters in air conditioning ductworks 3 Chrysotile: asbestos-cement 8 board on walls, painted 4 Crocidolite; fire-proofing on K steel structural members 5 Amosite: loose Roc in roof 59 space above ceiling 6 Amosite: spray insulation, 4 under-roof on lop Door 7 Chrysotile; spray insulation 8 residual on plenum wall after replacement of steel roof 8 Amosile; fire-proofing on steel 4 members on top floor 9 Chrysotile and amosite; asbestos-cement wall board 3 and compressed board, unpainted 10 Amosite; plant insulation, after s removal II No asbestos source identified 8 in building 12 No asbestos source identified 4 in building 13 Amosite; steam-pipe insulation 8 enclosed by metal or cloth binding 14 Amosite; fire-proofing on steel 4 members on top floor 13 Amosite and crocidolite: fire 4 proofing on steel members of building 16 Amosite: fire-proofing on steel 4 members of building and roof 17 Chrysotile; minor component 10 in under-roof spray insulation IK Chrysotile; minor component in spray-on ceiling finish: before removal 19 As in IK; after removal s s 20 Chrysotile. minor component s in spray insulation on concrete slabs 1 III 0 |2| 1 (II 0 (7| 0 |K| 0 (8) 22 (II .1 (2) 2 (4) 1 (9) 0 (43) 3 (II 1 (3) 1 ID 0 (71 1 (3) 0 (11 0 (31 n 4 (31 5 6 (31 1 4 (51 56 14 5K 14 10 16 59 04 (1) 17) (3) (5) (6) |I4| (39) 5 9 (2) 3 4 (2) 5 (2) 2 4 (6) 2 4 (4) 5 (1) 3 (2) 0 (2) 0 (8) 0 14) 0 (8) 2 ID 1 (II 0 (2) 0 (4) 0 (4) 0 (10) 0 (5| 1 III 0 III II 15) 5 (D 2 (D 1 2 (8) 2 4 |4| 5 6 (3) 1 4 (5) 25 5K III (3l 5 ID 1 4 131 5 7 (3) 4 (D 10 ID 5 6 |4| 0 4 |5| 5 III 0 4 (4| t 4 121 10 14 131 7 X i2l o 1 (31 (31 0 1 15) s (1) 0 1 (7) 2 3 (2) 0 1 (6) 5 6 (1) 2 4 (9) 0 1 1491 0 (4) 4 (II 0 1 (7) 0 1 (4) 0 1 (3) 0 1 (2) 7 ID 3 ID 0 1 (6) 0 1 (4) 10 ID 2 4 12) 0 1 (5) 0 1 14) 2 ID 0 1 (3) 2 3 (2) 0 121 2 III 0 1 191 s 121 0 1 131 10 III K ID 6 |2| 2 4 (31 UofUiniHil) 100031 SI .MiS Mineral ibre I."1 it 1 (31 "3 (31 o 1 (51 2 (II 'i i (7| 3 (2) i 1 (6) ; 6 (II : 4 (9) * i (49) 0 (4) 4 (II i 1 (7) i 1 <4| 1 (3) 1 (21 7 (1) 3 (1) 1 (6) 1 (4) 10 (1) 4 (2) 1 (51 1 141 x (1) 1 (3) 3 (2) (21 (11 1 (9) (2) 1 (3) > (I) HI 12) 4 (31 mintnul) Asbestos and other fibre levels in buildings 363 TaBI I I. AIRBORNE RESPIRABLE ASBESTOS AND OTHER FIRRF.S IN OFFIC I AND fl.ANT HI It DINGS iamlimicj| Building number Asbestos source type and location Number of samples Asbestos Organic fibre 1.'1 fibre 1.'1 (Number of samples) Mineral fibre 1."1 21 Amosite; fire-proofing on steel 5 members of building 22 Amosite; plant insulation, after 27 removal 1 <11 0 (4| 1 (II 0 (26) 5 24 59 63 10 20 59 14 (2) (31 (2l (3| (12) (10) 2 3 (3) 0 1 (21 13 (I) 5 8 (5) 2 4 (5) 01 (16) REFERENCES Asbestos International Association (1984) Recommended Technical Method No. 2 (RTM2): Determination of Airborne Asbestos Fibres and Other Inorganic Fibres by Scanning Electron Microscopy, London. Beckett, S. T. (1973) The evaluation ofairborne asbestos fibres using a scanning electron microscope. Ann. occup. Hyg. 16, 405-408. Cha.mpness. P. E.. Cliff, G. and Lorimer. G. W. (1976) The identification of asbestos. J. Microscopy 108, 231-249. Crow, E. L. and Gardner, R. S. (1959) Confidence intervals for the expectation of a Poisson variable. Biomelrika 46, 441-453. Hall. P. and Selinger, B. (1985) Simple statistical test for compliance of fibre counts with hygiene standard. Department of Statistics, The Australian National University. Canberra. Submitted for publication. Am. ind. Hyg. Assoc. J. Middleton, A. P. (1982) Visibility of fine fibres of asbestos during routine electron microscopical analysis. Ann. occup. Hyg. 25, 53-62. National Health and Medical Research Council (1976) Membrane filter method for estimating airborne asbestos dust, Canberra. Rajhans.G. S. and Suluvan, i. L. (1981) Asbestos Sampling and Analysis. Ann Arbor Science. Ann Arbor. Spurny, K. R., Store*, W., Opiela, H. and Weiss, G. (1979) On the evaluation offibrous particles in remote ambient air. Sci. Total Encir. II, 1-40. Walton. W. H. (1982) The nature, hazards and assessment of occupational exposure to airborne asbestos dust. Am. occup. Hyg. 25, 117-247. \0003152