Improved Methods for Clearance Testing and Visual Assessment of Asbestos Removal Operations. HSL/2001/11

Transcription

1 Broad Lane, Sheffield, S3 7HQ Telephone:+44 (0) Facsimile: +44 (0) Improved Methods for Clearance Testing and Visual Assessment of Asbestos Removal Operations. HSL/2001/11 Project Leader: Dr G Burdett Mr Peter R Stacey MRSC CChem and Dr G Burdett Environmental Measurement Group Crown copyright 2001

2 Summary Objectives 1. To further develop and define a standardised method for visual clearance and for disturbance (or aggressive) sampling inside the enclosure and after the site has been vacated. 2. To research and test a surface sampling method (by PCM and TEM) to collect surface dust as a measure of site cleanliness after a removal operation. 3. To investigate the current standard of visual clearance being carried out after asbestos removal and to document findings to inform and provide future guidance and training. 4. To assess any residual levels of asbestos remaining after the contractor has left by carrying out disturbance sampling before reoccupation and/or surface sampling. 5. To prepare a draft MDHS on clearance sampling and testing and publish results. Main Findings 1. Laboratory investigations to determine the detection limit of simple visual wipe tests, a torch test and quantitative assessment of surface fibre sampling methods were undertaken before evaluating them in a field survey. A laboratory investigation of the ability of different dust raising methods was also carried out along with measurements of the settlement rates and decline of asbestos concentrations with time. 2. A total of 20 site visits were made. Fifteen sites had enclosures still intact and clearance assessments were carried out. At two large sites, an additional visit was made to a second enclosure, making a total of 17 clearance assessments. Due to the difficulty in arriving at the start of the clearance, it was only possible to observe the site analysts clearance procedure at 11 of the 15 sites. A total of 12 clearances involving 11 different analysts from 6 different laboratories were observed. The removal sites varied in size from a small boiler room to a large multi-story car park. A range of materials had been removed: 4 sites involved the removal of ceiling tiles, 3 sites involved the removal of lagging, 4 sites involved the removal of AIB board and 5 sites involved the removal of sprayed asbestos. Experimental observations on surface tests 3. Wipe tests based on the density of the deposit and colouration of the wipe media by the surface dust were found to be a simple way to estimate whether surface dust levels are likely to present a problem and whether the site has been adequately cleaned.

3 4. The likely lower level of detection for aluminium oxide and amosite was measured in laboratory trials on different wipe media. The Whatman 540 filters and Mediwipes were found to give a visible contamination line after a 10 cm wipe at aluminium oxide particle concentrations of about 200 p/mm 2 and at amosite concentrations of 60 f/mm The wipe test appeared too sensitive for the current levels of cleaning in UK enclosures as 94 % of enclosures failed the wipe test. However, a similar percentage failed HSL s clearance air monitoring. 6. Although it was not envisaged that the project examine methods of cleaning, observations of working practices employed in enclosures suggest that the most common methods employed; vacuuming and wet mopping are not very effective or are not applied effectively. 7. The HSL operator found the HSL low angle of incidence torch test very useful to detect surface dust but could only be used on smooth surfaces. However, different operators are not consistent with their approach or judgments. 8. Adhesive tape was found to be a simple method to sample surface dust and phase contrast microscopy could be used to quantify the amount of asbestos fibres on the surfaces. Adhesive tapes can also be treated to allow further analysis by SEM or PLM to identify the fibres present. It does not, however, work well on rough or damp surfaces. 9. Forensic tape produces a clearer mount than adhesive tape for PCM analysis. However, it is limited, if further identification of fibres is required. 10. Forensic tape was found to be the most versatile and efficient sampling medium over a range of surface types. 11. Micro-vacuuming in the simplified form used, was found to be less efficient than tape sampling but gave a more integrated sample, as larger areas or multiple areas could be sampled. 12. A concentration of fibres on a surface of 100 f/mm 2, determined using forensic tape, produced a concentration in air of about 0.01 fibres/ml, when an area of about 50 m 2 was brushed with a hand brush for 5 minutes. This means that the visual wipe test which can detect down to 60 f/mm 2 of amosite will have the potential sensitivity to detect surface dust levels which may fail the clearance air monitoring. 13. The surface tests can only provide an indicator of the likely outcome as chrysotile asbestos will not discolour a white wipe media while other dust particles will. It should also be noted that the efficiency of surface sampling is dependent on the type of surface and the sampling media used. Also, the small areas sampled may not be representative of other areas.

4 Experimental observations on disturbance / resuspension of surface dusts and asbestos fibres 14. A survey identified that the most common method of disturbing dust in the enclosures was to shake a plastic bag in the air or to fill a plastic bag with air and hit it against the walls. These practices were shown in laboratory tests, to be very ineffective methods of dust disturbance (less than one tenth compared to brushing). 15. Leaf blower s were found in experimental comparisons to be the most efficient technique for resuspending and mixing settled dust, and were some 4-5 times more efficient than brushing. 16. The effectiveness of a leaf blower will dependent on the outlet velocity and the distance between the outlet nozzle and the surface. Leaf blowers also generally require an electrical supply inside the enclosure. 17. The efficiency of the leaf blower may mean that surface dust levels which may not be detected by a simple visual wipe test, may fail the clearance air monitoring. Also at some of the enclosures sampled in the survey, the airborne levels produced if a leaf blower had been used, would have been likely to exceed the control limit and will put clearance assessors at increased risk, as the respirators they use have low protection factors. 18. Brushing is also an effective method of resuspending the dust from the surface but it does not mix and disperse the particles as efficiently as a leaf blower. 19. Experiments with ground amosite asbestos and resuspension of the fibres with a leaf blower indicate that 40 % of asbestos fibres settle out in the first 60 minutes. With hand brushing, there was a sharper decline in the airborne concentration of amosite fibres, which had halved after about 10 minutes and over 90% of the airborne fibres had settled out within 30 minutes after disturbance. 20. The additional mixing action of the leaf blower had an important influence on the air measurements as the fibres lofted by the leaf blower took longer to settle out. This observation alone accounted for most of the difference between the relative efficiencies of brushing and leaf blowing. 21. This also suggests that for brushing, the sampling period should be short (30 minutes or less), or the surfaces should be subject to more frequent disturbance (e.g. every 15 minutes instead of every hour). Alternatively, fans could be used to further mix and distribute the dust in the air.

5 Field observations on visual assessment by site analysts 22. In most cases the analyst was involved in a remedial cleaning process rather than a separate visual clearance assessment. 23. The role of the analyst in the clearance process is ambiguous and they need a recognised independent role without any financial relationship with the removal contractor. At some sites visited the analytical firm did have a supervisory role. 24. Sites where the analyst had a recognised supervisory role, generally allowed a period of time for the enclosure to dry, before starting the clearance assessment. Analysts contracted by the removal contractor usually had less chance to do this and would wait a limited period (~1 hour) and start clearance when still damp or wet. 25. The spraying of the PVA was only observed at few of the sites and generally all equipment and surfaces which were to be removed or disposed of were liberally sprayed by the removal contractor. 26. There was however considerable variation in when the PVA spraying took place. Often this occurred before the clearance assessment and sometimes additional spraying took place after the visual assessment. 27. Analysts would often perform the visual assessment using a torch but were used only to help find relatively large pieces of debris and were not used to assess whether there was a fine layer of dust on the surfaces. 28. Two of the 17 enclosures observed failed the site analyst s visual assessment because of macro contamination with asbestos. One site had several failures before it was finally passed. Field observations on visual assessment by HSL analysts 29. Two thirds of enclosures did not use any covers on the floors which meant they were readily contaminated. 30. Some of the uncovered floors consisted of concrete, whose rough surface would make it almost impossible to clean up fine asbestos dust. 31. All but one of the floors of the enclosures were left visibly dirty with dust (as determined using a simple wipe test and low angle torch test) but nearly all had passed the visual assessment by the site analyst. 32. Many sites had sprayed the enclosure with PVA before the visual assessment, making the recognition of fine debris and dust more difficult.

6 33. Glass fibre insulation was left in place inside several enclosures. Potentially this glass fibre was contaminated, as it was left uncovered and it should have been removed at the same time as the asbestos. It also makes the analysts position more difficult as they do not disturb this material because it will produce fibres if disturbed during the air test. 34. Over half of the enclosures were found by the HSL analyst to be either wet (29%) or damp (24%). Often this was due to the liberal spraying of PVA solution before the clearance assessment and insufficient time being given for the enclosure to dry. Field observations on clearance air monitoring by site analysts. 35. All analysts disturbed the enclosure area for 5 minutes, regardless of the size of the enclosure. This made the method less sensitive for larger enclosures. 36. At some sites the enclosures consisted of several rooms. Again this made the method less effective as the air movement will be restricted and the disturbance time in each room reduced. 37. Two of the 16 enclosures failed clearance air monitoring on the first attempt, both of these passed on the second attempt. 38. Site analysts used a removal sack as the disturbance method in 10 of the 16 (63%) enclosures where clearance air monitoring was observed. 39. Of the 12 clearances assessments observed only at four was the floor of the enclosure deliberately disturbed, in spite of the fact that this is the obvious place for fine asbestos fibres and debris to collect. At the four sites where the floor was disturbed, this was done using an inflated plastic bag, where the amount of contact and the resuspension action is likely to be low. Only at two of these four sites was any serious attempt made to disturb the floor. Field observations on concurrent clearance air monitoring by HSL analysts. 40. The results from the concurrent samples were analysed off site at HSL. Only 7 of the 11 sites (64 %) and 8 of the 12 enclosures (67 %) monitored concurrently, would have passed the clearance air monitoring according to the HSL fibre counting results. Of the 33 samples taken and analysed by HSL only about half (17/33 or 52%) were found to be below the 0.01 f/ml clearance indicator. 41. The one site failed by the site analyst, also gave the highest fibre concentrations in the HSL analysis ( f/ml). There was a consistent bias in that the site analysts results were lower than HSL s. This bias was particularly significant in deciding whether

7 the site had passed or failed, and the percentage of samples recording fibre counts of <0.01 f/ml fell from >95% for site analyst samples to 52% for HSL concurrent samples. Field observations on clearance air monitoring by HSL analysts. 42. HSL carried out clearance air monitoring and disturbance at 14 of the 15 sites. One site was visually contaminated so was not sampled and one site produced such high levels of airborne particles, the filters were uncountable. 43. Only one of the sites passed the clearance assessment based on the HSL disturbance and clearance air monitoring. The site in question was very wet and had an underground spring which covered the floor with water. 44. Of the 53 individual air samples analysed 43 (80%) were > 0.01 f/ml. The average concentration inside the enclosures during clearance sampling was f/ml with a range of 0.23 to f/ml. The average air volume sampled was 306 L with a maximum of 600L and a minimum of 100L. These air volumes were lower than the recommended minimum of 480L due to the logistic problem of the Contractor wanting to take down the enclosure as soon as the site analyst cleared the site. Field observations on removal of enclosures / post clearance 45. Workers are often provided with paper masks 8810 type FFP2S with a protection factor of 10. However, many do not wear them when desheeting and some workers will break to smoke a cigarette whilst in the old enclosure area. 46. TWA personal exposures of about 0.1 f/ml were obtained from workers desheeting large enclosures. This work activity is probably when the removal workers are exposed to their highest levels of fibres since whilst removing the asbestos they would have been protected with powered respirators with high efficiency filters. 47. Post clearance visits were made to three sites. Two sites had visible asbestos-containing debris left after the site had been cleared and the enclosure removed. All sites were visibly dirty or very dirty based on a visual inspection. One site had a room full of the removed non-asbestos panelling. 48. Post clearance air monitoring was carried out at the site without visible asbestos debris and gave values of up to 0.08 f/ml with brushing. 49. Another HSL/HSE survey conducted at the same time as this report found visible asbestos debris at three of the six sites visited.

8 50. This suggests that no effective pre-cleaning takes place before the enclosure is constructed, and/or that no post-cleaning takes place after the enclosure is removed. Main Recommendations 1. Different HSE documents should be updated to be more consistent and use the same terminology. 2. It is suggested that the terms clearance assessment, involving a visual assessment and clearance air monitoring are used in future HSE documents. 3. More attention should be given to the detection and clean up of fine settled dust, including: i. Clearance visual assessments should involve a simple standard wipe test to detect the concentration of fine settled dust; ii.a standard method for tape sampling to measure the concentration of fibres removed from surfaces should be developed for field use by analysts so they can estimate if high airborne concentrations are likely to be produced during clearance air monitoring; iii.guidance on how to use low angle light beams to detect fine dust should be given by HSE. 4. A stronger message and attention needs to be given, that the entire enclosure should be dry before commencing a clearance assessment, including: i. The visual assessment should assess and record any areas of the enclosure which were damp or wet; ii.the ACoP states that enclosure should be, as dry as reasonably practicable which is interpreted as it does have to be completely dry. Allowing a suitable time for it to dry and the importance of this should be stressed in the ACoP and guidance; iii.the clearance certificate should state whether the enclosure was dry at the time of the clearance assessment. 5. The clearance air monitoring needs to be much more standardised: i. A standard disturbance should be specified based on brushing/sweeping (if a leaf blower is deemed unacceptable); ii.the time of disturbance should be adjusted to the size of the enclosure;

9 iii.five minutes should be used as a minimum disturbance time for enclosures with areas of less than 100 m 2, a further 5 minutes or part thereof should be added for each additional 100 m 2 up to a maximum of 15 minutes; iv.the floor of the enclosure and other flat horizontal surfaces should be disturbed for at least half of the disturbance period and the surfaces which formerly had asbestos materials attached should be disturbed for at least a quarter of the disturbance period. 6. All floors and surfaces should be pre-cleaned before the enclosure is established and then covered to protect them from further contamination. This covering could then be removed and the air test performed on the remaining surfaces which, in theory, should not be contaminated. Most sites would probably pass the air test if this simple measure was implemented to prevent the spread of asbestos. 7. The site analyst should be more independent from the asbestos contractor and should be employed by the client. 8. Current guidelines on the use of PVA are not being followed and are considered impracticable. A change in guidance is necessary. This is also important in obtaining dry enclosures. 9. Use of critical barriers and a double skin of polythene should be encouraged. This will allow the contaminated polythene to be locked down and removed before any clearance test takes place. 10. Guidance on enclosure removal should stress the risk of exposure due to residual contamination. 11. Hygiene units should be functional until the clearance certificate has been issued. 12. Materials which are likely to be contaminated with asbestos and cannot be covered should be removed. 13. A post-clearance assessment should be carried out after the enclosure has been removed but with the critical barriers still in place.

10 Contents 1. INTRODUCTION LABORATORY EVALUATION OF WIPE SAMPLING Quantitative evaluation of the detection limit Experimental Trials with asbestos fibres EFFECTIVENESS OF DISTURBANCE METHODS Experimental Settling time of disturbed 11 fibres Experiment 3:- Monitoring of settling in real time using FC2 Fibrecheck monitor FIELD TESTS Tests for surface contamination HSL clearance air monitoring SITE VISITS AND OBSERVATIONS Arrangements Personal protective equipment and decontamination procedures Observations inside the enclosures Clearance air sampling by analysts FIELD EXPERIENCE WITH THE SURFACE SAMPLING METHODS Wipe samples Low angle torch test Vacuum, filter swab and tapes samples RESULTS OF THE CLEARANCE AIR MONITORING Site analyst results HSL concurrent results HSL clearance air monitoring results

11 7.4. Reasons for differences between HSL and field analysts Relationship between Surface Sampling Technique and Air Concentration PERSONAL EXPOSURE WHEN REMOVING ENCLOSURES VISITS TO PREMISES AFTER CLEARANCE CONCLUSIONS Experimental observations on: Surface tests Experimental observations on: Disturbance / resuspension of surface dusts and asbestos fibres Field Observations: Visual Assessment by site analysts Field Observations: Visual Assessment by HSL analysts Field Observations: Clearance air monitoring by site analysts Field Observations: Concurrent clearance air monitoring by HSL analysts Field Observations: Clearance air monitoring by HSL analysts Field Observations: removal of enclosures / post clearance RECOMMENDATIONS REFERENCES FOR THE REPORT AND APPENDICES... A1. APPENDIX 1: CRITIQUE OF CURRENT CLEARANCE PRACTICE... A1.1. Problems with the current advice.... A1.2. Inspector, contractor, client relationships

12 A2. APPENDIX 2: REVIEW OF CURRENT HSE REGULATIONS, ADVICE AND GUIDANCE ON CLEARANCE ASSESSMENT... A2.1. CAWR (amended 1999)... A2.2. ACoP Procedure for site clearance testing... A2.3. Guidance on clearance testing EH10... A2.4. Guidance on clearance testing EH51... A2.5. MDHS 39/4... A2.6. MDHS A2.7. Assessment of HSE advice on clearance assessments... A2.8. Recommendations for changes in HSE advice... A3. APPENDIX 3: LITERATURE REVIEW OF VISUAL ASSESSMENT... A3.1. ASTM Approach... A3.2. Extent of ACM within the scope of the work... A3.3. Project work performance... A3.4. Inside enclosure... A3.5. Completeness of the removal and clean-up... A3.6. Completeness of clean-up... A3.7. Other guidance... A4. APPENDIX 4: LITERATURE REVIEW OF CLEARANCE AIR MONITORING... A4.1. Review of current regulatory methods.... A4.2. Site studies... A4.3. Methods of dust disturbance... A4.4. Effectiveness of dust disturbance... A4.5. Time of sampling... A4.6. Post - Clearance Reassurance Monitoring... A5. APPENDIX 5: REVIEW OF SURFACE DUST SAMPLING METHODS... A5.1. Types of method

13 A5.2. Sampling variables A5.3. Quantitative investigation - radioactive assessments A5.4. Whole surface analysis A5.5. Quantitative asbestos specific methods A5.6. Recommended methods for surface dust sampling A6. APPENDIX 6: SITE SAMPLING AND OBSERVATIONS A6.1. Site A A6.2. Site B A6.3. Site C A6.4. Site D A6.5. Site E A6.6. Site F A6.7. Site G A6.8. Site H A6.9. Site I A6.10. Site J A6.11. Site K A6.12. Site L A6.13. Site M A6.14. Site N A6.15. Site O Site P (Post removal investigation) USE OF WIPE TESTS IN INCIDENTS Site Q Site R A7. APPENDIX 7: SUGGESTED NEW TEXT FOR REGULATIONS, ACOP AND GUIDANCE

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15 1. INTRODUCTION Current HSE advice in the Approved Code of Practice (ACoP) for work with asbestos insulation, asbestos coating and asbestos insulation board (HSC, L28) refers to site clearance testing as an assessment at the end of a removal to check that the area is fit to be returned to normal occupation. It is described as a two stage process: clearance visual inspection and clearance air monitoring. The responsibility for the clearance is placed on the employer of those who carried out the work (usually the asbestos removal contractor). They must ensure that the enclosure or work area and immediate surrounding area is thoroughly cleaned and that, A thorough visual inspection of these areas should be carried out to make sure that all visible traces of asbestos have been removed as far as reasonably practical from the enclosure and the surrounding area. Once the work area and immediate surrounding areas have passed a visual inspection, clearance air monitoring is carried out to check that the concentrations of airborne fibres remaining in the areas affected by the work is as low as reasonably practicable. The clearance air monitoring should be accompanied by activities which raise dusts from the surfaces, at least to a level consistent with normal use of the area and possible future work activities. In most cases it is reasonably practicable to clean the work area thoroughly enough for the airborne fibre concentration in the work area after the final cleaning to be less than 0.01 f/ml when measured by methods set out in separate HSE guidance (i.e.mdhs 39/4). This report gives the results of project R which was commissioned by HSE to investigate the current practices and to develop improved methods for clearance testing and visual assessment of asbestos removal operations. The project had five objectives: 1. to further develop and define a standardised method for visual clearance and for disturbance (or aggressive) sampling inside the enclosure (and after the site has been vacated); 2. to research and test a surface sampling method (by PCM and TEM) to collect surface dust as a measure of site cleanliness after a removal operation; 3. to investigate the current standard of visual clearance being carried out after asbestos removal and to document findings to inform and provide future guidance and training; 4. to assess any residual levels of asbestos remaining after the contractor has left by carrying out disturbance sampling before reoccupation and/or surface sampling; 5. to prepare a draft MDHS on clearance sampling and testing and publish results. The first four are addressed by this report. As part of the initial work a number of literature searches and reviews were carried out and these have been appended to this report. A critique of the current UK asbestos clearance practice was 2

16 carried out in appendix 1, with a review of HSE s published guidance in appendix 2. Appendix 3 reviews clearance practices in other countries. A literature search and review of visual assessment and clearance air monitoring methods is given in appendices 4 & 5 and a review of surface dust sampling is given in appendix 6. These reviews are important as they put the research work undertaken in the main report in context. Appendix 7 outlines the suggested changes that are necessary to update the regulations, approved code of practices and guidance, based on the outcome of the work. Investigations on the detection limit and qualitative performance of simple visual assessment tests were undertaken in the laboratory. Quantitative assessments of the efficiency of surface fibre sampling methods and different dust raising methods were also carried out in the laboratory, along with measurements of the settlement rates and the decline of asbestos concentrations with time. Several of these methods were then tested in a field survey, which was also designed to observe current clearance practices used by site analysts. A total of 20 site visits were made. The results of the reviews, laboratory studies and field surveys are discussed and used to draw conclusions and recommendations for future clearance practice. 2. LABORATORY EVALUATION OF WIPE SAMPLING. Visual assessment of wipe samples provides a simple way to check whether the surfaces have been adequately cleaned. The presence of any visible dust on surfaces would suggest that the clean up has not been adequate but the sensitivity of the tests are important to see how low a concentration of dust can be detected and how method-dependent the limit of detection is. Dust colour will be a major factor in any wipe test and a predominantly white dust will not be detected by a white test media but a black dust will have a much lower limit of detection. For the laboratory evaluation a medium colour dust was chosen, similar to amosite asbestos and somewhat lighter than ordinary house dust. Particle size is also important and it is assumed that high efficiency particle arresting (HEAP) vacuuming would have been carried out and this would have removed most of the larger particles but have left many small particles which are likely to be bound more tightly to surfaces by Van der Waals attractive forces. Finally, surface type will also affect the sampling media so the tests were carried out on what is the most commonly found and extensive type of surface - polythene Quantitative evaluation of the detection limit Experimental design The efficiency of each material tested was assessed by settling a non fibrous powder on a 1000 gauge polythene sheet (the most common material found in enclosures). The particulate material used in all tests, was F1200 grade aluminium oxide grinding power Aloxite which had a similar colour as UICC* Amosite asbestos (Figure 1). The Aloxite powder is shown in the bottom left segment and the UICC amosite in bottom right tray. The powders in the two top trays are UICC chrysotile and UICC crocidolite. * Union International Contre le Cancer 3

17 Photo removed to reduce file size Fig 1: Colour comparison between Aloxite and common asbestos dusts The mass median diameter of the F1200 Aloxite was 4.5 µm and the mass median aerodynamic diameter 8.6 µm. Figure 2 shows the distribution of the particle size in terms of cumulative volume and indicates that about 80 % of particles, by volume, had a size of less than 5 µm in diameter. The sheeting was placed onto a 0.5 m 2 wooden board painted black that was segmented into 10 cm 2 areas with white lines. This was then placed into a rotating wheel in the lower of the two chambers of a dust box (Figure 3). 4

18 Fig 2: Per cent % Cumulative Volume Of Aloxite Powder 120 Per cent Cummulative Volume Particle size (µm) Figure 3: Diagram of dust generation apparatus Compressed air injector Impactor Air drawn through apparatus Upper chamber Air straightners Test surface Turn table The dust was sucked into the top section of the dust box from a rotating plate using a compressed air 5

19 injector. Any large agglomerates were removed by impacting the entrained particles onto a plate placed 5-6 cm in front of the inlet nozzle. The dust was dispersed in the upper chamber and drawn through a set of air straighteners into the main section of the dust box, before settling onto the polythene sheet. The distance from the impactor to the sample surface was approximately 2m Measurement of surface loading. The surface loading was measured in two ways. Firstly, nine pre-weighed, 47 mm diameter (18.1 cm 2 ) aluminium foil impactors were placed on the polythene surface before the Aloxite dust was generated and their increase in weight recorded. Secondly, nine 25 mm mixed cellulose ester gridded filters were placed alongside the foils before loading and the number of particles on the filters counted after the surface was loaded with dust. Figure 4 showed there was an approximate linear relationship between the average weight change recorded on the foils and the average number of particles counted by x500 phase contrast microscopy on the same number of filters. Fig 4: Comparision between average particles / cm2 counted and average weight on metal foil y = 7E+06x R 2 = Average particles / cm Average change in weight of foil mg/cm2 The counting technique was used by itself for surfaces where the weight change on the metal foil was less than 0.1 mg. Figure 5, below shows the variation of the particle counts on the filters placed at different areas on the sheet with each surface loading over the polythene sheet. 6

20 Fig 5:- Variation of counted particles/cm2 with sampling run Counted particles/cm Data Average Count Sampling Run Number At the lower loadings the variation in particles counted / cm 2 is about % RSD for about 95.5 % of the data. At the highest loadings the counting was more variable and ranged from about % RSD. The mass recorded on the foils showed that variations across the sheet were lower at the higher loading, than shown by the particle counts. Part of the increased variability was due to the hot acetone mounting method appeared to disturb the particle distribution on the filters and a smaller number of fields were counted when higher loadings were present Wipe testing Four different wipe media were chosen, either as they were used previously for wipe sampling or are readily available for use. These were: w 7 cm diameter Whatman 540 cellulose filters, w Medi-Swab swabs containing 70 % v/v isopropyl alcohol medical wipes, w baby wipes and w tissue paper. The damp or wetted material was rolled around a hexagonal shaped pencil and run once, along the surface in one direction along one side of a 10 cm grid outlined on the board behind the plastic sheeting. The material was then unrolled and illuminated with a light from behind. A torch was also shone at the surface of the polythene at an acute angle in order to assess if particles were visible by this method. Three repeat sampling exercises were made on each surface with each wipe material Results and discussion Material that possess a more rigid structure such as the Whatman filters and medical wipes were found to be more sensitive to detecting particles on the surface than the baby wipes or tissue paper. 7

21 This was because the dust accumulated along a narrower area of the filter surface and the greater density of particles was more visible. A line was only faintly visible with these medium when the surface had a average concentration of with estimated upper and lower 95.5 % confidence limits of and assuming a Poisson counting distribution. The line was easier to observe on the filters when they had dried out. The medical swabs dried quickest as they contain iso-propyl alcohol, while the Whatman filters, saturated with water, took 5-10 minutes. Table 1 compares the particle counts on the mixed cellulose ester filters with the visual detection of dust on various wipe media. Table 1: Results from tests with wipe material Average Concentration Baby Tissue Particles/cm 2 Wipes Paper 254, , , ,400 76,473 63,290 62,152 39,387 28,065 21,368 15,678 8,045 5,608 Mediswabs Filter Paper Torch This experiment was repeated and the surface loading evaluated using transparent forensic tape as the collection medium to estimate the density of the particles on the surface of the plastic. The advantage of the transparent forensic tape is that the particles deposited can be counted without any further sample preparation which could disturb the particles. The results on a plastic surface at three different concentration levels are shown in table 2 below. Table 2: Repeat of experiment using average counts on transparent forensic tape. Average (of 9 filters) surface level Whatman Cellulose 540 Filters Particles/cm 2 27,024 21,899 15,587 Using this medium, the end point of detection for cellulose filter paper was re-estimated as approximately particles/ cm 2, (with estimated upper and lower 95.5 % confidence limits of and particles/cm 2, assuming a Poisson counting distribution). At this loading, a very thin and faint grey line was only visible on the wipe material when it was completely dry. A much more visible 8

22 line was produced with the particles/cm 2 loading (with estimated upper and lower 95.5 % confidence limits of and particles/cm 2, assuming a Poisson counting distribution) Experimental Trials with asbestos fibres Ground amosite fibres were air dispersed using a leaf blower and allowed to settle out overnight onto a polythene sheet on the floor of a test chamber measuring approximately 2.5 x 2.5 x 3 m. Using forensic tape lift sampling, the average surface concentration of amosite fibres on the polythene surface was found to be 2010 fibres/cm 2 based on five samples taken at different locations on the floor with a sample standard deviation of 500 fibres/cm 2. Three separate single 10 cm wipes using the Whatman filters did not produce a visible line on the filter media. However, with multiple wipes a faint line was visible after the third wipe suggesting the detection limit of about 6000 fibres/cm 2. The lower detection limit for amosite fibres may be due in part to particle shape and surface area differences but the surface tape samples showed that some small particles of non fibrous dusts had also been suspended during the air dispersion and settled out with the asbestos. Therefore it appears that asbestos can be evaluated by a single wipe test. 3. EFFECTIVENESS OF DISTURBANCE METHODS The method of dust disturbance used by the analyst responsible for the assessment of clearance is not specified in MDHS 39/4 and analysts are at liberty to use any they consider appropriate. It was found from the site clearance visits that the methods most commonly used were; shaking a empty plastic bag, banging a large plastic bag full of air against surfaces, brushing a large plastic bag full of air against flat surfaces, banging a clipboard against flat surfaces, brushing a clipboard against flat surfaces, shaking a clipboard vigorously and brushing with a hand brush. The effectiveness of these methods may vary greatly and may not give a realistic estimate of potential asbestos levels that would be produced by a more representative activity, (e.g. sweeping the floor with a broom or brush, vacuum cleaning or dusting). It is important that the test method is effective so that the levels of asbestos fibres remaining on the site do not give rise to high airborne concentrations Experimental The laboratory experiments were carried out in a test chamber measuring approximately 2.5 x 2.5 x 3 m. The room has an air extraction facility, that was set to its lowest flow rate of 0.1 m/s (~ 0.5 m 3 /minute extracted) during the first experiment and was shut off during subsequent experiments. A 3 stage polythene air lock was built outside the entrance door to allow entry and exit to the chamber by an operator wearing full personal protective equipment. A glove port was used for some of the inside handling tasks. Two series of measurements were made to assess the different methods of raising dust. The methods investigated were identified from a survey of clearance assessments. Also, a US method based on the use of a leaf blower was tested. The leaf blower chosen was a Black and Decker GW250 Mastervac 123 and has a claimed exit velocity from the blower nozzle 180 miles per hour (80 m.sec -1 ). By far the most common method of dust disturbance identified in the survey was to bang a plastic bag full of air against surfaces. 9

23 Two air samples were collected at 8 L/minute over a 30 minute sampling period, which included the disturbance activity. The shorter sampling time was used to minimise differences in the rate of fallout of fibres. One sample was collected at each end of the test chamber with the membrane filter positioned face downwards in a conductive cowl about 1.5 m above the floor. The first set of experiments were made on the debris from the sawing of an board containing amosite. The floor area had been HEPA vacuumed of any large pieces of debris remaining. For the second test amosite fibres were dispersed in the chamber by blowing compressed air in a bowl containing the asbestos. In the first set of experiments the floor surface was disturbed for 5 minutes and in the second experiment for 2.5 minutes. Air concentration fibres/ml Fig 6:- Disurbance Experiment Shaking Empty Bag In Air Hitting Surfaces with Bag Filled with Air Banging surfaces with a clipboard Sweeping with Hand Brush 10

24 Fig 7:- Disturbance Experiment Air Concentration fibres/ml Shaking Clipboard Brushing Clipboard Brushing Bag Filled with Air Leaf Blower Sweeping with Hand Brush Results and discussion Results are given in figures 6 and 7. In the first experiment only banging the clipboard and brushing with a hand brush recorded airborne levels of >0.01 f/ml. Sweeping with the hand brush recorded the highest concentrations. In the second experiment the leaf blower appears most effective method for disturbing dust. However, this piece of equipment is relatively expensive, would requires a 240 V electricity supply and cable and would need to be disposed of after use. As the air jet caused the polythene covered floor to flex and bellow in a wave like manner, this may have produced a higher result than would have occurred if the floor was not covered. Brushing was the next most effective method followed by shaking a clipboard vigorously up and down at a surface. The shaking method would not be very practical when used in a very large area since it requires a lot of physical effort. Table 3 lists the methods tested in order of effectiveness relative to the brushing as this was performed in both experiments. Table 3: Order of effectiveness of disturbance methods Method Ratio Blowing air with a garden blower 4.5 Sweeping with brush 1 Shaking clipboard at surface (up and down) 0.75 Brushing with bag filled with air 0.63 Banging surfaces with a clipboard 0.56 Brushing with a clipboard 0.14 Hitting surface with inflated plastic bag Counts below detection limit Shaking empty plastic bag in the room Counts below detection limit 11

25 3.2. Settling time of disturbed fibres MDHS 39/4 requires a minimum sampling volume of 480 litres to be collect during clearance air monitoring to achieve a limit of quantification (LOQ) of 0.01 fibres/ml. The LOQ, assumes 20 fibres is the upper 95 % confidence limit for the number of fibres significantly above the blank fibre level recorded in 200 fields. Typically pumps are often run at 8 l/min for one hour to sample minimum volume of air required to achieve the LOQ Experiment 1 The rate of decline in airborne fibres was monitored in the test chamber using two pumps placed at each end of the room ~1.5 m above the floor. The air extraction was switched off. The asbestos was then disturbed by brushing with a hand brush for 5 minutes. Samples were taken, changing the heads on the 2 pumps every 10 minutes Results The rate of decline of the fibres in air is shown in the figure 8 in terms of cumulative percent of fibres collected on the sample filter at 10 minute intervals. In this experiment where brushing was used as the disturbance method, % of the fibres were collected in the first 10 minutes. Fig 8: Cumlative Percentage of Fibres Collected In Experiment Per cent of fibres Sampling Time (minutes) Figure 9 shows the measured concentration of asbestos fibres for each 10 minute sample. 12

26 Fig 9: Experiment 1 Average Amosite Concentration compared with time Air Concentration (Fibres/ml) Pump 1 Experimental Pump 2 Experimental Time (minutes) Experiment 2 A similar experiment was performed on a surface contaminated with amosite asbestos using the Black and Decker leaf blower, fig 10. Fig: 10 minute Air Samples for Rate of Decline Using Leaf Blower Average Air Concentration (fibres/ml) Time (minutes) 13

27 Discussion The membrane filter sampling data shows that with hand brushing there was a sharp decline in the number of airborne amosite fibres and had halved in about 10 minutes and over 90% of the airborne fibres had settled out within the first 30 minutes after disturbance. This means that the last 30 minutes of the 1 hour sampling period was essentially reducing the fibre concentration estimated by about one half. When a leaf blower was used to disturb the same fibres, a much lower rate of decline was found, with the airborne concentration of fibres only halving over 60 minutes. This shows that the method of disturbance was not only important in determining the number of fibres made airborne, but also had a large influence on how long they stayed airborne. The experimental evidence suggests that the amount of mixing is also important and methods that give a good mixing action will better disperse the fibres in the enclosure making a significant difference to the time taken to fall out. This factor alone was sufficient to explain an approximate factor of 5 difference between the hand brush and leaf blower over a 60 minute sampling period. A field exercise in 1988, carried out by HSL in a building used as offices also showed a similar effect. In the previous exercise samples were taken every 30 minutes but the data does show a sharp fall in the original air concentration. (figure 11) Fig 11: Results from field experiment in Average Air concentration (fibres/ml) PCM Results TEM Results Time (minutes) TEM Results PCM Results 3.3. Experiment 3:- Monitoring of settling in real time using FC2 Fibrecheck monitor The rate of decline was monitored using a real time fibre monitor the FC-2 Fibrecheck (Harley Scientific Instruments). This was connected to the test chamber through a sampling port 0.8 m above the floor. Settled amosite dust was disturbed for 2 minutes using a leaf blower and in a separate experiment for 5 minutes with a brush. A glove port was used to operate the blower from outside the room. The experiment was repeated twice at two different concentration levels with the 14

28 leaf blower. The FC2 Fibrecheck instrument is a continuous monitor which reports the cumulative concentration of fibres in air, so shows how the measured average concentration changes with time. Previous work (IR/L/MF/96/04) showed that there was a good linear relationship between the monitor and PCM counts for amosite fibre, with a small positive bias for the fibre monitor (slope = 1.14) Results and discussion Results are shown in figures and summarised in figure 15. The leaf blower was used to resuspend fibres at two different surface concentrations. The cumulative air concentration v time was monitored with the FC-2 Fibrecheck instrument. The decay of the airborne concentration with time after the disturbance are shown in figures 12 and 13. Fig 12:-Rate of Decline of Asbestos Fibres In Air When Disturbing the Air with a Garden Blower Cumualtive Fibres/ml Experiment 1 Theoretical Decline Time (Minutes). 15

29 Fig 13:- Rate of Decline of Amosite Asbestos In Air After Disturbance with a Leaf Blower 0.35 Cumulative Concentration In Air (f/ml) Experimental Data Theoretical Decline Time (minutes) The fibre raised by the second leaf blower test were allowed to settle out overnight before re-disturbing by brushing horizontal and vertical surfaces for 5 minutes with a hand brush. When using the same surface concentration the leaf blower was found to be more than 10 x efficient at disturbing fibres (Fig 14) than brushing at these low concentrations. Fig 14:- Comparision of the effectivness of the leaf blower with brushing 0.35 Cumulative Concentration In Air (f/ml) Time (minutes) Leaf Blower 2 5 mins Brushing 16

30 Fig 15: Percent Decline of Amosite Asbestos Fibres Compared With Time from Disturbance 120 Per cent (%) of Air Concentration Remaining Leaf Blower 1 Leaf Blower 2 Brushing 5 mins Time (minutes) Discussion The measured cumulative concentration with time with the leaf blower at two different concentrations confirmed the PCM assessments that this produces a much more well mixed sample which declines relatively slowly with sampling time and makes this much less of a variable. Figure 15 shows the percentage decline in air of the original average airborne concentration and shows that the two experiments with leaf blowers had very similar rates of settling at two different concentration levels. The data suggests that when using a leaf blower the concentration in air of amosite asbestos fibres is reduced by % and 45 % in 60 minutes. Both curves for the leaf blower experiments show a peak of asbestos concentration just after the disturbance has finished which takes about 10 minutes to start to decline. This is probably due to the momentum of fibres in air turbulence as a consequence of the disturbance. The experiment for the disturbance by brushing confirms the air sample results in the previous experiments. The decrease in air concentration is quicker with a reduction of about 55 % in the first 30 minutes and 71 % of fibres in 60 minutes. This rate of decrease for brushing would have implications for air sampling at low levels since it could not be assumed that a significant proportion of fibres remain airborne for the whole 60 minute sampling period. If brushing were chosen as a standard procedure for disturbance then it would be more effective if fans were used to circulate the air to keep the fibres afloat. The rate of decline has been compared with an empirical formula determined from observations on chrysotile asbestos levels in enclosures using TEM measurement. This empirical formula was derived from only 8 single time points in eight different enclosures with different disturbance methods (Ewing et al., 1992). This predicted a rate of decline of chrysotile asbestos at any point in time is defined as x y = where x is the initial geometric mean airborne concentration, t is the settling time (in hours) (1+t 2 ) 17

31 and y is the airborne concentration after time t. The leaf blower mixing, gave a slower rate of decline than found for chrysotile fibres even though it is a much thinner fibre and would be expected to stay airborne longer. The measured decay curves in figure 15 were fitted with various models and it was found that a simple linear relationship described the decay curves for the leaf blower with gradients of and with correlation coefficients (C.C.) of The brush test was more variable due to the low concentrations being measured and gave a poor linear fit (C.C. 0.86) with a steeper gradient and was better described by an exponential fit (C.C. = 0.95). The results showed that the decay curves are method dependent and it is wrong to assume a single equation can be used to predict the change in fibre concentration with time. The method of mixing of the air, as well as the method of dust disturbance, are both important variables when specifying the clearance air monitoring. Given the large differences between disturbance methods it would seem important to specify a standard disturbance method. 4. FIELD TESTS Seventeen clearance sites were visited during the project to test improved methods of assessing and quantifying surface contamination. The emphasis was to develop, test and compare practical methods 4.1. Tests for surface contamination The literature survey showed that a wide range of tests both qualitative (visual) and quantitative have been used for surface contamination. In this study six different methods were tested for assessing the surface fibre concentration; two visual and four quantitative tests. The quantitative tests used were all based on readily available equipment that the analyst would have on-site or could easily bring to site. This means that phase contrast microscopy (PCM) evaluation is used for quantification of the surface fibre concentrations, as this will be used for the clearance air monitoring. The counting rules in MDHS 59 should be used rather than in MDHS 39/4, as fibres may well be attached to particles in surface dust deposits and all fibres of respirable dimensions should be counted. An effective test method should be able to detect or indicate a level of fibre concentration on the surfaces that would fail clearance air monitoring. A practical test would save the removal contractor time and money, since clearance air monitoring failures may hold up the work for several hours and more importantly result in missed deadlines and penalties. The advantages and disadvantages of each technique are summarised in table 4 and the test protocols are described below Wipe test The filter wipe test evaluated in section 2 was used at all sixteen sites. A Whatman 540 cellulose filter paper was wetted with distilled water and rolled around a pencil. It was then drawn once for 10 cm 18