Clearance indicator air sampling for the certificate of reoccupation

7.132 Air sampling takes place once a thorough visual inspection has been carried out and the analyst is satisfied that all the asbestos in the POW has been removed, and there is no visible debris or layers of settled dust (see Figure 7.27). The lowest airborne respirable asbestos concentration that the method described in Asbestos: The analysts’ guide for sampling, analysis and clearance procedures8

can reliably quantify is 0.01 fibres/ml, for a sample volume of at least 480 litres passed through a filter with an effective diameter greater than 20 mm. In most cases it is reasonably practicable to clean the working area thoroughly enough for the respirable airborne fibre concentration after final cleaning to be below that limit, using the approved measuring method. Therefore a value of 0.01 fibres/ml is taken as the ‘clearance indicator’ threshold, and a site should not normally be regarded as fit for reoccupation until the asbestos in air measurements are below this level. 7.133 Details of the equipment to be used to carry out the sampling and analysis are given in Asbestos: The analysts’ guide for sampling, analysis and clearance procedures. 8 _{The strategy for sampling and dust disturbance and sample analysis}

are also given in Asbestos: The analysts’ guide for sampling, analysis and clearance procedures.8 _{Air sampling should be accompanied by sweeping the floor with a}

broom and brushing the surface from which the asbestos was removed and any other higher level horizontal surfaces (see Figure 7.28). Brushing should also take place on horizontal surfaces where the dust may have settled or collected or where there is suspicion of surface contamination, and on surfaces in close proximity to the sampling equipment. The broom and/or brush used should be made out of man-made fibre and should be used to give a representative simulation of cleaning activity. For enclosures with floor areas greater than 20 m2 _{a long-handled broom}

should be used to sweep the floor, for both ergonomic and practical reasons. 7.134 The dust disturbance should be carried out as described in paragraphs 7.135-7.136. The dust-raising activities undertaken and their duration should be recorded on the certificate of reoccupation. On some surfaces, brushing may generate significant amounts of particulate which may obscure the filter. If this is the case, sampling strategies may need to be modified to take this into account. Appropriate PPE should be worn by the person conducting the disturbance test. 7.135 The purpose of the disturbance activity is to ensure that workers,

occupants, cleaners and members of the public using the area in the future are not exposed to asbestos as a result of ineffective removal and cleaning. A realistic simulation of a possible future activity that may produce high airborne dust and fibre concentrations is the brushing or sweeping of surfaces. Brushing should be carried out in a manner that is consistent with normal cleaning activities in a building. Brushing should take place in all of the following locations: all surfaces from where the asbestos has been removed, horizontal surfaces where the dust may have settled or collected or where there is suspicion of surface contamination, and surfaces in close proximity to the sampling equipment. These dust-raising activities should be substantial enough to raise fine settled dust (if it is present) from surfaces, and should be commensurate with the size of the enclosure. They should take place for a duration of at least 1.5 minutes for each measurement point inside the enclosure, near the start of each full hour of sampling, or each time a new filter is used in an area. For larger enclosures there is likely to be more than one person carrying out the dust disturbance work, so the same total surface area will be disturbed but in less time. This means the total time of the disturbance is unlikely to exceed around 10-15 minutes each hour.

7.136 All brushes used for raising dust should be considered as being

contaminated and should generally be disposed of as asbestos waste. However some brushes may have detachable screw handles. Where the handle is

constructed from a material which could be effectively decontaminated (eg plastic) then this part may be reused after thorough cleaning. Brush heads, irrespective of the composition, should always be disposed of as asbestos waste.

Dusty enclosures

7.137 There may be occasions when the surface in the work area is a source of non-asbestos dust that would generate unreadable filters. The presence of non-asbestos dust would be noted at the thorough visual inspection. The analyst has to be satisfied that the dust is non-asbestos. However, the analyst should proceed with air sampling as normal. If this produces unreadable filters, the analyst should consider sampling for shorter periods with paired samplers, so that the dust loading on each filter is reduced. If the samples fail again because of the dust loading, then the spraying of surfaces with a sealant should be considered. If a sealant is used, the air test should not be carried out until the sealant is dry (see paragraph 7.125).

Figure 7.28 Analyst carrying out disturbance by brushing during clearance

Assessment of air sampling results

7.138 After air sampling, the analyst will check the final flow rate and collect samples for phase contrast microscopy analysis. The analyst will count the fibres in a minimum of 200 graticule areas and report the calculated fibre concentrations for each sample. The analyst will also produce a clear statement whether the enclosure has passed or failed, relative to the clearance indicator value (0.01 f/ml). Leaks in enclosures

7.139 Under normal circumstances, the air extraction equipment should be turned off and capped during the air test. The analyst should check that the pre-filter was changed before the final clean. However, if, in the opinion of the analyst, switching the air extraction system off would compromise the integrity of the enclosure, and there are people near the enclosure who may be exposed to airborne asbestos fibres above the clearance indicator as a consequence, the analyst can direct the contractors to leave the system switched on during the air test. Any decision to leave the air extraction system switched on should be recorded, with reasons why, on the certificate of reoccupation.