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continuous sample is taken across a period of time. This may be a 10 minute to an 8 hour sample.

6. DISCONTINUOUS SAMPLING i A series of samples, each preferably of at least 30 minutes" duration, may be taken; shorter sampling periods may sometimes be expedient, but the duration should be chosen to reflect the situation. The samples may

be collected on a single filter. 7. GENERAL CONSIDERATIONS AFFECTING CONTINUOUS SAMPLING

Where a reliable operator/employee can be persuaded to carry a personal sampler for the random sampling periods this procedure for assess~ ment of the dust concentration of the employee's personal environment is the most suitable. Extreme care should be taken to avoid contamination of the sample by anything other than airborne dust (e.g. direct contact with bags of asbestos). Where personal samplers cannot be used, it is recommended that a static sampler be placed as near to the operator/employee's working environment

-as possible, with the sample head in the operator's breathing zone. 8. TIME WEIGHTED AVERAGE SAMPLING

The TWA sampling is intended to provide an indication of the level of airborne asbestos dust to which an employee has been exposed during his working day. It can, therefore, be defined as the mean concentration of airborne asbestos dust occurring throughout an 8 hour period of continuous employment. A continuous and uninterrupted sample can be taken over the whole of the 8 hour sampling period using the standard equipment but over- : saturation of the membrane filter will preclude its assessment by microscopic counting. Its assessment will generally be made by means of other special 4 techniques. It should be emphasized that samples taken for assessment by a these mass sample methods should always be selected to include only the respirable fraction of the asbestos dust cloud. Such selection can only be

achieved by elutriating the airborne asbestos dust before collecting the