se The concentration of dust in the air to which a worker is exposed will vary, depending upon the nature of the operation and upon the type of work performed by the operator and the position of the operator relative to the source of dust. The amount of the dust inhaled by a worker can vary daily, seasonally and with the weather. In order to obtain representative samples of workers' exposures it is necessary to collect samples under varying conditions. The percentage of working time spent on different tasks will affect the concentration of dust the worker inhales since the different tasks usually result in exposure to different concentrations. The concentration of any air contaminant resulting from an industrial operation also varies with time. Therefore, a longer sampling time will better approximate the actual average. 3H TEN MINUTE SAMPLING PERIOD The 10 minute sampling period is to provide an initial assessment of the atmosphere in working environments and it is the agreed standard period of time to be used in routine primary sampliag for the purpose of assessing concentrations of airborne asbestos dust. A 10 minute sampling is defined. as a continuous sample which is collected over a period of 10 minutes. "Continuous" means that the flow of sample air through the sampling apparatus is at a constant rate. This is the British method. The U.S. method calls for a minimum sampling time of 15 minutes. 4. CONTINUOUS SAMPLING (STATIC SAMPLERS ) This is where a personal sampler is placed in a fixed location at the same height as an operator's breathing zone and a continuous sample is taken across a period of time. This may be a 10 minute to an 8 hour sample. Si CONTINUOUS SAMPLING (MOBILE SAMPLERS) This is where an employee carries a personal sampler with the sample head in his breathing zone traversing the designated area and a-