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FIGURE B-1 Normalized in-growth of decay-product activities of an atmosphere initially containing only 222Rn. at 3700 Bqm-3 (100 pCiL-1) is called a working level (WL). Thus, a 370 Bqm-3 (10 pCiL-1) equilibrium mixture represents 0.1 working level. The cumulative exposure to such activity can be expressed as the amount of activity in WL multiplied by time of exposure. In occupational exposure assessments for miners, this cumulative exposure has historically been given in Working Level Months, WLM, where it is assumed there are 170 hours in a working month. The WLM is calculated as 
where (WL)i is the average concentration of radon decay products during exposure interval expressed in WL and ti is the number of hours of the exposure. The cumulative exposure when spending all of the time in a house at a given decay product concentration is more than four times that for occupational exposure (8766 compared to 2000 hours worked on an annual basis).
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