Working Level Month (WLM)

The unit Working Level Month (WLM) is frequently used in the risk assessment of occupational radon exposure instead of a calculated dose in Millisievert. The advantage is, that the concentration can be measured directly. No further assumptions for the dose distribution in the body are necessary. To calculate the cumulative exposure to radon in WLM, the measured alpha energy concentration (unit: Working Level (WL)) in one litre air is multiplied by the time the miner has worked in this surrounding. 1 WLM equals an exposure of 1 WL (1.3 * 10⁵ Megaelectron-volt (MeV) potential alpha energy per litre air) over 170 working hours (monthly working time), or a half WL over 2 months (340 working hours), respectively.