The mining industry traditionally measures exposure using the working level (WL) index, and the cumulative exposure in working level months (WLM): 1 WL equals any combination of short-lived 222 Rn progeny (218 Po, 214 Pb, 214 Bi, and 214 Po) in 1 liter of air that releases 1.3 × 105 MeV of potential alpha energy;[1] one WL is equivalent to 2.08 × 10−5 joules per cubic meter of air (J/m3).[2] The SI unit of cumulative exposure is expressed in joule-hours per cubic meter (J·h/m3).