A unit of potential alpha energy concentration (i.e. the potential alpha energy per unit volume of air) resulting from the presence of radon progeny or thoron progeny, equal to 1.3 x 108 MeV/m3 (exactly). The term working level is now obsolete and its use is discouraged. In SI units, a working level is 2.1 x 10-5 J/m3 (approximately)