[en] In Spain, according to the 96/29/EURATOM Directive of the European Commission, the regulations of protection against ionising radiation (Real Decreto 783/2001) establish for the first time the need of monitoring those workplaces that might have high radiation levels from natural radionuclides. In this context, a systematic survey has been carried out in Catalonia in which the annual-averaged radon concentration has been determined in 13 underground workplaces. Two sites have been identified as having radon levels higher than the 1500Bqm^-3 suggested by the ICRP as the maximum value for an action level. In two other sites a more detailed study is required to determine whether the dose received by the workers constitutes a significant increment of the exposure to natural radioactivity from the radiation protection point of view