The Spanish law that transposes the European Directive 29/96 is based on the recommendations established by ICRP in 1991. This law requires that estimations of the doses due to radioactive emissions, from practices or installations in which ionising radiation sources are used, must be performed in a regular base and in the more realistic possible way. More recently, the ICRP, as well as the European Union, recommended that, in order to reach a realistic estimation, a gradual approach made by consecutive iterations should be made. However, in spite of the huge amount of documentation, realistic dose assessments are usually treated just for artificial radionuclides, being Naturally Occurring Radioactive Materials (NORM) industries only considered by regulation, regarding to the necessity of performing studies to determine if there is or is not a significant increase in the exposition to the workers or to the public. In this paper an adaptation of the general criteria used to carry on realistic dose assessments for the particular case of NORM is presented. Also the difficulties and implications, that applying such criteria involve, are analysed. In addition the main parameters, which could influence the uncertainty of the assessment, are discussed.