The endorsement process of the 2004/40/EC Directive (still in progress) has led to a critical analysis of the ICNIRP Guidelines, on which the directive is based. In particular, some known problems affect the applicability of the numerical techniques needed for checking compliance with limits at low frequency. A review of these open problems is presented in the paper, highlighting how such problems deal more with pre-processing and post-processing steps than with the core numerical calculation of the current density.