RESUMEN
Radon-222, a naturally occurring radioactive gas, responsible together with its progeny of around 50% of the average effective dose received by the population, has not been regulated by law until the recent Directive 2013/51 /Euratom. Its transposition into Spanish legislation was made in the recent RD 314/2016, which sets at limit value of 500 Bq l-1 for radon-222 in water for human consumption. Intercomparison exercises, such as those carried out by IPROMA SL and the Laboratory of Environmental Radioactivity of the Cantabria University (LARUC) in November 2015 and December 2016, represent the most useful tool available for detecting problems and taking corrective actions necessary for an efficient measurement by part of the laboratories. The participants in these exercises used three techniques: liquid scintillation counting, gamma spectrometry and desorption followed by ionisation chamber detection.