[en] This paper discusses the problems related with the safety of people exposed to radiation coming from base stations of mobile communication systems installed on the top of buildings in urban environments. After introducing the problem, the approach taken in the paper is presented: the issue is not to know if exposure to radiation is hazardous but to know what are the exposure limits that may constitute danger. Afterwards, the maximum values recommended by several international bodies are shown for the specific absorption range in the frequency range of mobile communications and, in particular, for GSM. Measurements done in a building in Lisbon, Portugal, with antennas on its top show that the signal is circa 50,000 times below the maximum values, hence perfectly within the safety margins. One can say that, in the large majority of cases, there is no danger for people working or living in buildings with base station antennas installed on their top (author)