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Radon gas is considered by the World Health Organization (WHO) as one of the most relevant indoor pollutants with proven relationship with higher lung cancer risk. Hence, indoor radon exposure in badly ventilated and extensively occupied rooms increases drastically the risk of health problems. Due to the geological nature of local soil, the Northwest region of Portugal is critical concerning indoor radon exposure. The new legislation is already in force to combat the problem by adopting a remediation strategy in order to reduce the occupants’ risk in local buildings. However, a remediation strategy involves not only manual or automatic mitigation actions to reduce indoor radon concentration, but also awareness-raising campaigns developed amongst buildings occupants and local community making part of an action plan to deal with the problem. In this paper we present a new case study of 15 public buildings assessed in 2018 through short-term measurements to characterize indoor radon gas concentration, in an inner city in the Northwest region of Portugal. Radon risk was assessed to evaluate occupants’ risk exposure and to define an Action Plan concerning radon gas remediation.