This article demonstrates the importance of quantifying the air quality with radon gas level as indicator in any heritage building, especially those intended for the use of people. The tourist activity or historical guide represents a typology where people spend a certain time, that is to say, in no case do they spend the same amount of hours as in their homes or jobs. Different gases that may be present in the environment must be controlled. The Séneca Square shelter, in Alicante, is a very important place for the history of the city during the Spanish Civil War that has recently been rehabilitated for exposure to people. The source of most radon gas inside a building is the ground. Many countries, including Spain, in which the building regulations, regarding the accumulation of radon gas, do not specify in their technical codes, the maximum dose that a building can sustain so that it is not harmful to people, or, the measures required to correct excessive accumulation. The possible existence of radon is verified in any underground building, regardless of the characteristics of the soil (whether granitic or not), the importance of defining and unifying the regulations that specify the different levels of radon in any architectural constructions is evident. Most of the scientific agencies in the field of medicine and health, consider that radon gas is a very harmful element for people. This element in its gaseous state is radioactive and it is present in almost all soils in which buildings are implanted, with granitic types of soil presenting higher levels of radon gas. Non-granitic soils have traditionally been considered to have very low radon levels. However, this work, providing the results of the research carried out in the underground air raid shelter in Seneca Square in Alicante (Spain), demonstrates the relevant presence of radon in non-granitic soils. This research addresses the constructive typology of the underground building and the radon presence in its interior obtained using rigorous measurement techniques.