## 4 Results
### Gaseous Composition of Caver Air  
Monitoring the composition of underground air in gases such as carbon dioxide (CO\({}_{2}\)) or radon (\({}^{222}\)Rn) is key to characterise the process of the renewal or mixing of air with the external atmosphere and, therefore, monitoring these tracer gases allows us to evaluate over time the degree of isolation and environmental stability of a cavity. Controlling CO\({}_{2}\) concentration is also essential in the study of speleogenetic processes, and its detailed monitoring allows us to evaluate the degree of environmental destabilisation generated by visits which, together with thermal disturbances, can activate the deterioration processes of the rock supporting the cave paintings. In particular, the CO\({}_{2}\) emitted by visitors' respiration can dissolve in the infiltration or condensation water that is in contact with the supporting rock, triggering corrosion processes [27, 29, 30, 31, 32, 33, 34]. As an example, the time evolution of the composition of the air in the El Castillo cave is shown with reference to these two tracer gases (Figure 5). The mean annual CO\({}_{2}\) concentration in the El Castillo cave remained stable during the period of 2021-2022 (1225 ppm and 1255 ppm, respectively). On an annual scale, the mean monthly concentration of the gas showed a seasonal evolution pattern, in general, with relative maximum values above the annual mean in summer and early autumn (in a range between 1400 and 1800 ppm, from July to October) and minimums in March-April (with mean concentrations below 1000 ppm). In 2022, the peaks in the mean monthly radon concentration were reached later (1586 ppm in October-November), which seems to respond to the significant increase in visits compared to the previous year, as evaluated in detail in the following:  
Daily variations in CO\({}_{2}\) concentration in El Castillo are greater than 100 ppm/day in the summer period (May to October), when the average outside temperature is higher than the temperature of the cave and the daily influx of visitors reaches its maximum values (\(>\)100 visitors/day in 2021 and \(>\)150 visitors/day in 2022). On the contrary, in the remaining months, daily variations in CO\({}_{2}\) concentration were recorded within a narrow range between 50 and 80 ppm/day. On an annual scale, seasonal variations in CO\({}_{2}\) have a range of around 1000 ppm in each of the years studied. The average concentration of radon ranged from 1183 Bq/m\({}^{3}\) in 2021 to 1100 Bq/m\({}^{3}\) in 2022. On an annual scale, seasonal variations in radon were recorded with an amplitude of approximately 1100 Bq/m\({}^{3}\) in both years, although there is not a defined pattern as in the case of CO\({}_{2}\). The minimum values in the monthly mean concentrations were recorded in July-August, whereas the maximum values do not follow a defined seasonal pattern. The opposite evolutionary trend between the two gases in the El Castillo cave during the period when the outside temperature is higher than that of the cavity (April-September) reveals the clear seasonal anthropic effect on CO\({}_{2}\) levels, which masks the summer ventilation process in this gas. On the other hand, the ventilation process is confirmed, since the levels of radon follow a decreasing trend during the summer period, indicating an active renewal of the underground air. Thus, from mid-June and throughout the summer months, the outdoor air temperature is clearly above the internal temperature. In this thermal situation, a convective ventilation process is triggered, evacuating the lower-temperature air from the inside and renewing it with warmer air from the outside. This aerodynamic exchange should preferably take place through discontinuities in the rock and karstified zones located at lower elevations with respect to the cavity and within the Monte Castillo karst massif.