## 3 Methodology  
### Environmental Control System  
Each microclimatic monitoring station (one in each cavity) was equipped with the following instrumentation:  
* Datalogger Tinytag TGP 4505 (measuring ranges \(-25+85\)\({}^{\circ}\)C and 0-100% RH) with external combined temperature and humidity (RH) probe: Thermistor 10 K NTC (accuracy: 0.35 \({}^{\circ}\)C, with resolution of 0.01 \({}^{\circ}\)C) and capacitive sensor (accuracy: \(\pm\) 3.0% RH at 25 \({}^{\circ}\)C and resolution \(<\) 0.3% RH). Recording rate of 10 min. * SeaBird (SEB56) high-performance Temperature Recorder Probe: accuracy: \(\pm\)0.002 \({}^{\circ}\)C (\(-5\) to +35 \({}^{\circ}\)C); temperature range: \(-5\) to +45 \({}^{\circ}\)C; resolution: 0.0001 \({}^{\circ}\)C. Recording rate: 5 min. * CO\({}_{2}\) monitor, Goodsell Systems CO\({}_{2}\)log, equipped with an NDIR sensor with a measuring range of 400 ppm \(-\)10,000 ppm and accuracy \(+/-\) 30 ppm. Logging every 30 min. \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**2020**} & \multicolumn{3}{c}{**2021**} & \multicolumn{3}{c}{**2022**} \\  & **Total** & **Mean** & **Max. ** & **Total** & **Mean** & **Max. ** & **Total** & **Men. ** & **Max. ** \\ \hline January & - & - & - & 134 & 9 & 18 & 337 & 13 & 31 \\ February & 436 & 21 & 50 & 169 & 12 & 25 & 337 & 16 & 32 \\ March & 173 & 19 & 39 & 252 & 13 & 27 & 409 & 17 & 4  
AlphaE (Bertin Instruments, Montigny-le-Bretonneux, France) radon (\({}^{222}\)Rn) gas logger monitor, equipped with a silicon diode diffusion chamber detector. Measuring range: 20 Bq/m\({}^{3}\) to 10 MBq/m\({}^{3}\). Detector sensitivity: 3 cph at 100 Bq/m\({}^{3}\). Recording every 1 h.  
The location of the micro-environmental monitoring stations inside each cave was selected based on the presence of the main cave painting panels of interest under the technical supervision of the cave curators and managers, as well as according to the route and timing of guided tourist visits in each cave (Figure 2). The meteorological data used in the study of the Cueva de El Castillo (Monte Castillo, Puente Viesgo, at approximately 187 m a.s.l.) correspond to the station ref. 1152 belonging to the State Meteorological Agency--AEMET--Spanish Government, located in San Felices Buelna, at 105 m a.s.l. and about 7 km to the west of Monte Castillo. In the case of the Covalanas cave (Ramales de la Victoria, 307 m a.s.l. ), the meteorological data records were compiled from the AEMET Station ref. 1089U, located in the same locality at 80 m a.s.l. and 1.8 km northwest of the cave. Assessment of the Impacts Generated by the Influx of Visitors on Air Temperature and CO\({}_{2}\) Concentration  
The use of a large amount of environmental data from different caves requires the use of statistical techniques to simplify and re-evaluate this environmental information. In this study, daily impact assessments of thermal and CO\({}_{2}\) concentrations from visitors to each cave were conducted using a standard and widely used graphical method. This was based on the frequency distribution of variations in each parameter (temperature and CO\({}_{2}\)), grouped on box-and-whisker charts for different increasing daily visits. It is a simple and practical approach to exploratory data analysis, which allows for an accurate assessment of the range of variation in control parameters based on the probability distribution (interquartile range) of the variations generated in these control parameters. Hence, this would be a useful method, easy to implement and interpret by cave managers  
Figure 2: Location of microclimatic monitoring stations. from their own microclimatic records and in confrontation with rigorous daily control of the number of visitors and access to their own caves.