High-Resolution Environmental Monitoring of a Prehistoric Rock Painting Cave for Preventive Conservation (Santián Cave, Northern Spain)

Sustainable public access to rock art caves requires an evaluation of how visitor presence alters cave microclimates. This study analyzed the response of Santián Cave (Cantabria, northern Spain) to controlled experimental visits conducted during the seasonal phase of reduced cave ventilation and elevated background CO2. Visitor impact showed a strong spatial contrast: Sector I exhibited only minor thermal anomalies (0.01–0.02 °C), whereas the inner decorated sector recorded mean increases of 0.11 °C in Conjunto I and 0.28 °C in Conjunto II, with a maximum of 0.37 °C. CO2 showed the clearest cumulative behavior, with daily increases of 268–368 ppm in Conjunto I and 327–376 ppm in Conjunto II, incomplete overnight recovery, and delayed propagation into connected sectors. Suspended particles also increased with visit intensity, from values below 300 particles L−1 for spaced groups of five visitors to a maximum of 686 particles L−1 and recovery times of 13.6 h for consecutive groups of 6–8 visitors. The results show that the most stable cave areas are highly sensitive to visits, cumulative effects become significant without adequate recovery time, and CO2 serves as the best short-term indicator for access management. The proposed thresholds should be considered preliminary and seasonally dependent.