Germán D. Padilla^1,2, Carmen López³, Nemesio M. Pérez^1,2, Rubén López³, Pedro A. Hernández^1,2, Luca D’Auria^1,2, Gladys V. Melián^1,2, Daniel D’Nardo¹, Alexis González⁴ and Juan A. Bermejo⁵

¹Instituto Volcanológico de Canarias (INVOLCAN), 38400 Puerto de la Cruz, Tenerife,

Canary Islands, Spain

²Instituto Tecnológico y de Energías Renovables (ITER), 38600 Granadilla de Abona, Tenerife, Canary Islands, Spain

³Observatorio Geofísico Central, Instituto Geográfico Nacional (IGN), 28014 Madrid, Spain ⁴Hidrolap Medioambiental S. L., 38750 El Paso, La Palma, Spain

⁵Cabildo Insular de La Palma, 38700 Santa Cruz de La Palma, Spain

Abstract: The 2021 Tajogaite eruption (La Palma, Canary Islands, Spain) generated an anomalous diffuse degassing process characterized by high concentrations of volcanic-hydrothermal carbon dioxide (CO₂) in the populated areas of Puerto Naos and La Bombilla, located about 6 km southwest of the eruptive center. Although these settlements were not directly affected by lava flows, elevated CO₂ concentrations (>5-20%) in indoor environments forced the evacuation of residents and restricted access for several years. CO₂ enters buildings through fractures, utility conduits and structural openings, leading to accumulation in poorly ventilated spaces. As an invisible, odorless toxic, and asphyxiating gas, high CO₂ concentrations may pose a significant risk to human health and safety. Immediate ventilation is required if the CO₂ concentration exceeds 2,000 ppm, meanwhile immediate evacuation of indoor spaces is recommended if the CO₂ concentration exceeds 1.5% (15,000ppm).

To support risk mitigation and emergency management, a high-density monitoring network was progressively deployed throughout both settlements. By June 2026, the network comprised 1,520 operational indoor stations financed by the ALERTA CO₂ project, integrated into a 24-hour monitoring room. This infrastructure constitutes the largest permanent volcanic CO₂ monitoring network installed in an inhabited area. Continuous real-time measurements allow rapid identification of hazardous accumulations and provide authorities with objective information for decision-making. The unprecedented spatial coverage of the network has revealed strong variability in CO₂ concentrations at neighbourhood, building and property scales, indicating that gas accumulation is controlled by highly localized geological and structural factors. While average and median concentrations generally show a decreasing trend since the eruption, episodic accumulation events continue to occur, demonstrating that the phenomenon remains dynamic several years after the end of eruptive activity. Monitoring results have played a key role in the progressive recovery of affected communities. As of June 2026, more than 1,100 residential and commercial properties had been authorized for occupancy under continuous surveillance, highlighting the value of dense real-time monitoring networks for protecting public health, reducing volcanic gas risk, and supporting the socio-economic recovery of communities affected by long-lasting post-eruptive degassing.

Keywords: Carbon dioxide (CO₂), monitoring, concentration, volcanic hazard, Puerto Naos, La Bombilla, La Palma, Canary Islands

Author Profile (first or corresponding author):

Nemesio M. Pérez, INVOLCAN’s Scientist-in-charge, mainly engaged in geochemical research applied to volcano monitoring, surface geothermal exploration, groundwater studies and environmental issues, etc.

E-mail: nperez@iter.es