Chemical and isotopic composition of the indoor ambient air of Puerto Naos and La Bombilla, La Palma, Canary Islands

During and after the end of the 2021 Tajogaite eruption (La Palma, Canary Islands), anomalous CO₂ degassing has been detected in the neighborhoods of La Bombilla and Puerto Naos, located around 5 km distance southwestern of the 2021 Tajogaite eruption vents. The aim of this study is to determine the indoor air quality of the houses of the aforementioned neighborhoods. For that purpose, from August 11 to October 24, 2022, air samples were taken, for further analysis, from indoors of 10 locations in Puerto Naos, on a weekly basis. In addition, on September 22, 2022, a discrete survey of the indoor ambient air was carried out in 10 houses of La Bombilla, consisting on in-situ measurements and gas sampling for further analysis.

Gas samples were taken for a complete geochemical characterization (i.e., He, Ar, Ne, H₂, N₂, O₂, CH₄, CO contents) by micro-gas chromatography (micro-GC) and quadrupole mass spectrometry (QMS) and, as well as for carbon isotopic analysis of the CO₂ (δ¹³C-CO₂) by isotopic ratio mass spectrometry (IRMS). In-situ measurements of CO₂, O₂, ²²²Rn, ²²⁰Rn, H₂S and Hg⁰ were conducted in La Bombilla with and without natural ventilation.

National Health Systems in the European Union reflect that the upper limit of the acceptable CO₂ concentration range for long-term exposure in the indoor ambient air of buildings for residential use should be of the order of 1,000-1,200 ppm to guarantee people health. The concentrations of CO₂ registered in the indoor ambient air of the 10 houses of La Bombilla determined by the in-situ measurements showed relatively high values -above 5,000 ppm- even reaching a maximum of 183,900 ppm in conditions without natural ventilation. In these conditions of absence of ventilation, a certain displacement of O₂ was observed, which dropped to 18.7% in the worst case. Under conditions with natural ventilation for a period of 2 hours, the range of CO₂ concentration fell to a range between 1,050 and 14,200 ppm and the O₂ concentration registered was 20.9%. These results reflect that natural ventilation, and even more forced ventilation, would contribute to reduce CO₂ concentration in the ambient air inside buildings. Regarding the results of the indoor gas samples analysis from Puerto Naos, the CO₂ concentration and the δ¹³C-CO₂ mean values ranged from 1,190 to 230,952 ppm and -7.9 to -4.8‰ vs. VPDB, respectively. These results of the chemical and isotopic composition of the indoor ambient air of Puerto Naos and La Bombilla demonstrate the importance of these studies to monitor and manage these silent hazards that pose a threat to the population and restrict access to their houses.