In vitro assessment of human alveolar epithelial responses to acute combined exposure to volcanic ash and sulphur dioxide gas.

Volcanic activity releases sulphur dioxide (SO₂) gas and fine particulate matter that may degrade air quality in populated areas, yet the effects of combined exposure remain insufficiently characterised. We examined the biological effects of acute combined exposure to volcanic ash and SO₂ in human alveolar epithelial cells (A549) maintained at the air-liquid interface. Cells were exposed to respirable ash from the 2021 Tajogaite eruption (La Palma; 100 µg/cm²) and SO₂ gas (nominal concentration 50 ppm; 1, 2, or 3 h), separately or sequentially. Cellular outcomes evaluated after exposure included cytotoxicity, pro-inflammatory and redox responses. Tajogaite ash alone had limited effects on cells, while exposure to SO2 altered inflammatory responses (increasing gene transcription while reducing extracellular cytokine levels). Co-exposure responses were largely comparable to SO₂ alone, with no statistically significant evidence of synergistic effects under the conditions tested. These findings highlight the importance of considering gas-phase exposures in volcanic environments.