<p>Volcanic eruptions represent significant episodic sources of atmospheric pollutants, emitting large amounts of particulate matter (PM), sulfur dioxide, and trace metals. These emissions are associated with adverse cardiovascular and respiratory health outcomes, yet the specific chemical constituents responsible remain insufficiently defined. This study investigates the chemical composition and biological effects of water-soluble organic matter (WSOM) isolated from fine particulate matter (PM_2.5) collected during a volcanic eruption and a non-volcanic reference period. Structural characterization of WSOM was performed using liquid-state nuclear magnetic resonance (NMR) spectroscopy. The cytotoxic, immunomodulatory, and oxidative effects were assessed in the human monocytic cell line THP1. A total of 25 distinct molecular structures in the two WSOM samples (volcanic event period and reference period), with 7 unique to the volcanic eruption period, including branched mono- and di-carboxylic acids, alkenes, nitro-aromatics, and 3-methyl-2,6-dinitrophenol. Both WSOM samples increased transcription of co-stimulatory molecules and proinflammatory cytokines in monocytes. WSOM exposure also elevated expression of adhesion molecules implicated in the progression of atherosclerotic plaques and increased cardiovascular disease risk. The results indicate that a short-term, long-distance volcanic event did not substantially modify the cytotoxic properties of fine aerosol WSOM in an urban environment influenced by anthropogenic emissions.</p>