Indoor Burning of Arabian Incense Generates Abundant Ultrafine Particles with Strong Oxidative Potential

Arabian incense (Bakhoor) burning is a widely practiced fragrancing and ceremonial activity, yet how the Bakhoor composition controls particle emissions and oxidative potential remains poorly constrained, especially under repeated use in low-ventilation settings. Here we characterized emissions from Bakhoor burning in a controlled chamber using a charcoal-assisted heating configuration representative of common practice and quantified aerosol oxidative potential using complementary acellular dithiothreitol (DTT) activity and a macrophage-based intracellular oxidative-stress response, with smoldering sidestream cigarette smoke as a protocol-matched indoor combustion reference. Normalized by the initial Bakhoor mass per burn, Bakhoor burning produced particle mass and number emission rates of 670–1690 µg min⁻¹ g⁻¹ and (6–7) × 10¹¹ particles min⁻¹ g⁻¹, respectively. Ultrafine particles contributed 70 %–75 % of the total particle number, and their emission rates substantially exceeded those from sidestream cigarette smoke. Across Bakhoor materials, emission magnitude followed a nonlinear power-law relationship with the loading of the hexane-soluble fraction, indicating that this fraction is an important control on particle production. In the acellular assay, the water-soluble particle mass-normalized DTT consumption rate (OP_{DTT, m}^WS) was approximately 32 pmol min⁻¹ µg ⁻¹, modestly lower than that of cigarette smoke particles, whereas Bakhoor burning particles elicited stronger intracellular oxidative-stress responses. Ozone aging increased oxidative potential for both sources, and the acellular and cellular responses remained evident after aging equivalent to days of indoor exposure. Overall, Bakhoor burning represents a previously underrecognized source of ultrafine aerosol with substantial oxidative potential. © 2026 Liyuan Zhou et al.