Unraveling the chemical complexity and secondary VOCs formation in indoor combustion: candles, incense, and cooking

Indoor combustion activities, including candle burning, incense use, and cooking, are significant but understudied sources of reactive nitrogen species and oxygenated volatile organic compounds (OVOCs). Using real-time high-resolution mass spectrometry, we comparatively characterize emissions from these sources in a residential setting. The tested unscented candles emitted 46.86 ± 26.27 ppb NO x and 1.63 ± 0.92 ppb HONO which is ten times higher than the specific scented candles while the tested incense combustion released biomass-burning tracers (e.g., methoxyphenols, acetonitrile) and nitrogen-containing heterocycle compounds (e.g., pyrroles). Cooking with vegetable oils generated aldehydes (e.g., hexanal, nonanal) via fatty acid decomposition, with peanut oil producing more oxidized OVOCs than olive oil. By comparing days with different OH level, we show that post-cooking aldehyde rebound occurs only when OH concentration exceed a threshold, providing direct evidence for OH-driven secondary formation. This observation is consistent with OH-initiated photochemical production of secondary aldehydes, though we acknowledge that physical transport from the kitchen to the sampling point cannot be fully separated in this real-world residential setting. The burning of the selected incense sticks emerged as an unanticipated source of isocyanates (∼0.38 ppb isocyanic acid), comparable to biomass burning. Ammonia (NH 3 ), an underreported byproduct, increased by 3–9 ppb above background during candle/incense combustion, reaching absolute concentrations of 30–38 ppb. Our comparative analysis reveals the distinct chemical complexity of indoor emissions, their dependence on source-specific mechanisms, and the corresponding need for mitigation strategies to reduce human exposure to toxic pollutants.