Time-dependent emission characteristics of terpenes from an essential oil diffuser and indoor exposure assessment...

Essential oil diffusers containing monoterpenes such as limonene, α-pinene, and β-pinene are widely used indoors, yet their impact on indoor air quality remains insufficiently understood. This study investigated the time-dependent emission behavior of monoterpenes from a commercial essential oil diffuser and assessed indoor exposure using a wearable passive sampling approach. The diffuser was operated in an office environment for two weeks, and terpene concentrations were measured with a pre-developed passive sampler. Emission rates were highest immediately after initial deployment and gradually declined over time. Limonene exhibited the highest emission rate, followed by β-pinene and α-pinene, reflecting their relative composition in the product. Within the first 2 h of operation, indoor concentrations increased from background levels of 1.8, 0.17, and 0.12 μg m^-³ to 5.8, 2.2, and 1.7 μg m^-³ for limonene, α-pinene, and β-pinene, respectively (corresponding to increases of 3.2-, 13-, and 14-fold). An additional unidentified compound exhibited a pronounced transient increase (up to 698-fold relative to background) during the initial deployment phase. However, its chemical identity and formation pathway remain uncertain, and both atmospheric formation and potential sampling artifacts cannot be excluded. These results demonstrate that essential oil diffusers can cause short-term increase in indoor terpene concentrations and may be associated with transient secondary chemistry, particularly during initial use. The findings highlight the importance of considering time-dependent emissions and secondary chemistry when evaluating indoor exposure associated with essential oil diffuser use.