Monoterpenes and terpene alcohols in indoor air

Monoterpene hydrocarbons and alcohols are major subsets of terpenoids, among the most common biogenic compounds in nature. Because of favorable fragrance and solvent properties, such compounds are found in products commonly used indoors. They are also emitted incidentally from foods, especially during cooking, and from plant-based materials, including wood. This review critically assesses the state of knowledge about nine terpene hydrocarbons (myrcene, d-limonene, terpinolene, α-terpinene, γ-terpinene, α-pinene, β-pinene, camphene, 3-carene) and three terpene alcohols (linalool, α-terpineol, terpinen-4-ol), addressing physicochemical properties, indoor concentrations, sources and emission rates, and dynamic behavior. The most prominent species are d-limonene and α-pinene, with central tendency residential concentrations of 25 µg m -3 and 16.5 µg m -3 , respectively. Average residential concentrations are lower for β-pinene (3.4 µg m -3 ), 3-carene (6.5 µg m -3 ), and camphene (0.6 µg m -3 ). Indoor concentrations vary substantially among different environments and over time within a given environment. Concentrations are much higher indoors than outdoors pointing to the importance of indoor sources. From a study of approximately 300 homes in three US cities, average indoor-to-outdoor emission rates are 3.5, 0.55, and 0.55 mg/h for d-limonene, α-pinene and β-pinene, respectively. Important dynamic processes affecting the concentrations and fates of monoterpenes and terpene alcohols indoors include oxidative decomposition and sorptive partitioning. Oxidative decay rates vary widely among terpenes. Ozonolysis is important, and reactions with OH, NO 3 , and chlorine radicals can also be meaningful. Of special concern is the production of reaction byproducts, including secondary organic aerosol in addition to formaldehyde and other oxidized organic compounds.