Aromatherapy uses fragrant essential oils, derived from plants, and the therapeutic technique is a popular offering at spas.  Der-Jen Hsu, from the National Kaohsiung First University of Science and Technology (Taiwan), and colleagues report that the use of essential oils in aromatherapy may emit potentially harmful indoor air pollutants such as volatile organic compounds (VOCs), the degradation of which by ozone present in the air can produce small, ultrafine byproducts known as secondary organic aerosols (SOAs), which may cause eye and airway irritation.  The team tested both fragrant and Chinese herbal essential oils for SOA formation in a controlled-environment study chamber under different test conditions. They also performed air sampling and analysis in spa centers that offer massage therapy using essential oils.   Writing that: “This study concludes that configuration and ventilation within spa centers can potentially affect the level of indoor air pollutants emitted during massage therapy,” the researchers warn that: “Indoor air quality in the environments using essential oils and the health effects caused by human exposure to volatile organic compounds and terpenes ozonolysis products, such as [secondary organic aerosols], in the spa centers are an area of concern.”