Beware of spas offering aromatherapy—they may have higher levels of harmful indoor air pollutants, including volatile organic compounds (VOCs) and ultrafine particles. These compounds in fragrant essential oils, derived from plants, react with ozone in the air to produce small, ultrafine byproducts called secondary organic aerosols (SOAs) which may cause eye and airway irritation. A study was conducted by Der-Jen Hsu (National Kaohsiung First University), Hsiao-Lin Huang (Chia-Nan University) and Shiann-Cherng Sheu (Chang-Jung Christian University) -- all in Taiwan. They tested both fragrant and Chinese herbal essential oils for SOA formation in a controlled-environment study chamber under different test conditions, the journal Environmental Engineering Science reported. They also performed air sampling and analysis in spa centres that offer massage therapy using essential oils, according to a statement of National Kaohsiung University. They concluded that the layout and ventilation within a particular spa may affect the level of indoor air pollutants produced during massage with aromatherapy.