This study investigated the secondary organic aerosols (SOAs) formation capability of various fragrant and Chinese herbal essential oils in the presence of ozone in a controlled-environment chamber under different test conditions. Air sampling was also conducted in two different types of spa center offering massage therapy using essential oils. The chamber study showed very strong SOAs formation capability in all fragrant essential oils tested, together with almost no SOA formation in the experiments using Chinese herbal oils. As particles smaller than 50nm dominated the ultrafine particles (< 100 nm) in number, this study speculated that SOAs from the reaction of ozone and terpenes initially formed in particle sizes smaller than 50 nm. Not only elevated level of total volatile organic compounds and limonene, but also a significant increase of ultrafine particles was found in spa centers during massage therapy. This study concludes that configuration and ventilation within spa centers can potentially affect the level of indoor air pollutants emitted during massage therapy. Therefore, 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 SOAs, in the spa centers are an area of concern.