This study aimed to examine the formation of secondary air pollutants (formaldehyde and secondary organic aerosols (SOAs)) in aromatherapy workplaces in which aromatic and volatile organic compounds were the dominant handling materials. A representative aromatherapy workplace with a split air-conditioner ventilation system was selected. Levels of various indoor pollutants, including total volatile organic compounds (TVOC), terpenes, ozone, particle size distribution, and formaldehyde, were measured. The results indicated that the levels of TVOC and terpenes that evaporated from essential oils increased considerably during periods of aromatherapy in the spa space. This study demonstrated that formaldehyde and SOAs were generated through ozone-initiated reactions with the major constituents of essential oils (terpenes) in an environment with low ozone concentrations (17.2-21.4 ppb). Variations in formaldehyde concentration (0.021-0.024 ppm) were in significant during the aromatherapy process. Secondary organic particles with diameters of 30-130 nm were present in significant levels during periods of aromatherapy. Generated formaldehyde and nano-sized SOAs should not be neglected in aromatherapy environmental settings because of their potential health effects. The results suggest that various control strategies for decreasing exposure to indoor secondary air pollutants may be used, such as improving ventilation systems, indoor decorations, or materials in aromatherapy workplaces. (C) 2012 Elsevier Ltd. All rights reserved.