Building and Environment 187 (2021) 107370 Available online 16 October 2020 0360-1323/© 2020 Elsevier Ltd. All rights reserved. Active ozone removal technologies for a safe indoor environment: A comprehensive review Marzieh Namdari, Chang-Seo Lee, Fariborz Haghighat * Energy and Environment Group, Department of Building, Civil and Environmental Engineering, Concordia University, H3G 1M8, Montreal, Canada A R T I C L E INFO Keywords: Ozone removal technologies Ozone decomposition mechanism MnO x -based catalysts ABSTRACT Ozone is a highly reactive gas and one of the critical air pollutants for both indoor and outdoor environments. The Occupational Safety and Health Administration (OSHA) determined that the permissible level of ozone is 100 ppb—for 8-h exposure at workplaces. Therefore, using an ozone removal technology can be crucial when outdoor ozone concentration is high and where active ozone emission sources exist. Activated carbon-based filters, catalytic decomposition, and photocatalytic decomposition are air treatment technologies that have been used for ozone removal. The catalytic decomposition approach showed higher efficiency and higher durability with no generation of considerable by-products, particularly manganese oxide (MnO x ) based catalysts, which can decompose ozone to oxygen at room temperature. The low cost, as well as high catalytic activity, are among the advantages of MnO x -based catalysts. High specific surface area, high density of oxygen vacancy, high reducibility, and low average oxidation state are desirable properties of the catalyst for ozone decomposition. Despite their excellent performance, their loss of activity in humid conditions challenges their widespread ap- plications. This review presents the recent studies on ozone decomposition technologies, particularly MnO x - based catalysts performance and modification techniques used to improve their performance, and potential future research directions in this field. 1. Introduction Ozone is one of the most critical inorganic pollutants due to its high reactivity and many adverse health effects [1]. These effects include respiratory irritation (e.g. cough, shortness of breath), chest pain, res- piratory inflammation, airway tissue damage, a decrease in lung func- tion, deadening the sense of smell, eye irritation, headache, and exacerbation of respiratory diseases and cardiovascular problems [2–7]. Ozone takes part in reactions with organic pollutants present in the in- door environment and generates C1 to C10 saturated aldehydes (e.g. formaldehyde, acetaldehyde), acetone, and organic acids [1,7–11]. Therefore, ozone removal from the indoor environments is crucial to avoid the adverse effects of not only ozone itself but also the wide range of hazardous chemical compounds that form through various reactions involving ozone. Despite the hazards of ozone exposure, it has various applications in the industry as an oxidizing agent [12]. As it cannot be stored nor transported in containers, it has to be generated in situ by ozone gen- erators. Ozone leakage occurs due to the malfunction of ozone generator components such as piping, gaskets, joint sealers, and valves [13,14], which puts a large number of workers and occupants at risk of exposure to high levels of ozone. To ensure the safety of workers, regulatory bodies and health organizations set exposure limits [13,15,16] (Table 1). An ozone removal technology is required to keep the ozone concentration level below these permissible limits or guideline levels. Additionally, meteorological conditions—high temperature, intense solar radiation, high humidity, low wind speed—and human activities that produce NOx and VOCs favours the ozone formation at ground-level. Natural or mechanical ventilation can introduce ozone to the indoor environment. Using ozone removal technology into the me- chanical ventilation system is a practical approach to prevent building occupants’ exposure. The increasing importance of ozone removal from indoor environ- ments, especially industrial workplaces, has recently motivated numerous studies on the effectiveness of various removal technologies. This paper reviews the current ozone removal technologies, ozone decomposition mechanisms, and the effect of relative humidity on the performance of each technology. * Corresponding author. E-mail address: fariborz.haghighat@concordia.ca (F. Haghighat). Contents lists available at ScienceDirect Building and Environment journal homepage: http://www.elsevier.com/locate/buildenv https://doi.org/10.1016/j.buildenv.2020.107370 Received 22 July 2020; Received in revised form 14 September 2020; Accepted 11 October 2020