2018 Yes https://linkinghub.elsevier.com/retrieve/pii/S0160-4120(18)30996-6 Salonen, Heidi North America Europe Asia Observational Exposure Study Ozone (O3) Air pollution Ozone (O3) Indoor sources Outdoor sources School environments Office environments The calculated median concentration of ozone both in school (8.50 μg/m3) and office (9.04 μg/m3) settings was well below the WHO guideline value of 100 μg/m3 as a maximum 8 h mean concentration. However, a large range of average concentrations of ozone was reported, from 0.8-114 μg/m3 and from 0 to 96.8 μg/m3 for school and office environments, respectively, indicating situations where the WHO values are exceeded. Outdoor ozone penetrating into the indoor environment is the main source of indoor ozone, with median I/O ratios of 0.21 and 0.29 in school and office environments, respectively. The absence of major indoor ozone sources and ozone sinks, including gas-phase reactions and deposition, are the reasons for lower indoor than outdoor ozone concentrations. However, there are indoor sources of ozone that are of significance in certain indoor environments, including printers, photocopiers, and many other devices and appliances designed for indoor use (e.g., air cleaners), that release ozone either intentionally or unintentionally. Due to significantly elevated outdoor ozone concentrations during summer, summer indoor concentrations are typically elevated. In addition, the age of a building and various housing aspects (carpeting, air conditioning, window fans, and window openings) have been significantly associated with indoor ozone levels.