Seasonal Indoor-to-Outdoor Ratio (I/O Ratio) of Fine Particulate Matter and Ozone Concentrations in Various Microenvironments in South Korea

Background: Exposure to fine particulate matter (PM 2.5 ) and ozone (O 3 ) poses potential health risks.The Indoor-to-Outdoor ratio (I/O ratio) is a valuable tool for understanding indoor air quality and identifying potential indoor sources. Objectives:The objective of this study was to determine I/O ratios of PM 2.5 and O 3 by different microenvironments and seasons in Korea.Methods: From December 2021 to November 2023, indoor concentrations of PM 2.5 and O 3 were monitored every hour in 13 microenvironments (residential indoor, office, school, restaurant, pub, caf, study caf, private educational institute, PC room, billiard room, screen golf center, supermarket, and shopping mall) in Korea.Hourly outdoor concentrations of PM 2.5 and O 3 were obtained from local air quality monitoring stations, provided by airkorea.or.kr.The hourly I/O ratio was calculated by the indoor and outdoor concentrations.Results: At the pub, billiard room, and PC room, the median PM 2.5 I/O ratio exceeded 1 in all seasons, except in spring at the PC room (0.9), suggesting indoor smoking as a potential cause.The median PM 2.5 I/O ratio at the restaurant exceeded 1 in winter, autumn, and summer, except for spring (0.9), indicating significant PM 2.5 emission sources in the restaurant.The median O 3 I/O ratio was below 0.5 in all seasons and microenvironments. Conclusions:This study provided useful data on relationships between indoor and outdoor pollution in various microenvironments by seasons.These I/O ratios could be applied for more accurate exposure assessment to protect health of human.