1. Introduction 1 National Institute for Environmental Studies 2 Environmental Science Analysis & Research Laboratory Biomass burning (BB) Global Environmental Research 27/2023 27-35 printed in Japan 16-2 Onogawa, Tsukuba, Ibaraki, 305-8506, Japan 1-500-82 Matsuo-yosegi, Hachimantai, Iwate, 028-7302, Japan *E-mail: Key words: agricultural open burning, dithiothreitol assay, heme oxygenase-1 assay, oxidative stress Oxidative stress due to exposure to PM2.5 that generates reactive oxygen species (ROS) is one of the pathways leading to morphologic changes and lung function decrements that are linked to exacerbation of asthma and chronic obstructive pulmonary disease. Biomass burning (BB) is one of the largest sources of PM with its expected induction of ROS through exposure. Open burning of agricultural residues is widely conducted in September and October in Japan, but it is unclear to what extent the oxidative stress induction ability (OSIA) is increased during the active BB season compared to other seasons, when the effects of BB are different. In this study we conducted heme oxygenase-1 assays on PM2.5 samples collected during the active BB season (October 2015) in Tsukuba, Japan, to clarify the influence of BB on OSIA. On average, BB particles were estimated to be responsible for 25% of PM2.5, and 80% of OSIA in October 2015. At that time, PM2.5, BB particles and OSIA were up to two times, eight times and seven times higher, respectively, than during seasons of low BB activity. Thus, elevated concentrations and OSIA are considered to arise mainly from BB particles. The results indicate that management of BB activity is important for good air quality and public health. Respiratory tract oxidative stress after short term exposure to PM2.5 resulting in reactive oxygen species (ROS) generation is one pathway leading to morphologic changes and lung function impairment, which are linked to exacerbation of asthma and chronic obstructive Yuji FUJITANI1*, Akiko FURUYAMA1, Seishiro HIRANO1, Akihiro FUSHIMI1 Katsumi SAITOH1, 2, Ayako YOSHINO1, Kei SATO1 and Akinori TAKAMI1 is a major source of atmospheric aerosols and a cause of global and regional issues, including adverse effects on human health (Johnston, et al., 2012) and climate system perturbations (Myhre, et al., 2013). In Japan, agricultural open burning is often conducted during the harvest season in rice fields in September and October to reduce agricultural residues, such as rice straw and rice husks, as well as to alkalize fields. Estimates of domestic annual PM10 and PM2.5 emissions (as of the year 2000) from open burning (not only field burning but also waste incineration) were 25 Gg and 18 Gg, representing 13% and 12% of total emissions in Japan, respectively (Kannari, et al., 2007). Abstract pulmonary disease (COPD) (U.S. EPA, 2019). Cells protect themselves against oxidative stress by generating antioxidant compounds such as glutathione and antioxidant enzymes such as heme oxygenase-1 (HO-1). The gene expression underlying this enzyme synthesis is the initial stage of oxidative stress (Hellack, et al., 2017). Thus, oxidative stress induction ability (OSIA) can be assessed in vitro through an HO-1 cellular assay (Li et al., 2003) and has been evaluated against PM samples. Acellular methods for determining oxidative potential (OP), such as a dithiothreitol (DTT) assay (Kumagai, et al., 2002), have also been widely performed on PM samples as a proxy indicator of potential of oxidative stress in the last two decades (Shiraiwa, et al., 2017; Bates, et al., 2019). These PM assays have revealed the emission sources and chemical components of PM that cause oxidative stress. Previous studies have shown that BB emissions contain high levels of humic-like substances (Hoffer, et al., 2006) which induce large amounts of ROS (Lin and Yu, 2011; Verma, et al., 2012) and that, BB ©2023 AIRIES 27 Oxidative Stress Induction Ability of Particles Emitting from Agricultural Open Burning in Japan

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