INTRODUCTION: The emergence of heated tobacco products (HTPs) has made it important to monitor HTP-generated aerosols in addition to combustible cigarette (CC) smoke as a source of secondhand tobacco (SHT) exposure. We investigated the trend of SHT exposure in school-aged children and assessed whether SHT exposure depended on household tobacco use status. AIMS AND METHODS: This repeated cross-sectional study from 2011 to 2021 (15 927 participants) was based on data from an annual survey of fourth-grade students (aged 10 years) in Kumagaya City, Japan. In addition to a questionnaire which includes questions about household tobacco use status, we measured the urinary cotinine levels of each participant by their first-morning urine sample to objectively assess SHT exposure. We defined the participants with urinary cotinine levels ≥5.0 ng/mL as being exposed to SHT. RESULTS: The prevalence of SHT exposure decreased over the 11-year period from 18.6% in 2011 to 5.3% in 2021. It was significantly higher in households with tobacco users than without tobacco users (t-test p<
.001). Prevalence of SHT exposure was 1.4% among the 68.1% of households not using tobacco, 22.9% among the 16.5% using only CC, 3.1% among the 12.3% using only HTP, and 27.6% among the 3.9% of households using CC and HTP. CONCLUSIONS: While the prevalence of SHT exposure showed a decreasing trend from 2011 to 2021, the prevalence of SHT exposure was higher in children with household members using tobacco products, regardless of the type of tobacco product, than in those without tobacco users. IMPLICATIONS: This study observed that the prevalence of SHT exposure was higher among children in households with tobacco users than among those without tobacco users, regardless of the type of tobacco product. Our findings highlight the importance of advocating that HTPs do not reduce the likelihood of SHT exposure to bystanders.