This study investigated residual aerosol characteristics and airborne transmission risk in a classroom associated with a COVID-19 cluster outbreak. Field experiments were conducted in an actual language institute classroom where a cluster infection occurred in 2021. The research focused on the possibility of time-shift airborne exposure caused by residual aerosols remaining after the conclusion of class. Dropletlike particle experiments were performed under actual classroom operating conditions consisting of a 55-minute class period and a 5-minute break period. Aerosol behavior and concentration changes were analyzed under two conditions: opening the door and natural cross-ventilation through simultaneous opening of windows and doors. The results showed that approximately 74% of the droplet-like particles remained in the classroom after the 5-minute break period when only the door was opened. Based on the measured residual aerosol concentration, the airborne infection risk estimated using the Wells-Riley model was 29.31%. In contrast, under natural ventilation conditions, the infection risk decreased to 3.04%, demonstrating significant aerosol removal and risk reduction effects. These findings suggest that limited ventilation during short break periods may be insufficient to remove residual aerosols in educational facilities, while natural cross-ventilation can effectively reduce airborne transmission risk associated with residual aerosols.

Abstract
1. 서 론
2. 연구 방법
    2.1 연구대상 시설현황 및 집단감염 사례 개요
    2.2 비말유사입자 거동특성 실험 방법
    2.3 공기전파 위험도 평가 방법
3. 코로나19 집단감염 사례조사 및 비말유사입자 거동특성 실험
    3.1 코로나19 집단감염 사례 조사
    3.2 비말유사입자 거동특성 실험
4. 코로나19 공기전파 위험도 평가
    4.1 감염발생 환경조건의 공기전파 위험도 평가
    4.2 자연환기 적용 시나리오의 공기전파 위험도평가
    4.3 종합토론
5. 결 론
References
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• 배상환(한국건설기술연구원) | Sanghwan Bae (Department of Building Research, Korea Institute of Civil Engineering and Building Technology) Corresponding author
• 유정연(한국건설기술연구원) | Jungyeon Yu (Department of Building Research, Korea Institute of Civil Engineering and Building Technology)
• 강수진(서울대학교 보건대학원) | Su Jin Kang (Graduate School of Public Health, Seoul National University)
• 이상은(질병관리청) | Sang-Eun Lee (Central Disease Control Headquarters for COVID-19, Korea Disease Control and Prevention Agency)
• 박영준(질병관리청) | Young-Joon Park (Central Disease Control Headquarters for COVID-19, Korea Disease Control and Prevention Agency)
• 조수현(질병관리청, 연세대학교 대학원 간호학과) | Soo Hyeon Cho (Central Disease Control Headquarters for COVID-19, Korea Disease Control and Prevention Agency, Department of Nursing, Graduate School of Yonsei University)