An overview of CO2 level as an infection risk proxy for different indoor environments in Istanbul

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Abstract
Indoor CO₂ concentration is widely recognized as an effective proxy for infection risk and ventilation efficiency. In addition to improving indoor air quality, the necessity of performing indoor ventilation correctly, effectively, and efficiently to prevent or minimize the spread of SARS-CoV-2 has gained even more importance during the COVID-19 pandemic. In this study, carbon dioxide (CO 2 ), fine particulate matter (PM 2.5 ,) temperature, and humidity were monitored in sixteen different indoor environments belonging to four categories: markets, restaurants, public transportation (Metrobus), and shopping centers in Istanbul, Türkiye, during March and April 2021. Measurements were conducted using calibrated low-cost sensors (± 30 ppm accuracy for CO 2 ) that recorded data every 20 s for 15 min on four different days. The lowest variation in CO 2 measurements was observed in shopping centers (533–663 ppm), while the highest was in restaurants (532–974 ppm). Humidity, temperature, and PM 2.5 values were also recorded and discussed. Results indicate that restaurants exhibited the highest CO 2 based infection risk due to limited or natural ventilation, whereas shopping centers with central mechanical ventilation maintained CO 2 concentrations below 650 ppm, indicating low risk. Ventilation management through continuous CO 2 monitoring is emphasized as a crucial strategy for controlling epidemic risks indoors. Recommendations include improving ventilation efficiency, using portable air cleaners with HEPA/UV filters, and visually displaying CO 2 levels via a “traffic-light” system to inform occupants about indoor safety. This study provides the first quantitative comparison of CO 2 -based infection risk across multiple indoor environments in Istanbul, providing baseline data for future research and public health policies.
Funding Information
  • Istanbul University Cerrahpaşa

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