Is exhaled carbon dioxide an appropriate tracer for assessing airborne transmission risk?

• CO 2 is a versatile indicator of general indoor ventilation conditions. • CO 2 is not an indicator of inhalation exposure. • Inhalation exposure to droplets and exhaled CO 2 is not analogous to a contact events. • Exhaled CO 2 can determine airborne infection risk, but is unreliable for wider IPDs. Understanding the inhalation exposure to exhaled carbon dioxide (CO 2 ) and respiratory droplets in close-contact interactions is critical for assessing airborne infection risks in indoor environments. This study presents a numerical analysis comparing the spatiotemporal transport and inhalation of exhaled CO 2 (modeled as a passive scalar) and virus-laden droplets (modeled using a Lagrangian discrete phase model) under various interpersonal distances, body postures, and respiratory activities (namely speaking and coughing). The results showed that although fine droplets (<5 μm) exhibit similar dispersion patterns to CO 2 owing to their aerodynamic behavior, larger droplets deviate significantly because of gravitational and inertial forces. The arrival time and inhalation amount of CO 2 were often earlier and higher, respectively, than those of the droplets, particularly in speaking scenarios. However, in coughing cases, the rapid dilution of the exhaled CO 2 jet led to an underestimation of droplet exposure. Furthermore, exhaled CO 2 did not account for dermal and nasal deposition, particularly for larger droplets. These findings highlight that CO 2 can serve as a useful tracer for visualizing airflow and aerosol-like behavior but not as a reliable proxy for droplet transmission risk. Collectively, accurate risk assessment requires the integration of particle-specific dynamics, evaporation, and realistic emission profiles.