Assessment of exhaled pathogenic droplet dispersion and indoor-outdoor exposure risk in urban street with naturally-ventilated buildings

Outdoor droplet exposure risk is generally regarded much smaller than that indoor, but such indoor-outdoor assessment and comparison are still rare. By coupling indoor and outdoor environments, we numerically simulate the ventilation and dispersion of exhaled pathogenic droplets (e.g., diameter d = 10 μm) within typical street canyon (outdoor, aspect ratio H/W = 1) and each room (indoor) of two eight-floor single-sided naturally-ventilated buildings. Inhaled fraction (IF) and suspended fraction (SF) between two face-to-face people are calculated to quantify and compare the human-to-human exposure risk in all 16 rooms (indoor) on eight floors and those at two outdoor sites. Numerical simulations are validated well by wind tunnel experiments. Results show that, the rooms in the 1st and 8th floors attain greater air change rate per hour (∼4.5–6.6 h−1) and the lower exposure risk (IF∼2–4 ppm) than the 2nd-7th floors (air change rate per hour∼1.6–5.3 h−1, IF∼4–11 ppm). Although inter-floor droplet dispersion exists, the room with index patient attains 2–4 order greater exposure risk than the other rooms without index patient. When the index patient stays outdoor, outdoor IF will change with locations, i.e. ∼55 ppm at leeward corner (even exceeding indoor IF∼2–11 ppm), and ∼7 ppm at middle street. Hence, the outdoor infection risk should not be ignored especially for people at leeward street corner where small vortex exists inducing local weak ventilation. Particularly, outdoor IF is decided by short-distance spraying droplet exposure (∼1 m) and long-route airborne transmissions by the main recirculation through entire street canyon (∼50–100 m).