Fig. 3.

The CFD simulation of particle transport and deposition to evaluate the risk under (A) an elevator setting with a simulated asymptomatic individual breathing under high ventilation of 30 ACH (supplementary movie S1), speaking under high ventilation (supplementary movie S2) and low ventilation of 2 ACH (supplementary movie S3), (B) a small classroom setting with a simulated asymptomatic instructor and the ceiling ventilation system located in the back (the top part of the figure, supplementary movie S4) and front of the classroom (the bottom part of the figure, supplementary movie S5), respectively, and (C) a small supermarket setting with a simulated asymptomatic shopper (his/her 10 stops along the dashed route are marked in the schematic) and the ceiling ventilation system located at the back corner (the top part of the figure, supplementary movie S6) and entrance of the supermarket (the bottom part of the figure, supplementary movie S7), respectively. Except for the low ventilation setting in A, the ventilation rate of each setting is designed to replace all the air in the space every 2 min, representing the upper bound of recommended ventilation condition of each setting. Under each setting, the risk of a person encountering virus-containing particles at one specific location ($I_{\text{risk}}$) is evaluated as the total particle number passing through this location during the simulation time, which can be interpreted as number of particles a person can inhale at this location during the simulated time. Note that such an estimate of the inhaled particle number only provides an estimate of maximum particle encounter since it does not consider the detailed flow processes involved in the inhalation of particles. In the 3D contour plots, the wall contours are the contours of $I_{\text{risk}}$ spatially averaged (denoted as ${\bar{I}}_{\text{risk}}$) along x, y, and z directions, respectively. In addition, assuming the height of each individual is ~1.75 m, selected horizontal slices of $I_{\text{risk}}$ contour at the height of human mouth are highlighted to show the $I_{\text{risk}}$ for individuals standing (1.6 m for A and C) or sitting (1.2 m for B) at different locations, and several representative locations are marked as safe or hot based on their $I_{\text{risk}}$ values (safe criterion: $I_{\text{risk}}\le 1$ for A; $I_{\text{risk}}\le 200$ for B; $I_{\text{risk}}\le 100$ for C; note that safe or hot zones are determined based on the overall $I_{\text{risk}}$ in each setting).