Exhaling indoors for hours

Contact tracing studies of who infected whom found many episodes in which people interacted indoors for hours, and few with transmission outdoors (Frieden, Lee; Nishiura, Oshitani, et al.; Qian et al.). The previous paragraph described patients transmitting to their families at home. In other episodes, people were breathing, coughing, sneezing, yelling, and touching each other (Asadi et al.; Atkinson et al.; Tang, Li et al.) in restaurants (Lu, Gu et al.), a telephone call center (Park et al.), choir rehearsal (Hamner et al.; Read), indoor sports (Dawson), and family gatherings (Bromage; Ghinai, Woods et al.). Close, prolonged contact indoors with presymptomatic and symptomatic cases caused several cluster outbreaks (Cevik; Cevik, Bamford, Ho). The relationships most likely to transmit were: friends, family members, taking the same transportation, living with each other, being within 6 feet of each other, and eating together, more likely than brief contacts or with non-family (Bagget et al.; Bi et al.; Burke et al.; Chen, Wang, et al.; Cheng, Jian et al; Danis et al.; Ghinai, McPherson et al.; Jing et al.; Li W et al.; Yong et al.) In superspreader events, large groups talked, sang, etc. indoors. But doing those activities outdoors, or keeping quiet or speaking softly indoors (such as theaters, class rooms, or offices), did not spread COVID-19 to large numbers (Kay). So, outdoor venues are safer, indoor places with flow-through ventilation are less safe, and indoor places with little ventilation are the least safe (Heil).