One study from China says that, of 7,324 reported infections, only one case with two infections was traced back to an open-air setting (Indoor transmission of SARS-CoV-2, Hua QIAN1,*, Te MIAO2,*, Li LIU3, Xiaohong ZHENG1, Danting LUO1, and Yuguo Li2,4,*).
This means that COVID-19 is the first and foremost disease that transmits indoors. An aerosol is a suspension of fine solid particles or liquid droplets in air. During breathing, exhaled aerosol particles are generated. These aerosols particles usually range between 0.2 and 0.6 μm and can transport viruses out of the lungs and be present in the room air for hours.