Infectious risk prevention and control methods of small open space in post-epidemic era: Risk analysis of airborne transmission of respiratory infectious diseases based on CFD wind environment simulation

In 2020, the COVID-19 broke out, the world adopted strict home quarantine to control the spread of respiratory infectious diseases. However, indoor isolation can hardly satisfy people's needs of outdoor life. Small open spaces that are largely diversified, widely distributed and easily accessible, have become the first choice of recreation for urban residents during the pandemic. At the same time, compared with indoor activities, outdoor space has a lower risk of infection and the space is more open, undertake more activities during the epidemic, such as small live show, vaccination sites, and entrance queuing space. However, the airborne transmission of respiratory infectious diseases is greatly affected by the wind environment. The transmission distance is long, and the virus concentration measurement is difficult in small open spaces. There are many gathering activities with high population density and frequent face-to-face interactions. Plus, people’s awareness of disease prevention in the outdoors is low and they may not able to keep social distancing as indoor. We still don't know how much of an infection risk there is, and whether small open space designs can response to that. According to the existing research on physical environment in urban planning, different building combinations in small open space can affect the wind environment, which will affect the direction of pollutant diffusion and the concentration. On this basis we overlay with the infection probability prediction model of airborne infectious diseases, Wells - Riley equation, and add the activity time as variable. So, the research expands the relationship between the building combination and the physical environment (wind environment), to infection risk. And then assessing the resilience of different types of small open spaces in response to respiratory infections. To be specific, this research firstly classifies small open spaces according to the forms of building combination. Then we analyze the mechanism of the airborne transmission of respiratory infectious diseases, and the wind environment is linked to the transmission risk. After that we discuss the risks under different building combination forms and activity scenarios, and divide the risk areas in small open spaces. Based on the comparison of different open space design and the temporal schemes, we come up with the spatial optimization layout to reduce the risk areas and the control strategies of outdoor small open space in the post-epidemic era. The results showed that the risk of infection is linked with the spatial and temporal attributes in small open space. In the space with good ventilation environment, the high-risk zone is elliptical and small along the main wind direction. In the space with poor ventilation, the high-risk zone is irregular and large. However, the ventilation is too poorly to be suitable for gathering activities in some specific form open spaces. From the perspective of time, the risk of infection is affected by both virus concentration and stay time, and the risk of infection increases exponentially in high virus concentration space for a long time. It is concluded from the research that a small open space with good ventilation has a low risk of infection, while in the winter with high incidence of infectious diseases, the greater wind speed may affect the comfort, and in the follow-up study we’ll add comfortability of wind for discussion. The time of activities also significantly affects the risk of infection. Even in areas with low virus concentration, prolonged stay still carries a greater risk. Therefore, it is also very important to control the time of activities in small open space.