Indoor transmission of SARS-CoV-2

Hua Qian; Te Miao; Li LIU; Xiaohong Zheng; Danting Luo; Yuguo Li

We cannot pinpoint the exact transmission routes from these identified outbreaks. Most health authorities advised that the COVID-19 virus is transmitted mainly by close contact and via the fomite route (e.g., China NHC 7 and CDC 8 ). The China NHC also suggested that longrange aerosol transmission may occur when certain conditions are met, such as in crowded enclosures or spaces with poor ventilation. Frequent close contact occurs and high touch surfaces exist in buildings. 9-12 We do not have data on the hygiene conditions and human density of the infection venues of the 318 outbreaks studied here. The exact location of the infection venues and the necessary parameters such as the floor area or the number of occupants were not provided in the case reports. Instead, we reviewed the current design standards of thermal and ventilation conditions, occupant density and close contact behaviour in the various indoor environments discussed here (Table S3 ). The required ventilation rates vary significantly among homes, offices, trains, and buses. For example, the required ventilation rate is only 3·9 L/s per person in shopping malls and 2·8 L/s per person in public buses, whereas a ventilation rate of 8 to 10 L/s is required for good indoor air quality. 26 An international systematic review showed that a rate as high as 25 L/s per person may be needed. 13 Many existing buildings are crowded, poorly ventilated, and unhygienic. A comprehensive review of ventilation conditions in Chinese indoor environments by Ye et al. 14 showed that the CO2 concentration can reach 3,500 ppm in some buildings. The design and operation of buildings have also been under pressure to reduce energy use 15 and increase human productivity. Balancing the need for energy efficiency, indoor environment, and health in both urban planning and building design has not been easy. 16 The quality of indoor environments might be sacrificed by putting a greater focus on cost than on health.