Research On The Risk Of Aerosol Transmission In Canteen And Control Measures

The haze caused by COVID-19 has shrouded human society for more than two years,and the frequent outbreaks of regional pandemics in China reminds the pandemic prevention situation is still very severe.On-site investigation found that the current Chinese university canteens generally lack mechanical ventilation facilities,the dining area is relatively closed,the diners are highly dense during the meal period,and chewing food without wearing a mask will aggravate the generation of aerosols.Therefore,university canteens are places that possess high aerosol transmission risk.However,there are currently few relevant studies on aerosol transmission of respiratory infectious diseases in canteens,and commonly implemented measures,such as wearing masks,maintaining separation distance,using desktop partitions,increasing ventilation rate,etc.,have implementation obstacles and lack scientific exploration of their effectiveness.Therefore,it is of great practical significance to systematically study the aerosol transmission risk of respiratory infectious diseases in canteens,explore the effectiveness of existing measures,and put forward targeted guidance schemes to help pandemic prevention and control.This study firstly conducted on-site investigations on two university canteens in Chang Sha to analyze the risk scenarios of aerosol transmission during the whole process of dining,and collect the important human behavior information,such as queuing time,queuing form and queue size,dining time,the occupancy rate,etc.,environmental parameters such as CO₂ concentration and air temperature in canteens were also monitored to establish boundary conditions for subsequent research.Then,aiming at the queuing scene,CFD simulation was used to study the aerosol transmission risk under the different queuing distance,queuing form and wearing mask or not.For the dining scene,the method combing field experiment and CFD simulation was used to study the prevention and control effects of linear partition and cross partition according to the state of diner,and targeted improvement measures and optimal implementation strategies were proposed.Based on the quantum value of the COVID-19 infected persons,the Wells-Riley modified model was used to assess the infection risk.The main conclusions are as follows:（1）Due to the fact that the exhaled air flow is affected by strong thermal buoyancy,the aerosol transmission risk in the winter queuing scene is low.In summer,exhaled aerosols gather in the poorly ventilated space between pe ople.The smaller the distance,the more serious the accumulation phenomenon.In the linear and cross queuing scenes with a separation distance of 0.2 m,the highest COVID-19 infection risk due to location replacement is 1.16%and 1.08%,respectively;in the cross queue with a separation distance of 0.2 m,there is a risk of direct aerosol exposure for the exposed person in front of th e infected person to breathe on the side of the head,in the scenes with queuing time of120 s,200 s,and 277 s,the average infection risks of infected person wearing mask or not are 0.65%,1.25%,1.76%and 0.81%,1.80%,2.83%,respectively.The use of masks can reduce aerosol transmission risk and alleviate the phenomenon of aerosol accumulation between persons.Therefore,maintaining a linear queue with sufficient separation distance,shortening the queuing time,requiring the queuing personnel to wear masks and instructing them to avoid side head orientation can significantly reduce the risk.（2）A partition that is not lower than the breathing level of the infected person by 3-5 cm can effectively block the horizontal development of the expiratory jet.Compared with no partition,the use of cross partition and linear partition results in an increase of the exposure index in the source compartment by 97.2%and 25.5%,respectively.Therefore,if the occupancy rate is strictly controlled to 50%,a linear partition should be used instead of a cross partition.（3）Even in winter,it takes,on average,11 minutes for the aerosol concentration in the source compartment to decay back to the background level.Considering that a single seat is usually occupied continuously for many times during the meal period,the follow-on diner is at high aerosol transmission risk caused by his/her former diner.The average infection risk for the latter occupant sitting in the same seat for 15 minutes immediately after the infected person leave is as high as 2.87%,the meal time is shortened by 7 minutes,the infection risk is only reduced by 0.48%;while the latter wait for 3 minutes and 7 minutes and then sit for 15 minutes,the average infection risk is reduced by 1.13%and1.78%,respectively.Therefore,the interval between occupants before and after a single seat should be guided as an important pandemic prevention scheme,or in the dining area where the occupancy rate is controlled to 50%,the next diner should be guided to sit next to the previous diners.（4）When the ACH is increased to 1.0 h^-1 in winter,the background aerosol concentration could be significantly reduced;when the ACH is increased to 3.0 h^-1 in summer,there is still a high risk of aerosol transmission in local areas.Install a 20 cm diameter local exhaust close to the top of the partition and operate at a negative pressure of 5 Pa,which can exhaust the aerosol exhaled by the infected person in time,thus controlling the background concentration at a low level and controlling the high concentration area in a small range;for dining scenes where it is inconvenient to use desktop partitions,only local exhaust facilities can also play a better prevention and control effect.Therefore,in qualified dining places（such as restaurants,hotels）,installing local exhaust facilities is an effective measure to cope with the spread of infectious aerosols.