3.1 Transmission risk of indoor pathogens
(1) The differences in virus concentration of different particle sizes
In the indoor environment, the source of the risk of virus transmission is that when there are virus carriers in the room, toxic droplets exhaled through coughing, sneezing and other behaviors may be inhaled by susceptible people, thus causing the spread of the viruses(Olsen et al., 2003; Qu et al., 2020). We need to understand the viral load of the mouth, nose, and throat swabs of infected persons (or asymptomatic patients) through the medical measurements, as well as measuring the particle size range and volume distribution of droplets released by respiratory activities into the room. It is the basis for the air-conditioning and ventilation system to transmit the virus-containing droplets and nuclear particles, and investigating the risk of virus disease transmission.
In medicine, the real-time RT-PCR virus nucleic acid test of the swab of the mouth, nose and throat of the population or patient is generally used to judge the infection and its severity(Guarnieri & Balmes, 2014). Since the outbreak of the COVID-19 epidemic, there have been many reports on the viral load data of patients. Zou et al. (Zou et al., 2020) and Cao et al.(Cao et al., 2020) tested 6 patients and 199 SARS-CoV-2 patients who participated in the treatment of throat swab viral load concentration in the early stage of the epidemic outbreak. The results showed that individual differences were significant in different patients, and viral load differences were significant on different dates after admission. Pan et al.( Pan et al., 2020) found that viral load reached peak value 5-6 days after the onset of symptoms in the throat swabs of confirmed patients in Beijing. Wang et al.(Wang et al., 2020) tested saliva samples from 23 patients of different ages in two hospitals in Hong Kong and found that the viral load peaked in the first week after symptom onset and then decreased over time. Figure 2 presents a representative comparison of viral load changes in mouth, nose and throat swabs from 5 medical reports involving 51 patients. It can be seen that the viral concentration of the COVID-19 patients (asymptomatic) varies greatly among different patients, and the same patient varies greatly at different time periods after infection, generally reaching the peak at 4-6 days after onset, with the maximum of 109copies/ml. This suggests that if there are asymptomatic patients in a centrally air-conditioned indoor space, the virus concentration and transmission risk of the sprayed droplets or saliva droplets greatly differ.
Affected by the degree of infection, time and other factors, although the virus concentration exhaled from different parts of the respiratory tract of an infected person is different, for a certain respiratory activity, the virus concentration in the exhaled droplets of various particle sizes can be considered to be the same. Therefore, when discussing the transmission risk of exhaled droplets, the number of viruses contained in droplets of different particle sizes is positively correlated with the total volume of droplets in the range of particle sizes, which can represent the initial level of virus concentration and transmission risk of such droplets.
Figure 3 shows the analysis of total volume and volume ratio of the exhaled droplets in different particle size ranges based on five medical reports, such as breathing, sneezing and coughing (Chen et al., 2020; Duguid et al., 1946, 1954; Lindsley et al., 2012; Morawska et al., 2009). It can be seen that although the size distribution of droplets reported in different studies is different due to differences in instruments and methods(DuguidJP; "Guidelines for preventing the transmission of Mycobacterium tuberculosis in health-care facilities, 1994," 1994; LoudonRG & RobertsRM, 1967), the overall rules are similar. Infected persons exhale a large number of small-size particles, but a small number of large-size droplets, with a large particle size range (1~2 000μm). Although the number of small particle size droplets is much higher than that of large particle size droplets (60~100μm), because of their small single size, the cumulative volume of small particle size droplets is much smaller than that of large particle size droplets. Therefore, the risk of virus transmission of large droplets is much higher, with a difference of up to 3 to 4 orders of magnitude. It reminds us that the number of small droplets floating with the air in the air conditioning system may be much larger than that of larger droplets, but the total volume of the small droplets is much smaller than the total volume of the large droplets, so the small droplets are not the main contradiction of propagation.
(2) Survival rate of spittle with different particle sizes
The droplets with a smaller particle size emitted by indoor infected persons float in the air, while those with a larger particle size settle on the surface. What are the effects of air conditioning on the survival and transmission risk of virus droplets with different particle sizes? Only through medical experiments, can this question be answered. In medicine, the virus culture solution which is similar to human saliva is usually atomized by a nozzle and then released into a small chamber, or titrated to the surface of an object. After a certain period of time, samples are taken to detect changes in concentration to find the decay behavior.
Figure 4 shows the change of relative survival rate of respiratory viruses in the air with atomized droplet size according to 6 groups of experiments conducted by 5 scholars(Ge et al., 2014; Ijaz et al.,2018; Pyankov et al., 2012; Sattar et al. , 1984). In order to eliminate the coupling effects of other factors, the selected experimental conditions are all air-conditioned environmental experimental conditions with a temperature of 18~25℃ and a relative humidity of 40~60%, and are basically similar. The value C in C/C0 is the concentration value that aerosolized for 1 hour after the virus atomization. It can be seen that the survival rate of the atomized droplet viruses in the air is positively correlated with the particle size, that is, the larger the atomized droplet size, the higher the survival rate and the greater the risk of transmission. The larger the particle size of the atomized droplets, the more viruses they can carry. Moreover, the longer the water takes the evaporate due to the heat and mass transfer between the droplets and the environment, the slower the survival environment of the virus deterioration, leading to a significant increase in the risk of transmission. The results suggest that although a small particle size droplet nucleus can float in the air for a longer period of time, and the transmission risk is far inferior to the initial stage just after leaving the host.
Figure 5 shows the relative survival rate of respiratory viruses obtained from 12 groups of experiments in 6 reports with the change of titration volume on different surfaces(Chin et al., 2020; Doremalen et al., 2020; Doremalen et al., 2013; Greatorex et al., 2011; Sakaguchi et al., 2010; Warnes et al, 2015). In order to eliminate the coupling effects of other factors, the selected experimental conditions were all air-conditioned conditions with a temperature of 18~25℃ and a relative humidity of 40~60%, which are basically similar. C in C/C0 is the concentration value that aerosolized for 1 hour after the virus atomization. It can be seen from this figure that the volume of the venomous droplet has a very obvious effect on the survival of the viruses on different surfaces: as the volume of the surface titrated liquid decreases, the survival rate of the virus on the surface drops rapidly. Specifically, the survival rate of the virus in a 1μL droplet is 3 orders of magnitude lower than that in 500μL droplet. These medical measured data strongly prove that the larger the volume of the surface droplets, the greater the risk of transmission, and therefore they are the focus of epidemic prevention and control. The mechanism is that the size of the droplet volume directly affects the physical process of the viruses on the surface, and then has a significant impact on virus decay. According to related literature research and testing, it takes only a few minutes for 1μL droplets to reach equilibrium on the surface, while it may take several hours for 500μL droplets to reach equilibrium, which creates better conditions for the survival of the viruses.
3.2 Temperature and humidity control method based on the medical experiments
Controlling the indoor temperature and relative humidity is the most important aspect of air-conditioning systems. At present, we already know what temperature and relative humidity is most conducive to human health and comfort, and we have accumulated a wealth of knowledge in the field of building environment regulations. However, what temperature and humidity environment is the most unfavorable to the survival and transmission of respiratory viruses is a question that can only be answered through evidence-based medical experiments.
(1) Medical experiments on the effect of temperature on virus survival
In terms of the influence of environmental temperature on virus viability, Casanova et al.(Casanova et al.,2010) inoculated 10μL of TGEV and MHV containing 104~105mpn into cell culture medium similar to human secretion at 4 ℃, 20 ℃ and 40 ℃, respectively, and then placed them on stainless steel surface carrier. After that, an appropriate sampling interval (1-24 hours or 7-28 days) was selected for each working condition, and then the samples were taken out for physical examination of virus survival. Prussin et al. (Prussin et al., 2018) titrated 10μL Phi6 virus solution with the concentration of 108 ~ 1010 pfu·ml-1 into the polystyrene cell culture dish, and cultured at different temperatures for two hours, then the samples were taken out to analyze the survival of the viruses. They all found that higher temperature can reduce the relative survival rate of the viruses, and low temperature was more conducive to the survival of viruses, and this rule was also verified in the virus survival experiment of Chin(Chin et al., 2020). Chin et al.(Chin et al., 2020) first incubated SARS-CoV-2 in virus medium for 14 days (the final concentration was 106.8 TCID50·ml-1), and then they were placed in the environment of 4 ℃, 22 ℃ and 37 ℃, respectively, and the stability of sars-cov-2 at different temperatures was measured.
Figure 6 presents the medical comparative experiment results of virus survival rate under five different temperature working conditions with 50-70%relative humidity which is a relatively comfortable air-conditioned environment. It can be seen that when the relative humidity is at a certain condition and the temperature is low, the survival rate of the surface viruses is very high. With the increase of temperature, the relative survival rate decreases significantly, and when the temperature rises to above 35 ℃, most of the viruses have lost their threat. This is mainly because the higher temperature leads to the faster evaporation rate of the surface droplets; and the more rapidly increase of various salt concentrations that is suitable for the survival of virus droplets, and acid-base imbalance, causing the decline of the viruses. The effect of temperature on the virus survival rate of virus in air is similar, that is, with the increase of temperature, the survival rate of viruses decreases significantly, but in the air, when the temperature reaches above 25 ℃, the virus is basically inactivated(Colas de la Noue et al., 2014; Happer, 1961).
(2) Medical experiments on the effect of relative humidity on virus survival
In terms of the influence of relative humidity on virus viability, Lin et al.( Lin et al., 2020) carried out surface titration experiments on MS2 and Φ 6 phage viruses. They titrated 10μL high concentration virus suspension with concentration of 1010~1011pfu·ml-1 to polystyrene cell culture plate, cultured for 1 hour under seven levels of relative humidity conditions in the range of 23%~100%, and compared the ratio of virus concentration after exposure to that before exposure in the fixed droplet. The results showed that the relative survival rate of the MS2 virus was the lowest when the relative humidity was 55%, and the relative survival rate of Φ 6 virus was the lowest when the relative humidity was 75% ~ 85%.Prussin et al.(Prussin et al., 2018) and Casanova(Casanova et al., 2010) also found similar behavior for virus survival in the range of 20% - 98% RH. Aiming at SARS-CoV-2, Smither(Smither et al., 2020) and Biryukov(Biryukov et al., 2020) respectively tested the survival of SARS-CoV-2 on tissue culture medium and different pore free surfaces under different relative humidity conditions. The study also found that the survival rate of the viruses was low at medium relative humidity, but higher at high relative humidity.
Figure 7 presents the results of medical comparative experiments on the effect of relative humidity on the relative survival rate of surface viruses obtained from five reports. In order to exclude the influence of other complex factors, the indoor temperature selected in the diagram is in the laboratory conditions with the temperature from 19 ℃~25 ℃ which is more comfortable for the human body. It can be seen that the influence of relative humidity on the relative virus survival rate is generally U-shaped, although the virus types are different. In other words, the relative virus survival rate is high when the relative humidity is low or extremely high. When the relative humidity is in the range of 50%~70%, the relative survival rates of most viruses are low, which indicates that controlling the relative humidity at a medium level (50%~70%) is beneficial to reducing the virus survival and inhibiting the virus transmission.
People spend about 90% of their time indoors, and the respiratory disease infection is realized by the infected and susceptible individuals in the process of indoor social activities. However, due to the human ethics restriction, it is impossible to carry out human experiments on virus infection, so animal experiments are widely used in medicine. Generally, infected animals and susceptible animals are exposed in a closed space with comfortable temperature and humidity to simulate social behavior and observe the infection status. For example, same cage experiments(Mubareka et al., 2009; Sia et al., 2020)、adjacent cage experiments(Kim et al., 2020; S et al., 2009), short-distance exposure downstream(S et al., 2009), exposure to virus liquid smearing on the surface(Deng et al., 2020; Sia et al., 2020), and aerosol exposure of virus suspension(Kim et al., 2020; Sia et al., 2020), respectively correspond to common close contact, close propagation, media transmission and air transmission that are common in interpersonal social activities. Although in air-conditioned indoor spaces, the activity space for interpersonal social behavior is not as limited as in animal experiments, social distance and contact behavior are more random. Infected persons who enter the public buildings after investigation by various means are fewer than those who have participated in the experiment, and the virus concentration is generally not that high, but the evidence-based medical experiments can help readers understand the risk of virus transmission in the operation of the air-conditioning system from another level. Some scholars have also carried out medical experiments on the influence of the temperature and humidity environment on the transmission of infection between animals, exposing a number of experimental animals that have been diagnosed with the viruses and a number of healthy animals in a limited box space with the same temperature and adjustable relative humidity. The changes in the viral load, infection rate or mortality of the animals on each day after exposure were observed. It was found that the infection rate or mortality rate with the change of indoor air temperature and relative humidity has an influence similar to that of the virus survival rate(Herlocher et al., 2004; Marr et al., 2019).
(3) Temperature and humidity control strategy based on medical experiment
The above medical experiments of the indoor temperature and humidity environment on virus survival and animal infection transmission suggest that during the outbreak of respiratory viruses, especially in the winter, raising the indoor air temperature through the air-conditioning or heating systems can achieve thermal comfort for the human body, which is more conducive to inhibit virus activity and reduce the risk of infection between people. The suggestion to stop air conditioning or heating in winter will cause the indoor temperature too low, which is not good for the health of indoor personnel, and may increase the risk of virus transmission. It is recommended that when the air-conditioning or heating systems are running, opening windows for ventilation or increasing the amount of fresh air may cause the indoor temperature too low or even resulting in the existing system having difficulty in bearing the heavy load. At this time, the indoor temperature is close to the outdoor, causing a great waste of energy and may increase the risk of virus transmission. Since most air-conditioning or heating systems, especially split air-conditioners, do not humidify, the relative humidity in the room drops rapidly after heating, which may increase the risk of virus transmission. Therefore, an appropriate increasing in relative humidity is a very important means. In summer, the operation of the air-conditioning systems reduces the room temperature, although it may increase the virus activity and transmission risk to a certain extent, but due to its dehumidification effect, the indoor relative humidity can be brought to a moderate state, thereby generally inhibiting the virus activity. In particular, the pathogens released indoors by infected persons are in the form of droplets, so dehumidification can collect the potential pathogens in the air from the source, avoiding them from floating in the air, greatly reducing the harm to people indoors and reducing the spread risk.
Although based on the evidence-based medical experiments and combined with the basic principles of indoor environment regulation, the temperature and humidity regulation strategies of the air-conditioning or heating systems during the outbreak of the epidemic has been establishes, due to the coupling complexity of the problem, there are still many issues worthy of more in-depth study.
3.3 One-time removal effect of the filter layers on particles containing pathogens
Whether in distributed or centralized environmental control equipment and systems, there are filters of various levels including primary efficiency, medium efficiency, and high efficiency. In some areas with more serious pollution, the owners of residential and public buildings will also install air purifiers to meet the requirements of higher indoor air quality. From the previous analysis, it is not difficult to find that the size of the droplets exhaled by indoor infected persons has a wide distribution range, and the droplet nuclei are formed after the water evaporates rapidly and floats in the air with the airflow generated by the air conditioner, which leads to the risk of transmission. Since the risk of virus transmission of droplet nuclei of different particle sizes is different, searching for the influence of filters (filter layers) on the removal of particles with different particle sizes based on related experiments is an important scientific basis for risk assessment of the spread of air-conditioning system filters or air purifiers during the epidemic.
Some scholars(Emmerich et al., 2013; Miller-Leiden et al., 1996; Williamw et al., 1998) have found that particle filters commonly used in Heating, Ventilation and Air Conditioning (HVAC) systems can also be used to reduce the risk of airborne infectious diseases. For particles less than about 50μm in diameter, the evaporation usually takes place within a few seconds(Chen & Zhao, 2010). After rapid evaporation, the droplet core containing mixed solid particles (including any infectious particles) still exists, and its particle size is related to the initial droplet diameter, which is generally 26~35% of the initial diameter(Nardell et al., 2001). Relevant experimental studies have been carried out(Blachere et al., 2009; Lindsley et al., 2010; Lindsley et al., 2010; Noti et al., 2012; Yang et al., 2011), testing the one-time removal efficiency of different grades of filters for infectious droplet nucleuses with particle sizes ranging from 0.3 to 10μ. The results show that HVAC filters can effectively remove infectious droplet nuclei in the environment, and some high efficiency filters can even reach more than 90%. Figure 8 summarizes the data of one-time filtration efficiency of HVAC filters of different grades on infectious droplet nuclei in the above studies. It can be seen from the figure that: 1) The average filtration efficiency of infectious droplet nuclei ranges from 10.5% of MERV 4 filter to 99.9% of HEPA filter, and the filtration efficiency is significantly different.2) The higher the filter level is, the better is the filtering effect. However, when the filter level reaches a certain level (MERV 13 and above), the filtering efficiency will be basically stable at above 90%. Therefore, regardless of the level of the air conditioner used indoors, it can achieve the effect of removing infectious droplets and reduce the risk of infection.
The filter efficiency of different filter layers varies with the particle size, as shown in Figure 9(Davies et al., 2013; Lee et al., 2008; Macintosh et al., 2008; Oberg et al., 2008). From this figure, it can be further seen that different filter layers have different filtering effects on particles of different sizes in actual use. It can be found that for the same filter layer, the smaller the particle size, the lower the filtration efficiency. As the particle size increases, the filtration efficiency increases rapidly until a certain particle size where it becomes stable. Therefore, this figure can fully illustrate that the range of indoor pollutant particles removed by filters and air purifiers in air conditioning systems is mainly for larger particle sizes, and their filtering and purification effects are very significant, with an average efficiency of more than 80%.