In recent months, several studies have provided details on cases of transmission of SARS-CoV-2, the coronavirus that causes COVID-19, through the saliva drops that we produce when we speak out loud, sing, cough or sneeze. The largest drops (droplet) remain suspended in the air for a short time and then settle on the surfaces, which can become a vehicle of contagion if touched by uninfected people who then touch their faces with dirty hands. Smaller droplets remain suspended in the air longer (aerosol), and there is the suspicion that in particular conditions of humidity and poor air exchange they can cause new infections, especially if they are shared for a long time time the same closed spaces with a contagious person.
Poor ventilation of rooms has long been known to be one of the causes of transmission of infectious diseases. According to several studies, for example, it is a factor in the spread of seasonal flu, which reaches its peak in the colder months where we tend to stay indoors longer, sharing the same spaces with other people. As with other infectious diseases, also in the case of COVID-19 the role of room ventilation in the spread of the disease has not yet been fully clarified, although the first research indicates an increased risk factor for offices, restaurants and other places where you remain indoors for a long time.
Research carried out in China, the country from which the epidemic started, analyzed the origin and evolution of 318 small epidemic outbreaks, noting that all but one had developed indoors. Other research has highlighted the presence of contagions in events with a large outdoor audience, where it is obviously more difficult to practice physical distancing, a circumstance that also involves indoor environments, for example the tunnels crossed by the public to reach the stands of a stadium.
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The amount of time spent in an environment with poor air exchange also seems to be a significant factor in the increased risk of coronavirus spread. A study reconstructed the chain of contagions in the United States among members of a choir, who had spent about two and a half hours sharing the same environment during rehearsals. Among the 61 participants, 32 cases of direct contagion and 20 other probable cases of secondary contagion were detected. During the singing a great quantity of is emitted droplet and researchers estimate that this has contributed to the significant number of new infections.
Offices and restaurants
Research in a call center in South Korea, where more than 40 percent of employees had contracted coronavirus, had been highly commented in recent months. Most of the 216 employees worked in a rather crowded environment and, reconstructing the dynamics of the infection, the researchers concluded that the long-term stay in the same environment had influenced more than other factors in the spread of coronavirus.
Starting from these researches, from other studies and previous knowledge, some researchers have tried to understand if the presence of ventilation systems, such as those for air conditioning, is an additional risk factor. The study that received the most attention, and was most cited, concerned a restaurant in China, where 10 infections were detected between three different families. They had all dined on the evening of January 23 in the restaurant, and then developed symptoms of the disease between January 26 and February 10.
The tables in the restaurant were more than a meter away from each other, therefore compliant with the recommendations for practicing a minimum of physical distance. The contagious person, who would then transmit the coronavirus to all the others, at the time of dinner had very mild symptoms and would have developed more relevant ones (cough and fever) only at the end of the evening. The people then infected were all along the air flow produced by an air conditioner, while the other guests of the restaurant not involved in the air flow had not been infected.
Transport and events
Another research carried out in China, and disseminated in a preliminary form (therefore to be taken with some caution), has traced the origin of two outbreaks to air conditioning, equipped with air recirculation systems. In one case it had been activity around a religious event in a temple: the contagious person, coming from Wuhan (the epicenter of the pandemic) still had no relevant symptoms and would have developed them only in the evening after the meeting in the temple. He had made a 100-minute bus trip with 67 other passengers, 23 of whom would later become infected.
The researchers noted that passengers closest to the infected did not have a higher risk of being infected than those seated at a greater distance. Those with seats near the window were not infected, which seems to indicate that the air flow from the bus air conditioner had affected the spread of coronavirus only in certain areas of the vehicle.
The other outbreak had instead involved a training course, also in China, in which 30 individuals from different cities were involved, who had stayed in different hotels and had not eaten together during the course. The training was divided into four group sessions of 4 hours each, organized in two rooms of 49 and 75 square meters respectively. The air conditioning system was set to automatically activate 10 minutes every four hours, using an air recirculation system. Half of the participants contracted the coronavirus and then showed COVID-19 symptoms starting a couple of days after the end of the course.
The most advanced air filtering systems, such as HEPA (High Efficency Particulate Air), have proved useful for retaining at least part of the viral particles of SARS-CoV-2. These filters are quite widespread and have been used for some time in the ventilation systems of hospitals and in those of airplanes, where the recirculation of the same air is inevitable during the flight phases. However, to date there is not enough solid scientific evidence to claim that HEPA filters alone play a role in preventing the spread of infectious diseases.
A study carried out years ago on SARS-CoV-1, the coronavirus that causes SARS, assessed the risk factors between different types of aeration in larger airliners, detecting how the ventilation systems that circulate air in the entire cabin are more risky than those which require the presence of the vents (to be opened, closed and adjusted) above each individual seat. However, the research left several questions unanswered about the dynamics of the spread of infectious agents through air conditioning in planes.
Based on these and other research, the European Center for Disease Prevention and Control (ECDC, an independent agency of the European Union) has summarized the scientific evidence available so far on aeration systems and coronaviruses:
• The transmission of the COVID-19 usually takes place in closed spaces.
• To date, there is no evidence of SARS-CoV-2 infections caused by infected aerosols distributed via the air conditioning system ducts. The risk factor is indicated as very low.
• Well-maintained ventilation systems, including air conditioners, quietly filter out the largest droplets containing SARS-CoV-2. It is possible that the aerosols (very small drops) are spread through the ventilation systems inside a building or vehicle and through the air conditioners, if they maintain air recirculation.
• The air flow produced by a ventilation system could facilitate the spread of droplets produced by infected people at great distances, inside closed environments.
• Ventilation systems could have a role in reducing the risk of transmission in closed environments as long as they increase the rate of air exchange, reduce recirculation and increase the introduction of air from outside.
From the ECDC summary, but also from other documents such as the recommendations produced by the Istituto Superiore di Sanità in Italy (no new specific standards have been produced), it is evident that at the moment the council is taking as many precautions as possible, in waiting to have clearer elements from those involved in doing research.
Reduce the risks
To reduce the risk of contagion in closed environments, the practices recommended for some time and which we are well acquainted with first apply: apply physical distancing, wash your hands often and well, use masks where a sufficient distance from the neighbor is not guaranteed, reduce residence times in closed rooms, especially if sufficient air exchange is not guaranteed.
If a ventilation and air conditioning system is maintained according to the manufacturer’s instructions, with regular maintenance and cleaning cycles, no additional extraordinary interventions are necessary to reduce the risk of spreading COVID-19.
In rooms and large offices there are usually centralized systems for air conditioning, which provide for the possibility of introducing air from the outside, offering a discreet change. This mode should be preferred over that of air recirculation, which could increase the risk of coronavirus transmission.
Individual air conditioners, such as wall-mounted air conditioners for small rooms, usually cool the air in the room in which they are installed, but do not draw any more from the outside. This entails a greater risk and therefore makes it advisable to periodically open the windows to ensure greater air exchange.
ECDC also advises that ventilation systems do not create currents directed towards the occupants of the rooms, because this could favor contagion, as probably happened in the restaurant in China we were talking about earlier. Even in this case, however, ensuring an exchange of air with the outside should significantly reduce the risks.
The quality of indoor air is often neglected, although it is estimated that pollution of indoor environments is among the causes of death of at least one million people every year. Few precautions, such as the correct maintenance and cleaning of the systems, could help to significantly reduce the problem, as well as the design of more efficient ventilation systems that guarantee adequate air exchange.
The greatest risks of contagion indoors are often underestimated, as well as the importance of ensuring a good air exchange, and for this reason an effective analogy proposed by the doctor James Hamblin on theAtlantic: “Coughing in a well-ventilated room is like peeing in a river compared to doing it in a bathtub: ideally you wouldn’t do either, but one is definitely worse than the other.”