On June 26, 2020, NBC released a story that raised the possibility that air conditioning (AC) could spread coronavirus COVID-19 disease. The question is of central importance as businesses and public health officials look for safe ways to reopen aspects of the economy. It is also, sadly, a question that lacks an easy or actionable answer, and one that reveals the limits of our understanding of how, when and where COVID-19 spreads.
NBC News highlighted comments from New York Governor Andrew Cuomo, who said at a press conference on June 24 that “there are some reports … that the air conditioner may not be cleaning the air from the virus, simply recirculating the air with the virus … We “are studying that, and as soon as we get more information, we will make an informed decision.”
NBC reports also highlighted an April 2020 study published in the Centers for Disease Control and Prevention (CDC) magazine Emerging Infection Diseases that raised the possibility of an air conditioning-related COVID transmission case. in a Hong Kong restaurant. In broad brushstrokes, the study authors reasoned that an outbreak that saw COVID-19 (also known as SARS-CoV-2) spread from one family to three other families in an air-conditioned restaurant could only have happened if “strong air conditioning air flow … droplets spread. “
Most of the experts cited in the NBC report, including infectious disease doctor Amesh Adalja of the Johns Hopkins Center for Health Safety and virologist and NBC collaborator Joseph Fair, were skeptical. The American Society of Heating, Refrigeration, and Air Conditioning Engineers, an “Organization dedicated to promoting the arts and sciences of heating, ventilation, air conditioning, and refrigeration to serve humanity and promote a sustainable world,” issued a statement arguing that , “Ventilation and filtration provided by heating, ventilation, and air conditioning systems can reduce the concentration in the air of SARS-CoV-2 and, therefore, the risk of transmission through the air.”
Understanding the variety of opinions on the subject of COVID-19 and air conditioning requires an understanding of the physical dynamics of aerosol transmission. It is conceivable that air conditioners facilitate the spread of COVID-19 by transporting viral droplets from one point to another, but it is also conceivable that air conditioning can reduce the risk by replacing and diluting the air in an infected space. However, one thing is for sure, it is that no amount of modifications to an interior space can equal the comparative security of an exterior space in terms of COVID air transmission.
COVID-19 and air transmission
Generally speaking, three categories cover possible person-to-person transmission of COVID-19: “droplet transmission” in which large droplets are expelled from an infected individual “with enough momentum to directly impact the mouth, nose, or eyes” of receivers; “Contact transmission” that occurs when those large droplets fall onto a surface that is then touched by a receptor that then physically carries viral particles into your own respiratory system by touching your face; and what is known as “air transmission”. Six-foot social distancing and regular hand washing, practices widely promoted in the early days of the pandemic, are designed to reduce the risk of droplet transmission and contact transmission, respectively. Masks also effectively reduce the travel distance of these larger droplet particles.
Unlike the other two routes that involve “drops” that are large enough to fall to the ground under the influence of gravity, the route in the air involves “aerosols” that are particles small enough to keep floating in the air. air. These broad classes are not mutually exclusive, Rajat Mittal, a Johns Hopkins University professor with joint appointments at the Department of Mechanical Engineering and the School of Medicine, told us in a phone interview.
In the COVID-19 literature, the distinction between gout and spray is generally set at 5 microns. “From a functional point of view,” Mittal told us, “any drop that does not fall directly to the ground under the influence of gravity. [which] can be airborne and airborne [is] in a sense … a spray. Some of those particles could be larger than 5 microns. “
Scientists are increasingly confident that aerosol transmission can be an important, if not the most important, mode of person-to-person transfer of COVID-19. “As a scientist and as someone working on fluid dynamics,” Mittal told us, “there is overwhelming evidence that the airborne route is primarily responsible for this pandemic.” A June 2020 outlook article published in the journal Science, citing the experimental work done by Mittal and his colleagues, also supports this conclusion:
Recent studies have shown that in addition to drops, SARS-CoV-2 can also be transmitted through aerosols. A study in hospitals in Wuhan, China found SARS-CoV-2 in aerosols more than 6 feet away from patients, with higher concentrations detected in more crowded areas. Estimates using an average sputum viral load for SARS-CoV-2 indicate that 1 minute of speaking aloud could generate> 1000 aerosols containing virions.
A study by a separate research group, also published in June 2020, found that aerosols containing SARS-CoV-2 could maintain infectivity for up to 16 hours, “that SARS-CoV-2 is resistant in the form of aerosol “and that” The transmission of aerosol SARS-CoV-2 may be a more important route of exposure transmission than previously considered. ”The behavior of these aerosols in indoor and outdoor environments is at the center of questions about the possible role that air conditioning plays in the diffusion of COVID-19.
Indoor versus outdoor sprays
“It all starts with drops coming out of an infected person’s mouth and nose,” Mittal told us. The air exhaled from a human body is generally warmer than the ambient ambient air and as a result the air and aerosol particles contained in it will rise. In outdoor settings, “air currents tend to dilute the concentration of the virus that carries aerosols in the air significantly … as your breath carries these aerosols upward, it immediately mixes with the environmental flows and it is diluted and carried away. “
Hot air also rises indoors. Here, however, aerosols have limited opportunities to dissipate. Instead, they will rise to the ceiling and increase concentration. Some particles will dry out and become inactive, but some of these potentially infectious particles “can remain airborne for hours.” The fate of these particles is difficult, if not impossible, to predict. In their article published in the Journal of Fluid Dynamics, Mittal and colleagues argued that “indoor environments such as homes, offices, shopping malls, airplanes, and public transportation vehicles pose a particular challenge to disease transmission.”
Air Conditioning and Transmission COVID-19
The argument presented in the April 2020 document regarding air conditioning in a Hong Kong restaurant was made assuming that the only plausible route of transmission was the transmission of droplets:
Virus transmission in this [Hong Kong restaurant] The outbreak cannot be explained by droplet transmission alone. The largest respiratory drops remain in the air only for a short time and travel only short distances, usually <1 m. The distances between patient A1 and persons at other tables, especially those at table C, were all >1 m. However, a strong flow of air from the air conditioner could have spread droplets from table C to table A, then to table B, and then to table C.
This analysis does not consider aerosols, which potentially remain in the air for hours and migrate with or without the flow of an air conditioner. Even the movement of people entering and leaving the restaurant could affect their movement, Mittal told us. This was an explicit limitation of the Hong Kong study. “Our study has limitations,” the authors wrote. “We did not conduct an experimental study that simulates the air transmission route.” Part of the problem, as highlighted in the Journal of Fluid Dynamics, is the complexity of indoor air flow:
Interior spaces can have extremely complex flows, due not only to the presence of recirculatory flows driven by ventilation systems, but also to the thermally driven anthropogenic flow effects. Transmission of COVID-19 from asymptomatic hosts makes it more critical than ever that we develop analytical methods that provide a better prediction of these effects.
The air conditioner, outside of a hospital setting, is not designed to clean the air of viral aerosols. Some systems can effectively reduce risk by diluting and replacing air in a similar way to an outdoor environment, but others can exacerbate it, Mittal told us. The effectiveness of air conditioning in this regard would depend on how quickly the system replaces the air in a space and in which location it ventilates the exhaust materials. “The air conditioning in my house,” he said, replaces indoor air with outdoor air, “but it is also replacing air in one room with air in another room. So that’s not a complete replacement, it’s just an air shift. “Theoretically, such a shift could lead to an airborne transmission from an infected person to another person” upwind “of air currents.
These questions are almost impossible to study in real time and can only be analyzed through limited case studies like the Hong Kong restaurant. As a result, in Mittal’s opinion, the question about air conditioning and the spread of COVID-19 is problematic. “You can only ask the question in terms of overall security improvement,” Mittal said. “You can ask the question, ‘If a person was sitting in that particular corner of the room is it their opportunity to go up or down,’ but that becomes a much more difficult question. [and] it may not be a relevant question. “
“I think overall air conditioning with good replacement rates should help, but … one has to go into a little more detail to understand what kind of air currents are being set up,” Mittal told us.
