Air Conditioning in the times of COVID-19

The world is facing a pandemic caused by a novel coronavirus, named COVID-19 by the World Health Organization (WHO). Since it started from Wuhan in China, it has spread across the globe now. The pandemic has upended the traditional business practices and forced us to re-think our decision-making criteria and strategies keeping safety at its core. Almost every industry is impacted by this pandemic and the air conditioning industry is no exception. Keep reading and we will understand what role air conditioners play in our health and safety, how air conditioners can influence virus transmission, and what changes will be required in air conditioning systems to safely navigate such virus outbreaks.

Evolution in the role of air conditioning

In 1902, Willis Carrier was working for Buffalo Forge Company in the US and was assigned the task of controlling humidity as it was warping the pages of a client magazine company in Brooklyn. Through a series of experiments, he would then design a system that could control the air humidity level by cooling the air, which later became known as the air conditioner.

The use of these types of systems was limited to industrial applications until around 1930 when we realized the usefulness of temperature and humidity control for human comfort and productivity. Subsequently, the use of air conditioners in residential and commercial applications started to pick up and air conditioners slowly became ubiquitous.

Later in the 1960s and 70s, studies pointed towards the importance of the quality of the indoor air (measured by the concentration of pollutants like gaseous compounds like carbon monoxide, and the presence of fine particles) for long-term health and well-being of occupants. The air conditioning design engineers started focusing on making the quality of the indoor environment better. Air filtration, air movement, and fresh air intake started becoming critical components of the air conditioning system design. American Society for Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) published the first air quality guidelines in 1989 known as Indoor Air Quality (IAQ) standard 62.

After the 2000s, we have seen a lot of infectious disease outbreaks like SARS, MERS, H1N1, and COVID-19. Some infectious diseases are known to spread by aerosols. The risk of pathogen spread can be positively or negatively affected by the installed air conditioning system. This has forced the engineers and designers to improve the design and operation of the air conditioning systems to minimize the risk of cross-infection. The COVID-19 pandemic has accelerated the transition to design-for-safety.

Air conditioning and COVID-19 transmission

Historically, researchers have classified infectious particles into 2 categories. When the infectious particles are smaller than 5 microns, they are called aerosols, and infectious particles of size 5–10 microns are classified as respiratory droplets. These particles are generated by coughing, sneezing, breathing, talking, etc. by the infected person.

Until March 2020, World Health Organization (WHO) was of opinion that there is not sufficient evidence to support the possibility of airborne transmission of the COVID-19 virus. So, the guidelines emphasized preventing droplet-based transmission by resorting to masking — social distancing (to prevent droplet transmission) and disinfecting guidelines (to prevent fomite transmission) for limiting the COVID-19 spread.

In April 2020, the Centre for Disease Control and Prevention (CDC) published a study of an air-conditioned restaurant in Guangzhou, China where 9 diners contracted the COVID-19 virus without coming in close proximity of the index COVID-19 positive patient, a 63-year-old lady who returned from Wuhan a few days back. The contact tracing of the newly infected people found no other source of exposure to the virus.

The larger respiratory droplets (>5 μm) can remain in the air for only a short time and travel only less than 1m generally. The distances between index patient A1 and persons at other tables, especially those at table C, were all greater than 1m. However, virus-laden small (<5 μm) aerosolized droplets can remain in the air and travel long distances, >1m. So, the researchers concluded that strong re-circulating airflow from the air conditioner and no provision for fresh air was the reason for the spread.

Since then, there were concerns around the possible spread of coronavirus by the use of air conditioning systems. Later, various incidents of people getting infected due to prolonged stay within air conditioned spaces like call centers, ships, and choir rehearsals were reported. This led to growing unrest from the scientific community and WHO finally updated its guidelines in July 2020 for the modes of COVID transmission. The modes now included airborne transmission in addition to droplet transmission and issued a caution against closed poorly ventilated spaces.

As the air conditioning systems control the airflow, ventilation, and air filtration, it determines the trajectory, travel distance, and concentration of the COVID-19 virus particles in the air if an infected person is present indoor. Thus its role is critical in mitigating the risk of cross-infection within indoor spaces like commercial buildings, hospitals, schools, and also within planes, trains, and automobiles.

Risk mitigation strategies

Following are some of the recommendations from ASHRAE and other global health agencies to make air conditioning systems better equipped to prevent the indoor spread of COVID-19.

Upgrading air conditioning systems

For commercial and public buildings, increasing outdoor air ventilation can reduce re-circulation and dilute the virus concentration in the air. Increased air change rates (passing the air more frequently through filters) is also recommended as it increases the probability of virus particles removed by the filters. CDC recommends that indoor spaces should have between 6 to 12 full cycles of new air per hour to prevent airborne infections. But most buildings are only capable of only one to two changes per hour which should be increased.

Home air conditioners don’t typically have any means of allowing outdoor fresh air inside the space which is a common misconception among people. It keeps recirculating the same air within the room. In this case, opening windows at regular intervals would reduce the risk of contamination.

Better air purification

The effectiveness of filters is measured with Minimum Efficiency Rated Value or MERV rating. Higher the MERV rating, better the filter is at trapping smallest particles. Air conditioning systems in homes as well as in commercial buildings generally have filters rated MERV 12 or lower. Commercial buildings would require to be retrofitted with better air filters, preferably MERV 13 or higher rated filters to remove small virus particles. For home air conditioners, changing the filter is not recommended. In that case, a good air purifier or a germicidal ultraviolet lamp can be used in the room.

Humidity control

Research suggests that controlling humidity levels reduces the transmission of airborne infectious microorganisms. 40% to 60% relative humidity levels are most unfavorable for the survival of microorganisms. In addition, there is evidence of improved immunity against respiratory infections at mid-range humidity levels (50%). So, utmost care must be taken to maintain mid-range indoor environment humidity levels by proper selection, operation, and maintenance of air conditioning equipment.


Evidence suggests that COVID-19 virus particles can transmit as droplets as well as aerosols. Properly designed, maintained, and operated air conditioning systems can reduce the risk of cross-infection within indoor spaces. But it has to be kept in mind that even the most robust air conditioning systems cannot guarantee complete prevention of COVID-19 infection as there are multiple modes and circumstances under which the virus transmission occurs. Mitigating the risk of infection spread is a collective responsibility and will require collaborative efforts from owners, operators, and occupants of the commercial and public buildings.

Note: The facts and recommendations mentioned here are based on currently available research and guidelines and might change in the future as this is still an evolving space and a considerable amount of research is still undergoing on various aspects of COVID-19 disease transmission and prevention.



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