Do air conditioning and ventilation systems increase the risk of virus transmission? If so, how can this be managed?


Updated: 22 May 2020


Air conditioning and ventilation systems that are well-maintained and operated should not increase the risk of virus transmission. Fans are safe in single occupancy rooms. Fans for air circulation in collective spaces should be avoided when several people are present in this space.


All air conditioning and industrial ventilation systems for both residential and high occupancy buildings (government buildings, schools, hotels, and hospitals) should be inspected, maintained, and cleaned regularly to prevent transmission. Even in well-ventilated environments, people should continue following recommendations of physical distancing and frequent hand hygiene. Set temperatures between 24oC/75 oF and 27oC/ 80.5oF for cooling during the warmer weather, and RH between 50% and 60%.


If the use of fans is unavoidable, increase outdoor air exchange, and minimize air blowing from one person directly at another should be taken to reduce the potential spread of any airborne or aerosolized viruses.

What does this mean?

High-occupancy publicly-accessed commercial properties such as stores, hotels, schools, shopping malls, restaurants, office buildings, etc. generally operate on centralized and ‘closed-system’ climate control and ventilation systems. Air conditioning may also be routinely used in homes during the summer months.


Air conditioning and ventilation are considered effective control strategies for preventing workplace infection and ill health in the hierarchy of controls (a framework used in occupational health to prioritize the controls needed for protection of human health, which is applicable also when considering health risks to the public in indoor spaces). If a ventilation system is well maintained the risk of transmission should not be significant. By well maintained, we mean that the system is inspected periodically, the most efficient filters are used, the filters are changed according to manufacturer recommendations, and the duct systems are cleaned periodically (Quian and Zheng, 2018).


If the air conditioning or ventilation system is not well maintained and operated, there are two potential mechanisms through which it could contribute to virus transmission: the system itself could recirculate contaminated air; and/or could create indoor conditions (temperature and humidity) that support virus survival.


Buildings that use a central ventilation and/or climate control system, should use the most efficient filters. Consider installing a higher efficiency filter (MERV 13 to 16, EPA 2019), or in healthcare facilities, a HEPA filter which captures viruses effectively (Perry, 2016), and where the air handler is rated for such a filter. Filters should be installed and maintained according to manufacturer recommendations. High efficiency filters are also available for residential use.


To reduce the number of days that the SARS-COV-2 virus can remain viable in the indoor environment, avoid setting climate control systems to low “cold” temperatures (below 70 F/ 21C) (Chin et al., 2020) and “dry” low humidity settings (below 40%) as these are optimal conditions for the virus to survive(Chan et al., 2011;Van Doremalen et al., 2020).


In summary, air conditioning, ventilation, or other climate control systems that are well-maintained and operated should not increase the risk of virus transmission.

What can be done?

Reduce virus circulation in public spaces. As a precautionary measure, people should continue engaging in physical distancing until a vaccine or tests for immunity are available, considering the large proportion of pre-symptomatic transmission (Gandhi 2020). Universal facial protection will also help prevent droplets (from speaking and coughing) from reaching surfaces or others. Facial protection will not prevent the person wearing it from becoming infected, but it will reduce the potential for spreading from pre-symptomatic individuals (Koehler and Rule, 2020).


Suspected or confirmed COVID-19 patients should be isolated and quarantined and should not be present in public spaces. They should wear a mask to reduce droplet shedding and further transmission within the household. Rooms/areas with a suspected or confirmed COVID-19 patient should be well ventilated and properly cleaned periodically. Clinical facilities that house COVID-19 patients should have negative-pressure and HEPA-filtered ventilation to prevent the virus from spreading to other areas of the facility.


Building and business owners and facility managers should take steps to ensure air conditioning and industrial ventilation systems are inspected, maintained, and regularly cleaned to reduce inadvertent transmission risk. Set temperatures between 23.8oC/75oF and 26.9 oC/ 80.5oF for cooling during the warmer weather, and RH between 50% and 60% to reduce mold growth (CDC 2015). They should also undertake additional regular cleaning of spaces frequented by staff and the public, even when masks are worn and physical distancing is being respected. The virus does not reproduce outside living cells but can survive on surfaces and air (van Doremalen 2020, Fears 2020) for a sufficient duration to facilitate transmission, it is important to insist on appropriate and frequent sanitization of surfaces to prevent transmission.


In non-hospital buildings where ventilation systems function in a closed circuit, (such as in office buildings, restaurants, hotels, shopping malls, senior housing facilities, and schools), a high-efficiency particulate air (HEPA) filtration (or the highest MERV rating allowed by the HVAC system) should be used to clean recirculated air. Another strategy would be to install distributed HVAC (heating ventilation and air conditioning) units such as packaged terminal air conditioners (PTAC) that do not depend on a central mechanism to distribute and recirculate air (Dietz et al. 2020).


Building occupants should practice safe hygiene practices, including wearing a face mask (N95 when possible), frequent hand washing with soap and water, and practicing physical distancing as much as possible to avoid further transmission until otherwise indicated by health authorities. Facility managers should encourage these practices by posting reminders and making hand washing facilities available, when possible.


When air conditioning or ventilation are not available, (as is the case in many homes and buildings in warm climates) an air purifier, fans, and/ or opening windows to recirculate and allow fresh air into closed spaces is good practice (WHO 2009). If you use an air purifier, you must also ensure proper maintenance by following manufacturer recommendations. If fans are used, they should be installed where room air can be exhausted directly to the outdoor environment through either a window or door (WHO 2009).


The use of fans is advised where there is only one person in a room. In collective spaces, when several people are present in this space, the use of fans for air circulation/cooling is not advised particularly in small volume, closed or partially open spaces with minimal outside air exchange (HCSP, 2020). If fans are used, take steps to minimize air from fans blowing from one person directly at another to reduce the potential spread of any airborne or aerosolized viruses (CDC-2020).


A. The role of air conditioning and ventilation in transmission via indoor re-circulation of virus:


Direct droplet transmission (i.e., contact within 2 meters of an infected person) is an important route of transmission, but airborne transmission (at a distance >2 meters) cannot be ruled out (Santarpia 2020, van Doremalen 2020, Fears 2020). In addition, contamination of surfaces (fomites) may contribute to continued transmission of infection, particularly in indoor climate-controlled environments where ventilation systems may not be functioning properly and create conditions that may be favourable for the virus to survive longer time periods on surfaces.


At present, there is no strong evidence to suggest that a well-maintained air conditioning, ventilation, or other type of climate control system will contribute to the transmission of COVID-19:


Two recent studies, based on air sampling in the immediate vicinity of COVID-19 patients in a hospital with significant viral load in their respiratory secretions, reported that the SARS-CoV-2 virus was not detected in the air samples (Chen et al 2020, Ong et al 2020) likely due to a well-maintained and efficient ventilation system in a hospital setting. Another study carried out on a long-distance flight reported no evidence of transmission of the virus to passengers seated near people with COVID-19, also attributed to efficient ventilation and filtering of cabin air (Schwartz et al 2020).


The existing evidence suggesting that central ventilation and / or air-conditioning in large buildings may help spread some viruses (Zhao et al., 2003; Li et al., 2005) either reported problems with the ventilation systems or is inconclusive. Another recent report from China claims that COVID-19 was spread via an air-conditioning system in a restaurant (Jianyun et al, 2020). However, major limitations have been noted in this study (Dietz et al., 2020).


If a ventilation system is well maintained and operated the risk of transmission should not be significant in either residential or commercial buildings. Thus, an important prevention measure is to have all air conditioning and industrial ventilation systems for both residential and high occupancy buildings (government buildings, schools, hotels, and hospitals) inspected, maintained, and cleaned regularly.


B. The role of indoor low temperature and relative humidity (RH) in helping virus transmission:

The purpose of air conditioning, ventilation, or other climate control systems are to create thermally comfortable environments for people, while often also improving the quality of circulating indoor air. Studies that have examined different levels of indoor temperature and relative humidity on the transmission of SARS-CoV-1, H1N1, and MERS-CoV viruses show that lower temperature and relative humidity (below 70°F/ 21°C and below 40% RH) increase the survival time of both coronaviruses and influenza on dry surfaces (Otter et al., 2016). For example, SARS-CoV-1 can survive at least two weeks in an air-conditioned environment where temperature is typically 22-28°C and relative humidity is usually controlled at 30-60% (Chan et al., 2011). However, the same virus is inactivated within 15 minutes at 56°C in a laboratory setting (WHO, 2003). Recent studies confirm that SARS-CoV-2 survival is comparable to SARS-CoV-1 (Van Doremalen et al., 2020). Therefore, it is plausible that findings for SARS-CoV-1 are also true for SARS-CoV-2 (Sun et al., 2020).


Studies of the influence of indoor and outdoor ambient conditions on COVID-19 should not be conflated. The WHO recommends avoiding exposure to sun or to ambient temperatures higher than 25°C as there is no evidence that this prevents or cures COVID-19 and increases risk of sunburn and heat related illness (WHO, 2020).


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