Publié 01/07/2021

Is your HVAC system protecting you against COVID-19?

Several factors that are HVAC-related might be of significance when it comes to the way COVID-19 is spread. That spread can be minimized through enhanced humidification, increased filtration, improved ventilation, and upgraded mechanical hygiene. General guidance was issued by ASHRAE for HVAC operations during the pandemic. This article will break down what we know about SARS-CoV-2 (a.k.a. COVID-19), including the virus that caused it. We will also go over other viruses, and the role that HVAC plays to control and spread infection.

Airborne Transmission Significance

Respiratory infections transmitted through air could result in immediate contact (within 3 m). Until it becomes inactivated (degraded), COVID-19 is infectious. How long it stays infectious while in the air remains to be seen, though. Since viruses typically come with minimum doses capable of causing infection (as per the dose-reaction relationship), health risks are associated with air concentration and exposure duration. Such factors remain unknown for COVID-19.

SARS-CoV-2 response guidelines set forth by the WHO and CDC assume that critical transmission routes involve direct contact made with patients, droplet exposure at short ranges, and transfers from surfaces in places where aerosols are settled. As per this assumption, public health agencies have recommended certain response measures, which are generally restricted to surface sanitization, handwashing, face coverings, and social distancing. With that said, evidence suggests small aerosols stay suspended while airborne. In doing so, occupants are exposed to them (airborne transmission).

Chinese and Australian researchers believe there is significant proof of SARS-CoV-2 being transmitted through the air. As such, improvement of filtration and ventilation systems are justified, as doing so could minimize the exposure to COVID-19. This position has been adopted by ASHRAE. Environmental epidemiology establishes how important airborne transmission is when COVID-19 is being spread. It also establishes efficient measures to minimize airborne exposure.

Health scientists, environmental epidemiologists, and engineers are encouraged to collaborate in order to gauge the timing and location of cases, as well as the conditions that are associated with them.


Examples of Poorly operational HVAC system

A comprehensive engineering assessment was conducted involving conditions when an outbreak of COVID-19 happened (at a China-based restaurant). After case locations were mapped out, conditions that were experienced by unaffected and infected staff and customers were considered. The role that various transmission routes played was assessed by contrasting factors. Study findings included the following:

  • 10 of 73 restaurant patrons were infected.
  • The ten infected individuals were sitting down at a few adjacent tables situated on a single side of a dining room (about 3 feet to 16 feet, or 1 meter to 5 meters) from an individual who had recently arrived from the city of Wuhan. This happened before the spread throughout the remainder of China happened.
  • HVAC was comprised of 5 FCUs (fan coil units) without any exhaust fans or outside air. If exhaust fans were present, they were switched on at that time.
  • Filtration and ventilation rates were measured and prioritized by magnitude below the ASHRAE standard of 62.1-2019.
  • Airflow pattern modeling determined that one “bubble” was created by each fan coil unit, separating the room directly into five divided zones, which contained contaminants that were released back into that very zone.
  • There were three tables impacted, and all of them were inside of the exact same zone.
  • Results from the modeling also indicated that a fan coil unit’s discharge directed air toward breathing zones between customers.
  • Air transmitted from the zone that was contaminated didn’t integrate with the remainder of that room. As such, no other customers became infected that were in hotspot areas of the room.
  • Surveillance footage revealed how close proximities between people – as well as fomite contact – weren’t significant.
  • Some waiters had been in contact with customers that were infected, albeit briefly. It wasn’t enough time for these workers to become infected, though

Inefficient HVAC systems caused COVID-19 lockdown in infected areas.

A Restaurant That Was Poorly Ventilated

The conclusion of the study revealed that COVID-19 became spread by a mix of aerosol exposure that reached past a 2 m distance (extended airborne short-range) and close contact (droplet exposure within 7 feet). Lackluster ventilation resulted in minimal virus dilution, which was regarded as a notable factor. The study was inconclusive, and couldn’t determine how relevant airborne transmission was (because of the HVAC’s atypical configuration). Insufficient customer infections in certain adjacent zones indicated that COVID-19 was isolated, stopping transmission of disease. Because waiters weren’t infected, the indication is that contact of a short-term nature was not enough for COVID-19 transmission.

A Bus That Was Poorly Ventilated

126 people had taken a trip to China in a couple of buses, both of which had an AC system that had recirculating air (but no exterior air). An individual that was infected – who originated from Wuhan – happened to be on 1 of the 2 buses. Each passenger with close contact with the individual was infected, along with more than 170 other patrons who hadn’t set foot on that bus. Of them:

  • Zero bus passengers that the individual infected didn’t end up contracting COVID-19.
  • 7 attendees that hadn’t been on the bus ended up contracting the virus. However, they were in very close proximity to the individuals that became infected at an event everyone attended.
  • 23 bus riders that were on the same vehicle with the individual infected had contracted the virus. As far as location goes, more cases of infection happened with individuals who sat more than 7 feet away from the Wuhan-based traveler.

A Conference Center That Was Poorly Ventilated

Thirty people went on a 3-day trip to China. A person from Wuhan that happened to be infected within the same building, which happened to have significantly poor ventilation (for instance, HVAC was cycled on for 15 minutes only every 4 hours). What wasn’t established was who was in close range with the individual infected, so determining who was within the droplet range was not possible. 15 attendees later became infected. Investigators believe that several infections happened because of airborne exposure. They came to this conclusion after contrasting this incident’s infection rate directly to outbreaks that were similar. One major contributor seem to be poor ventilation, which impacted effective dilution.

Study Samples of COVID-19

A Cruise Ship That Was Well Ventilated

An epidemiological evaluation of 696 SARS-CoV-2 cases that took place on the cruise ship known as “Diamond Princess” offered a chance to investigate the effectiveness of an HVAC system recirculating air. It was said to be running with a ventilation rate that adhered to ASHRAE standards. Classification of infection rates fell into a few categories:

  • People engaging without restrictions (for instance, passengers before being quarantined).
  • Passengers that were quarantined within their respective cabins without any COVID-19 positive people present.
  • Passengers that were quarantined in cabins who were exposed directly to infected individuals.

Infections occurred only in the first two classifications. Passengers that were quarantined inside of a cabin were not infected because of recirculated air coming from areas that infected people were in. Because there were no cases in the second category, the assumption is that dilution and circulation of air – by way of HVAC systems – didn’t contribute to infection.

A Call Center in Korea

There were 1145 occupants of a building that received COVID-19 tests. Several cases were discovered on a single floor – a popular call center that was densely occupied. 44% of employees there had tested positive. 94% of people who received positive test results were situated in one portion of that building. 5 cases were discovered on the remainder of a floor where most employees worked out of. Contact that transpired between occupants on the floor’s affected side happened to be quite short. Uniform spread that transpired throughout the area infected indicates that airborne transmission was a factor. There wasn’t sufficient information provided to establish any relationship between HVAC zoning and case location.

Other studies sampled the virus on surfaces and in the air, but they didn’t correlate with these infection patterns. Measured concentrations of contaminants determined that airborne exposure was significantly away from individuals infected, but it did not determine if that exposure transmitted infection to other people.

A Hospital in Oregon

Surfaces were evaluated for COVID-19 within an HVAC system. The recirculating system was placed in rooms that had SARS-CoV-2 patients. Sites that had positive examples included a pre-filter, which received mixed air (outside and return), as well as supply air dampers, post-filtration. Results that were recently reported established that COVID-19 was capable of being transmitted through an HVAC system. These findings didn’t establish if COVID-19 remained infectious (airborne viruses gradually inactivate themselves). Air-quality also wasn’t tested.

Two Hospitals in Wuhan

Airborne COVID-19 was tested at a couple of hospitals in China, as were outdoor locations nearby. Investigators classified several samples as per their particle size, as well as their predicted surface deposition rate.

Included Findings

  • Airborne detection of the virus was prevalent in a majority of sites that had patients present.
  • Airborne concentrations that were elevated were discovered in one bathroom (possible fecal contribution).
  • In a temp hospital, concentrations happened to be lower. Air filtration here was much greater in comparison to a long-term hospital.
  • Deposition tests linked fomite contamination with particle settling.
  • Viruses that were airborne settled on various surfaces that were outside of the immediate space around the source. They were subsequently re-suspended, which contributed to the airborne exposure further.
  • Elevated concentrations of air were quantified in an employee changing area. PPE (personal protective equipment) was used, indicating settled virus resuspension.
  • Concentrations were much lower in employee changing areas after implementation of rigorous sanitization practices.
  • Distribution of particle sizes varied. Droplets greater than 1 µm were dominant at one particular site. Airborne particles lower than 1 µm dominated another. There was another site that was divided equally between small particles and droplets.

Nebraska Hospital

Airborne particles 7 feet away (or more) from patients were discovered inside an adjacent hall in a study recently. COVID-19 wasn’t detected in air surrounding infected patients situated in Iranian and Singapore hospitals. There wasn’t enough information to establish if negative outcomes were because of methodological limitations.

For infectivity’s sake, a virus that was similar to COVID-19 was generated artificially before being measured. Detection of this infectious virus happened after a few hours of it being airborne. On surfaces, detection occurred after three days. Another study discovered infectious airborne viruses after a dozen hours.

These sampling studies are representative of occupant exposure, however, disease transmission is not necessarily clear. PCR (polymerase chain reaction) is what most of the analyses were based on, as it measured COVID-19 RNA totals. That included inactivated viruses no longer capable of causing infection. Methods used measured infectious viruses only.

How HVAC operates against COVID-19?


The above studies establish how COVID-19’s airborne potential goes past the immediate surroundings of infected people. They also establish how concentrations are able to be diluted when ventilation is increased. During the 2003 SARS pandemic, investigators proposed the theory that increased exchange of air might have minimized spread of the disease. However, they didn’t have enough data to support ventilation rate minimums. One investigator cited ventilation as being a key player in the way COVID-19’s spread will inevitably be contained.

While window openings can minimize risks, there are not enough studies relevant to this topic in order to confirm this.

Distribution of Air

Field evaluations performed by the authors involving FCUs mounted on walls identified situations involving air supplies blowing right on occupants. This could potentially transmit the virus from individuals who are infected to others around them. Our assessment of various airflow patterns (that were stated in the restaurant outbreak’s study) indicates that this may have been significant. Air blown right on surfaces may re-suspend aerosols that contain the virus once settled. Distribution of air can establish if infectious droplets either concentrate locally or disperse. Relatively speaking, pressurization can either spread or contain contamination. There is a lack of data out there pertaining to patterns of airflow and their effect on transmission of COVID-19.


Surface sampling within HVAC systems recirculating inside of a hospital offers an indication of a filter’s efficiency, based on a recent report. The virus collected by the supply’s air damper, pre-filter, and final filter makes way for virus comparisons within mixed air – not just before pre-filters were used, but after as well (MERV 10). It also compared final filter discharge (MERV 15). SARS-CoV-2 gathered on surfaces was minimized by about 70% once they went through pre-filters. However, final filters didn’t decrease them any further.

An influenza spread modeling study revealed that air filters with higher efficiency could lower the risk of flu infection. While SARS-CoV-2 response measures currently include filter installation (ones with high MERV ratings, specifically), no studies exist to establish if filter ratings make a difference, as far as disease transmission is concerned.

With appropriate positioning and design, HEPA filters inside an area can minimize exposure to COVID-19 directly. It does this by capturing air within proximity to patients that are infected, or by eliminating airborne viruses that are close to vulnerable individuals. With that said, designers must be mindful that a discharge of air could blow the virus in-between occupants, re-suspending settled viruses lingering on surfaces.

Disinfection of Air

UV (ultraviolet) light systems to accommodate infection control may be either in the duct (HVAC system) or in a space (an upper room). Such systems are sometimes utilized in hospitals for the sake of controlling infectious agents that are airborne. However, they are seldom used in the average non-healthcare facility.

One study revealed that upper room ultraviolet had the ability to prevent measles, chickenpox, and mumps from spreading. However, it may not be very effective in protecting vulnerable individuals from other pathogens. A lack of research was found relative to UV use, as far as controlling COVID-19 is concerned.

Control of Humidity

Coronaviruses are able to survive even under the driest conditions. Occupants are susceptible to potential respiratory infections at lower RH (relative humidity). As a way of controlling the flu, humidification has been recommended. Contrasting the original cases of COVID-19 by country or region suggests that rates of infection may be higher in places with high humidity. With that said, COVID-19 has spread during the pandemic’s initial stages in areas that were humid.

Continuous high COVID-19 rates in the USA this past summer suggest humidity isn’t a key factor. In fact, the NAS (National Academy of Sciences) stated that humidity differences only explain small fractions of COVID-19 variations, as far as transmission rates are concerned.


COVID-19 has been discovered in air, toilet surfaces, sewage, urine, and feces in restrooms. Droplets containing viruses that are released from restrooms are probable, however, no trackable results were evident.

Mechanical Hygiene

COVID-19 lingering on surfaces within HVAC systems were recently studied – particularly in hospitals where air was recirculated in COVID-19 wards. SARS-CoV-2 was found on pre-filters that received mixed air, air damper supplies that were installed after final filters, and final filter surfaces that received air after pre-filtration. Another study revealed that an exhaust outlet’s hospital surfaces in Singapore received positive test results.

HVAC preventions‘ role against COVID-19


More study is required to establish if HVAC systems are capable of transmitting the infection of COVID-19. Further study will also be necessary to determine if changes to HVAC systems can actually control the spread. There is little-to-no detail about such information at the time of writing.

Airborne Transmission

There haven’t been enough samples taken to establish that COVID-19 is airborne. There are also very few samples suggesting that it can expose occupants past 7 feet (2 m), though exposure doesn’t necessarily mean infection. Recent findings have established that SARS-CoV-2 is capable of circulating through various HVAC systems. Although multiple SARS-CoV-2 infection outbreaks have implied airborne transmission, it is typically in spaces with minimal ventilation. That may be because building ventilation dilution (as per building code requirements) hinders transmission of disease.


The increase of outdoor air can dilute SARS-CoV-2’s concentration if it’s airborne. Minimum ventilation rates haven’t been established yet for building operations where infected people might be present. With that said, it hasn’t been determined whether the increase of ventilation rates over ASHRAE minimums could actually reduce the transmission of disease. National ventilation hasn’t been evaluated for SARS-CoV-2.

Distribution of Air

Research suggests that air, once discharged, that circulates inside spaces could establish if exposure is enough to stimulate disease. For instance, FCUs that are wall-mounted are capable of discharging air into breathing zones, which could direct the virus to another occupant. Relative pressurization is also capable of spreading or containing contamination. There is a lack of field data about the effect of air distribution when it comes to exposure of COVID-19.


Reports of sampled surfaces within HVAC systems that were recirculating were recently reported. According to the data, a pre-filter (MERV 10) minimized the quantity of COVID-19 that was deposited by almost 70%. Also, a final filter (MERV 15) might not remove extra viruses. Field data was insufficient with regards to filter selection and COVID-19. However, filtration inside an area (for instance, compact HEPA units) can minimize virus exposure inside local spaces with appropriate placement and design.

Ultraviolet Disinfection

While ultraviolet light can potentially minimize virus exposure inside certain situations, a lack of data is available to back up widespread application.

Control of Humidity

Coronaviruses have the potential to survive longer. Occupants are therefore more vulnerable to respiratory infections inside dry conditions. While SARS-CoV-2 initially spread within areas with dry conditions (such as during winter), community spread also happened in humid places (like southern hemispheres and tropical climates in the summertime). Continuous high COVID-19 rates in the USA this past summer suggest that levels of humidity are not a relevant factor. The available information doesn’t support humidification if the RH happens to be under 40%, as far as controlling COVID-19 goes.


COVID-19 was discovered in urine and feces, making sewage and sewer gas a potential exposure source. Further studies are required to establish impactful strategies capable of preventing occupants from becoming infected.

Mechanical Hygiene

COVID-19 was found on surfaces within HVAC systems that were recirculating air. However, it is unclear if the virus is capable of contaminating the airstream.

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Publié 01/07/2021

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