How ventilation controls can affect the spread of COVID-19

Two new research papers advocate the use of engineering controls in public buildings, along with other measures to limit the risk of infection from covid-19.

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This is because to prevent the spread of the coronavirus and other infections in public buildings, handwashing and social distancing are not enough; they do not prevent infection by inhalation of the virus.

Both papers conclude that the use of ventilation controls would be an “additional important measure globally to reduce the likelihood of transmission and thereby protect healthcare workers, patients and the general public”.

“Existing evidence is sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit infection risk indoors,” 35 authors of one  of the papers conclude.

Despite this evidence, says one of the authors Dr. Morawska, “no countries or authorities consider airborne spread of COVID-19 in their regulations to prevent infections transmission indoors.

“It is therefore extremely important,” she insists, “that national authorities acknowledge the reality that the virus spreads through air, and recommend that adequate control measures be implemented to prevent further spread of the SARS-CoV-2 virus, in particular removal of the virus-laden droplets from indoor air by ventilation.”

How to control the virus spread indoors

1. Ventilation

Ventilation is the first step, according to Morawska, Shelly Miller and 33 other authors from around the world of the second of the two papers.

Hospital buildings typically have good ventilation, but the authors note that ventilation in shops, offices, schools, kindergartens, libraries, restaurants, cruise ships, elevators, conference rooms and public transport, can vary from purpose-designed mechanical systems to simply opening doors and windows, which may not be sufficient. They say:

“In a mechanically ventilated building, ventilation air is typically provided by a heating, ventilating and airconditioning (HVAC) system. Sometimes, ventilation air is provided by dedicated fans or outdoor air units.

“HVAC system control strategies can usually be modified to increase ventilation to a certain extent in the occupied zones, with relatively little additional cost, to reduce the risks of airborne transmission between occupants.

“However, this is not via a simple ‘flick of a switch’, as HVAC systems are complex and usually designed for individual buildings within standard specific operating parameters. Many requirements need to be considered apart from the ventilation rate, including control of temperature, relative humidity, air flow distribution and direction.”

Such systems can be specifically customised as needed by HVAC engineers, e.g. to reduce the risks of airborne transmission.

2. Avoid recirculating air

Avoiding air recirculation is the second step, which is also recommended by the European and American engineering associations.

This is obviously not easy in hot climates.

In many systems recirculated air is taken from one space and distributed to other spaces connected to the same system, potentially increasing the risk of airborne infection. One of the papers advises:

“Particulate filters and disinfection equipment in recirculated air streams can reduce this risk, but they need to be purposely designed to control risk of airborne infection and need regular service to maintain their effectiveness.

“Many systems are designed for filters that are intended to remove larger particles that may affect the functioning of equipment and that are not effective at removing small, sub micrometre or micrometre size particles associated with adverse health effects.

“Filter ratings by test methods, such as ASHRAE Standard 52.2 (ASHRAE 2017) that give an indication of performance as a function of particle size should be utilized in choosing appropriate filters.”

During an epidemic, including the current pandemic, air should not be recirculated as far as practically possible, the authors advise.

Systems which have no fresh air intakes should be turned off or supplemented by lots of fresh air through opening windows, preferably with cross ventilation. Again, this is not easy in hotter parts of the country and fans may be required that direct interior air outside.

In every case, equipment should be sized in relation to the room, temperature, the airflow and number of occupants.

3. Air cleaning, ultraviolet light and disinfection

Air cleaning and disinfection devices may be beneficial too. This includes ultraviolet light systems. Portable air filters and cleaners “could help”.

Portable consumer air cleaning devices may be beneficial in smaller rooms, although again it should be recognised that such devices must be appropriately sized for the space.

However if UV lamps are integrated into the HVAC system, “the main benefit is to keep organisms from growing on the surfaces, especially the coil and drain pan,” says Allison Bailes of Energy Vanguard. “It’s not going to kill much coronavirus or other baddies that get pulled into the ducts.”

The International Ultraviolet Association says UVC will help prevent COVID-19 “based on existing evidence”:

“All bacteria and viruses tested to date (many hundreds over the years, including other coronaviruses) respond to UV disinfection. Some organisms are more susceptible to UVC disinfection than others, but all tested so far do respond at the appropriate doses…UV light, specifically between 200-280nm (UVC or the germicidal range), inactivates (aka, ‘kills’) at least two other coronaviruses that are near-relatives of the COVID-19 virus: 1) SARS-CoV-1 and 2) MERS-CoV.”

4. Minimise the number of building occupants

It’s also important to minimise the number of people within the same indoor environment.

“In a school or a supermarket, for example, if the number of infected students or shoppers is low, and the ventilation rate is high, the risk of airborne transmission can be low.”

Crowding people into a subway or bus is definitely not advised.

Lessons for architects

There are lessons here for architects.

Co-author Lloyd Alter adds that the design process needs to be changed:

“Architects can’t just design a building and then toss the plans over to an engineer. The mechanical systems and the building design are inseparable – how the air moves, how much is needed. Instead of designing for aesthetics or value or comfort, we have to design for health.”

This will not be the last pandemicTo hit humanity.There will be many lessons to learn for engineers and architects, not to mention planners, health officials and building control. The way we control internal air-quality must evolve, factoring in not just the effect on climate change and energy costs, but more dimensions of human health than have been previously considered.

David Thorpe is the author of Passive Solar Architecture Pocket Reference, and ‘One Planet’ Cities: Sustaining Humanity within Planetary Limits. He also runs the online course, a Post-Graduate Certificate in One Planet Governance. He is based in the UK.