Can better indoor air fight Covid-19?

Offices are typically air conditioned by recirculating air systems. These might be centralized air-handling units in dedicated mechanical equipment rooms, or local distributed fan coil units positioned above the ceiling or around the perimeter of a space. Both function in basically in the same way – they draw in air from the space, mix it with a proportion of fresh air from outside the building, pass it through a filter, then through a coil containing chilled or warm water, and finally blow it back into the space at high velocity to create a general mixing of room air. “The issue is that increased air motion may cause the virus to carry further than it normally would in still air,” says David Cooper, a mechanical engineer and president of global property and buildings at WSP. “So instead of the virus dropping out of the air within the 6ft social distancing guideline, it may well travel 10 or even 20ft or more before coming to rest on a surface.” So sitting the recommended distance away from someone else may not provide the expected level of protection if your workspace is fitted with these systems, even if they are fitted with enhanced filtration.

Traditional displacement or underfloor air distribution (UFAD) systems can minimize air currents and the horizontal movement of air, says Cooper. “UFAD systems basically create an air zone per occupant workstation.” Similarly, chilled beams or radiant ceiling panels also stimulate very little airflow and therefore reduce the risk of increasing the horizontal travel of coronavirus through the air.

Taking it to another level, “laminar air” systems in operating theatres and clean rooms are designed to minimize horizontal air movement to the greatest extent possible. “The concept is that you supply air up high and it moves vertically down through the space slowly, taking germs and contaminants out of the breathing zone, bringing them straight down to floor level where it is exhausted,” says Todd See, a specialist in laboratories and high-performance buildings with WSP in San Francisco. “It would be harder to put that into an existing building, but it’s certainly something we could consider as we’re designing new spaces.”

As SARS-CoV-2 particles tend to drop rather than travel through the air like an aerosol, increasing the proportion of outside air or leaving systems running 24/7 to purge spaces would probably not directly impact the spread of this particular disease. However, it could dilute other contaminants, improve air quality more broadly and certainly make for a more pleasant environment – which would in turn be reassuring to returning office workers.

This again comes with an energy penalty, as it takes more to cool or heat the air to the right temperature, and equipment would need to be sized to accommodate higher demand, as it is for hospitals. Exactly how much additional energy is needed depends on where you are. Using a greater proportion of outside air is already an established sustainability strategy in mild climates. In the UK, for example, the external air temperature is below 18C for 60-70% of the year, says Austin Wikner, WSP’s head of building services in London, so propelling that through the building using a displacement system can be up to 40% more energy efficient than fan coil units. “It’s not particularly new or innovative, but interest has been increasing as we’ve realised how energy efficient it can be. It’s the thing we default to now.”

Similarly, in San Francisco, the outside air can be used for most of the year, whereas in New York City (or generally on the eastern seaboards of continents), it is very hot and humid in July and August and can be very cold in the winter months, which is typically the flu season. “With a clean sheet of paper, I would try to minimise the amount of extra energy or at least the times when you’d have to spend extra energy,” says See.

See designed a novel system for a fully air-conditioned space using 100% outside air without expending any fan energy, which was installed at a research facility for the US National Oceanic and Atmospheric Administration in Hawaii. This was inspired by the ancient Middle Eastern principle of allowing a space to stratify so that heat rises and cool outside air enters at a low level. “It uses chimneys where air is heated by solar energy to create some small forces that drive more and more air through the building.” The system has been installed in several other buildings, including a two-storey office for the Hilton Foundation in California.

Could we just throw open the windows? “Everyone talks about natural ventilation,” says Pomerantz. “I think it’s great – as long as I get to sit by the windward window. Someone’s going to be downstream, and they will get everybody else’s germs as the air migrates across the space before exfiltrating the building.”

Any strategy for resilience against a pandemic needs to take into account other heightened threats too. Sometimes the outside air is the problem, as in Australia when bushfires raged from October 2019 to January 2020. “In Sydney, the office smelled like smoke for three months and outside you could not see 100m away,” says Jonathan Ramajoo, head of healthcare at WSP in Australia. “They actually shut off outdoor air to the building for a number of hours. If you had a bushfire during COVID, that would really compromise the air quality in the space.” In a new building for the Australian National University, WSP has designed the air-conditioning systems with room for carbon filters that can be fitted when needed to remove smoke from incoming air, so that outdoor air could still be provided to the spaces. “This space provision could allow for other filter types to combat the environmental hazards that are presented to us,” adds Ramajoo.

It is believed that the right relative humidity (RH) levels – ideally between 40-60% RH – decreases the infectivity of viruses, makes them settle out more quickly, and improves our ability to fight them.

That’s good news – but not necessarily the easiest strategy to implement. “Controlling humidity outside of a healthcare environment is almost impossible,” says Tomer Zarhi, mechanical manager in the healthcare team at WSP in Canada. “It would be very expensive to monitor and control in an office.”

It partly depends on local conditions. It’s not so much the humidity of the outside air as the temperature, says Jack Maynard, who leads WSP’s mechanical and electrical business in Canada: “As you bring air inside and warm it up, the percentage humidity decreases.” To compensate, water has to be added. Relative humidity of 40% would be a high target for an office, and achieving that would again require greater energy and water use.

Another complication is that many existing buildings are not designed for higher humidity levels, warns Cooper. “Humidification in the winter will result in condensation on windows and mullions, potential for moisture freezing inside wall cavities and damaging them, greater potential for mould and mildew if not controlled properly, and of course high energy costs,” he says. Even non-medical buildings that are designed to accommodate indoor humidification for comfort and general health are not usually able to maintain 40-60% RH during peak winter months. At that level, condensation would still be experienced even on high-performance glazing and thermally enhanced mullions.

If COVID-19 spurs the nascent trend for more healthful office environments, it will be welcomed by many. But it may, in the end, come down to economics. Just as the pandemic is forcing us to reconsider established strategies for controlling the office environment, it is also shining a very powerful spotlight on the ways that we have traditionally occupied and valued those spaces. That’s something WSP will be exploring next in the series. 

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