Can better indoor air fight covid-19?

Over the coming weeks, WSP will be looking in detail at how the office will evolve in a post-COVID world, from how we’ll behave around our long-lost colleagues to how a working from home revolution will affect demand for commercial space. How can employers transform their spaces into FOMO-inducing, must-go destinations, and what role will smart technologies play in all of this? Will sustainability be boosted by the adaptations we’ve made, and what does a “flexible” office mean as we consider resilience to future pandemics?

But one of the first questions for returning office workers will be: is it safe? Last week, we looked at how disease spreads in offices and how we can limit this through design and cleaning. This week, we turn to the role of air-conditioning and ventilation systems – one of the hottest topics, and one of the most complex.

There are many tried-and-tested strategies from healthcare and high-performance buildings that could be transferred to offices. These come with varying levels of disruption and viability, and there are always trade-offs in performance versus energy, cost or experience. There are also strategies that can promote health more broadly within a building, but may not tackle coronavirus specifically. The conundrum for building owners and employers will be how much they need to change to make us feel comfortable, and what compromises are worth it. We’ll discuss how this will interplay with the economics of changing occupier demand later in the series. But first we need to understand what can be done.

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.”

Fan coil units do contain filters to catch dust and particles, but these must be maintained. “Over time, the filters get clogged and they need to be cleaned or replaced, and quite often this isn’t done properly,” says Justin Turnpenny, who leads WSP’s fit-out team in London. “Good maintenance is a key thing we need to manage going forwards.” (The role that smart technologies can play in remote monitoring of building health will be covered in a forthcoming article.)

Improving filtration can improve the general quality of the air, but higher grades of filtration add resistance to the system: the finer the mesh, the more energy it takes to push the air through – which increases the energy consumption of the building. “All of the guidance and regulations are pushing us into a low-energy office, so improving air quality in this way would run contrary to that” says Turnpenny. “It would definitely go against where we currently are in Building Regulations, so it requires a balanced approach.”

Gary Pomerantz, a building systems specialist and executive vice president at WSP in New York, has also been exploring options for cleaning air within air-handling units. One way to do this would be to install larger air-handling units with better filters and UV light. “HEPA filters are probably the only ones that are proven to remove viruses – they’ll get things down to 0.3µm.” High MERV rated filters, which are typically specified in central air-handling units, can remove 90-95% of bacteria and small particulates, but not particles the small size of most viruses. A UV light could kill the rest – “but it would have to be really bright because there’s very little contact time.” Pomerantz’s rough calculations found that on a project in New York, equipping a 25,000 CFM (cubic feet per minute, equivalent to 11,000 litres per second) air handler with HEPA filters would add US$5,000 to the annual operation cost. Adding UV lights would add half as much again. “So it’s not small numbers and that’s just for one air handler. On a large project, there might be 75 of them.” That additional cost could be reduced by increasing the size of the air-handling units, reducing the impact of the added resistance, though that would increase the space required for each. 

But a lot of particles never make it to the air-handling units. How to address those? One proposed solution is ultraviolet germicidal irradiation (UVGI), which uses short-wavelength UVC light to kill or deactivate microorganisms. However, this comes with its own risks – “humans are lifeforms too,” Pomerantz points out. Strong UVC light can damage our skin and corneas, as well as degrading plastics. It could perhaps be installed at high levels only, away from people – but that would not clean the air in the breathing zone.

Pomerantz has been exploring an alternative solution for capturing the droplets as close to the source as possible, talking to furniture manufacturers about integrating a clear screen and an exhaust slot into desks. “So when we talk, anything coming out hits the screen and gets drawn into the desk, and into a small unit with a HEPA filter and a light source.” The desks could be arranged in a line, all exhausting into the same unit.

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.