As the pandemic forced people to shield themselves indoors, many office spaces around the world were left empty for months.
Desks went unused, printers and computers were untouched and office plants were left to battle the elements on their own.
Two years later, employers are encouraging workers to come back to the office. But many are reluctant to return.
One report has found roughly six-in-10 US workers who say their jobs can mainly be done from home (59 per cent) are working from home all or most of the time, while half of British workers (50 per cent) revealed in a survey they are still working from home at least some of the time, up from 37 per cent before the pandemic.
Workers are reluctant to return to the office for a number of reasons. Working from home has allowed them to claw back time wasted on commutes. Others are saving money they would have otherwise spent on transport to work or buying lunch.
One other factor playing on people's minds is COVID-19. While the risk of getting the coronavirus may be significantly lower than it was two years ago, largely thanks to vaccines, we are not completely out of the woods yet.
So, how can businesses help ease the minds of people taking those tentative first steps back into the office?
Experts suggest a mix of approaches, including monitoring CO2 levels, using air purifiers and staggering rosters or allowing more hybrid work.
How does COVID-19 spread in the air?
An aerosol physicist at the Queensland University of Technology, Lidia Morawska, has been studying COVID-19 for more than two years and says the science on how it moves in the air is clear.
SARS-CoV-2, the virus that causes COVID-19, is mainly spread through larger particles called respiratory droplets or smaller particles called aerosols.
But the debate around aerosols has been contentious.
Early on in the pandemic, the World Health Organisation had insisted that the coronavirus spread through contaminated surfaces and in larger, heavier droplets
That prompted Ms Morawska to lead a group of 239 scientists to appeal for public health organisations like the WHO to address the "overwhelming" research on the dangers of microdroplets.
"There's basically no doubt about [droplets and aerosols] being the major transmission of the virus," she says.
Droplets and aerosols can spread when an infected person breathes out, coughs, sneezes, speaks, shouts or sings.
While larger particles can disappear quickly, these tiny particles can stay suspended in the air indoors from minutes to hours and inhaled by someone.
"You're at a greater risk if you're closer to someone because the virus in aerosols behaves like cigarette smoke particles. And if you're sitting closer to a smoker, you're going to breathe in more of that smoke, it's more concentrated close to the person,' says Linsey Marr, an expert in airborne viruses at Virginia Tech.
Ms Morawska recently discovered the perils of indoor meetings during the pandemic first hand while travelling Europe for work.
While she doesn't know exactly where she caught COVID-19, she suspects she may have been exposed while attending a conference.
"When I walked into that conference, I had that feeling that I should have left, that I shouldn't go in there, because of the proximity of people," she says from Helsinki.
But while poor indoor air quality may increase the risk of transmission, experts suggest more research is still needed on how the coronavirus moves in places like offices and other shared spaces.
How COVID can spread indoors
As part of a study last year, a group of researchers observed how the air is likely an important transmission route for COVID-19 in hospitals.
The study was limited to hospital wards, although public health organisations have acknowledged how workplaces can be risky settings in a global pandemic.
In open-plan offices, conference rooms or with hot desking arrangements, it can be difficult to maintain social distancing for an extended period of time.
And while vaccination reduces the chance of infection, indoor air still poses a risk. Yet enforcing targets for air quality is difficult, experts say.
In contrast to the US, Australia currently has no specific controls on indoor air quality — apart from in certain workplace situations under the National Occupational Health and Safety Commission.
One impediment is a lack of awareness in society about the harmful effects of air. Another issue is that, unlike with contaminated water which can often be traced back to a particular source, air is all around us.
"Let's say there is pathogen virus and bacteria in an office space or a concert hall [for example], people will not get contaminated, not get sick immediately, it will take a few days, and then be dispensed," Ms Morawska says.
"So where did they get that virus? Did they [get] it somewhere else? That's the nebulousness of indoor air."
Even so, public health experts agree there are some ways employers can make office spaces safer for employees during a pandemic or virulent flu season.
Keep fresh air flowing
One of the best ways to make offices and other public spaces less risky is by increasing ventilation and filtration, according to Ms Marr.
These strategies alone won't stop the spread of the virus, but should form part of a suite of infection control measures in workplaces, she suggests.
Small offices do not always adequately provide enough airflow to dilute a pathogen virus. This can be a problem when there is an infected person in the office, as they are likely to leave higher concentrations of viral particles, which can build up over time.
Experts say employers could start by monitoring carbon dioxide (CO2) levels, since they can be a useful indicator of the relative infection risk of COVID in an indoor space.
High CO2 levels indicate poor ventilation and could be used as a trigger to reduce occupancy and/or increase ventilation and filtration measures in the workplaces, according to Stephen Duckett, the director of health and aged care program at the Grattan Institute.
Safe Work Australia advises that CO2 concentration measurements averaging between 800-1500 parts per million over an occupied period could be an indicator to take action to improve indoor ventilation.
If CO2 levels are tracking higher, experts suggest natural ventilation methods like opening windows, doors or air vents could be useful.
"During the Victorian period and beyond, one of the main treatments for TB, was people going to Switzerland, and breathing in mountain air, and if you look at [older] sanatoriums, they're all kind of light and airy with fans, and they have the windows open, so we've known this for some time," Mr Baker says.
"But the way that modern buildings have been built — even in my building now — there's no windows [to] open."
The other option is to use mechanical ventilation, which describes air-handling systems that bring in fresh air from outside.
This is traditionally part of a building's heating, ventilation and air conditioning (HVAC) system.
"There are adjustments you can make to these systems in [office] buildings, including bringing in more outdoor air," Ms Mar says.
"You can also upgrade filters that are in the system [if] you have a mediocre filter that removes 20 or 30 per cent of particles from the air ... as long as your system can handle it."
Air filters can also be useful
High-quality air filters, like HEPA filters, may also be effective in workplaces where it is not possible to change HVAC settings, experts said.
"If you don't want to mess around, or you can't mess around, with a whole building's central HVAC system, you can get portable air cleaners, which have HEPA filters in them, [that can] remove particles from the air that passes through that machine," Ms Marr says.
While these filters have been sold as a way to reduce the concentration of SARS-CoV-2 particles in the air, the usefulness of these devices depends on the number of people in the room, how the filter is set up, where the air is coming from, how many filters there are and the position of the filter in the room.
It can also depend on the number of air exchanges you have in a room compared to the size of filtration device you have.
"So if you have a massive room with a small filtration device in it, then it's probably not going to do anything.
"Whereas if you have a large filtration devices in a small room, it's going to be super effective at cleaning more air."
Mr Baker says while more research is needed, having these HEPA filters in offices, hospitals and in schools could have a "positive impact" by providing cleaner air.
Get creative with rostering and hybrid work
Despite the unwinding of social distancing restrictions, one of the most effective protections against COVID-19 continues to be limiting interactions with infected people.
Throughout the pandemic, this was encouraged by advising employees to work from home when possible.
Working from home doesn't completely eliminate the risk of COVID. But it does lower the risk of contracting (and transmitting) COVID in the workplace.
If it's not possible to work from home, experts also suggest staggering work hours to reduce the number of people congregating in small spaces like conference rooms and lifts.
Finally, while mask wearing has fallen out of favour with the unwinding of restrictions, experts say they can still be a useful tool in combating the spread of the virus in an indoor setting.
These measures could also help to reduce the spread of other viruses, particularly the flu and certain colds in office settings.
"[Things like] ventilation, filtration and disinfection will also help reduce transmission of the flu and colds that are probably transmitted in the same way that COVID-19 is transmitted," Ms Marr says.
It could also help ease conditions for those who are immunocompromised and people with asthma.
"There are a lot of benefits to having cleaner indoor air in terms of absences from work and school and productivity and academic performance," Ms Marr says.
