Air-conditioning ceiling unit

The air that I breathe

Image credit: Getty Images

Can we trust air-conditioning and ventilation systems to provide us with safe, virus-free air?

Before the pandemic, open-plan offices and hot-desking were de rigueur in many industries. But, as we start the bumpy journey to a post-lockdown ‘new normal’, details like where people sit and workplace ventilation have come under fresh scrutiny, meaning our workplaces could look and feel very different when we all eventually return to them.

A growing number of experts believe ventilation should be improved to help prevent the spread of coronavirus, which, according to the World Health Organization (WHO), is primarily transmitted through large respiratory droplets that settle on surface areas or are spread through human contact, rather than being ‘airborne’ per se.

“Shops, offices, schools, restaurants, cruise ships and, of course, public transport, is where ventilation practices should be reviewed and ventilation maximised,” Lidia Morawska from Queensland University of Technology writes in an article in the Environment International journal. So, employers and companies may not only have to mark out two-metre distances and create safe working zones, but check out the ventilation too.

Ventilation plays a role in how the virus can spread through indoor spaces, especially if they are crowded and contain people who have Covid-19. Luckily, the European Federation of Heating and Ventilation Engineers (REHVA) has developed comprehensive guidance for building services engineers to help to minimise the spread of the virus through HVAC or plumbing systems.

The guidelines are based on the idea that dilution of internal air should reduce any risk of potential airborne viral transmission by cutting exposure time to any airborne viral aerosols, and therefore minimising the chance of germs settling on surfaces.

“The advice for workspaces is to increase the rate at which outside air is drawn into the building,” says Dr Hywel Davies, technical director at CIBSE (the Chartered Institution of Building Services Engineers). “The most important consideration is to get a good air exchange and ensure the outdoor air is supplied throughout the space, to dilute and exhaust pollutants.”

This means that any ventilation or air-​conditioning system that normally runs with a recirculation mode should now be set up to run on full outside air wherever possible.

Deadly dining

Serving up a virus

Scientists believe an asymptomatic diner at a restaurant in Guangzhou, China, unwittingly infected nine people eating at the same location because of his/her position in relation to an air-conditioning unit. The authors of a letter published in the Emerging Infectious Diseases Journal hypothesise that respiratory droplets were blown around the windowless room.

“Droplet transmission was prompted by air-conditioned ventilation. The key factor for infection was the direction of the airflow,” they wrote.

There is no epidemiological evidence that coronavirus spreads through air-conditioning systems themselves, explains Paul R Hunter, professor in medicine at the Norwich School of Medicine, University of East Anglia. “The risk is not sitting downstream of an air-con unit, but downstream of an infected person. If the air con is sufficiently aggressive that it blows that person’s droplets your way, then you need to worry,” he says.

Numerous experts say that simply opening a window is the best solution, but it’s not always that easy for buildings without openable windows or those in polluted areas. “In some cases, the mechanical ventilation and sufficient filtration may actually offer a better, cleaner solution in some buildings, depending on their location,” says Julian Tang, consultant virologist at the Leicester Royal Infirmary and honorary associate professor in the Department of Respiratory Sciences at the University of Leicester.

Of course, you can’t just open a window on an aeroplane, but most, if not all, commercial planes have high-efficiency particulate air (HEPA) filters, which are also used in hospital operating theatres to trap dust particles, bacteria, fungi and viruses. Air flows past millions of particle-grabbing layers in the filters and is blended with air sucked in through the engines, creating a 50-50 mix of fresh and recirculated air.

In fact, research has shown there is little risk of any communicable disease being transmitted on board an aircraft and a total change of air is achieved 20 to 30 times an hour. However, WHO warns that transmission of infection could occur between passengers who are seated in the same area of an aircraft, usually as a result of the infected individual coughing or sneezing or by touch. “This is no different from any other situation in which people are close to each other,” it says, suggesting airlines will have to introduce additional measures to keep passengers safe.

People sitting on an aeroplane, fiddling with light and air, inline

Image credit: Getty Images

In most buildings, filters are too coarse to filter viruses, so why not fit HEPA filters instead? “It isn’t possible to simply retrofit a HEPA filter as they are unlikely to seal properly,” says Davies. “Such filters will also reduce the ability of the system fans to move air around the building due to increased resistance from the HEPA filter.”

Other air-cleaning devices, like using light in the UV-C spectrum, may be an option. Germicidal ultraviolet devices use light to inactivate coronaviruses and can be added to an upper-room system or a standalone consumer unit by specialists, REHVA says. However, they can be dangerous to humans.

“Care needs to be taken in the installation of air-cleaning devices to ensure they are safe and effective. It is a delicate balance to calculate the rate at which cleaning can take place versus the rate at which the virus is being generated versus the effect of dilution,” Davies says.

Ultraviolet light could destroy viruses without any risk of hurting humans. The technology uses lamps that emit continuous, low doses of a particular wavelength of ultraviolet light, known as far-UVC, which can kill viruses and bacteria without harming human skin, eyes and other tissues, as is the problem with conventional UV light.

“Far-UVC light has the potential to be a game-changer,” says David Brenner, professor of radiation biophysics and director of the University of Columbia’s Centre for Radiological Research. “It can be safely used in occupied public spaces, and it kills pathogens in the air before we can breathe them in.”

While HEPA filters and light devices may boost safety, “the ideal set-up for avoiding infection is to work from your own home”, says Davies. However, if you do have to go back to the office or into shared buildings, there are things everyone can do that help.

Tang says the layout of the office space is arguably less important than the ventilation rate per person that can be achieved. “But, for any deficiencies in the ventilation system, opening windows, masking and maintaining social distancing, will all help to reduce the overall exposure risk to the occupants.”

Airborne transmission

Cruise ship crisis

In February, the Diamond Princess cruise ship was quarantined with 3,711 passengers and crew members on board after a passenger who had disembarked the week before tested positive for the Covid-19.

More than 700 people on board became infected and 14 died. There were concerns that the ship’s air-conditioning was partially responsible for the spread, but Professor Hunter says: “When you look at the timing of clinical illnesses in the passengers it is almost certain that everyone got their infection before quarantine was imposed.”

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