Once coronavirus restrictions lift, we face huge changes in the way we go about our lives ― it’s been suggested that social distancing will continue until we have a vaccine (or treatment) for the virus, which could be another year.

When we eventually return to our workplaces, start commuting by bus, train and even plane, shop more regularly, and begin gathering at restaurants and bars again, should we be mindful of the air conditioning that is commonly used in these environments?

Opening windows and doors helps to keep fresh air circulating in enclosed spaces, which is why people who are self-isolating are encouraged to do both in their own homes. However, many offices, shops and restaurants (not to mention public transportation) rely on air conditioning to circulate air and regulate temperature. Can air conditioning systems spread coronavirus?

How does COVID-19 spread?

To answer this question, we first need to talk about how the virus spreads. At the moment, we know COVID-19 can spread from person to person through small droplets expelled from the nose or mouth when a person coughs, sneezes, exhales or speaks. These droplets land on objects and surfaces around the person, according to the World Health Organization.

We can catch the virus by touching these objects or surfaces, then touching our eyes, nose or mouth. People can also catch COVID-19 if they breathe in droplets from someone the virus who coughs out or exhales smaller droplets (known as aerosols). This is why we’re urged to keep a distance of 6 feet from other people.

Whether or not the virus is airborne is fiercely contested. The World Health Organization suggests there isn’t enough evidence to show the disease is airborne ― in an analysis of 75,465 COVID-19 cases in China, airborne transmission was not reported.

But airborne transmission might be possible in healthcare settings, especially during certain procedures or treatments (for example, disconnecting someone from a ventilator). This is why personal protective equipment (PPE) is so vital for healthcare workers.

Aerosols can travel in the air due to how tiny they are, carrying potential viral content quite some distance from where they originated.

But Paul Hunter, an infectious diseases expert and professor in medicine at the University of East Anglia, tells HuffPost UK: “Although droplets actually have to move through the air [to infect someone], they don’t tend to be airborne.”

Airborne, in this sense, would mean the viruses could stay in the air for a long period of time and travel long distances to infect somebody, Hunter said. “Although it is possible that COVID-19 is in certain circumstances airborne, it probably doesn’t happen very often,” he stresses.

Scientists in Finland have used modeling to show aerosol particles can remain in the air for several minutes after a person expels them – either through coughing, sneezing or speaking “then end up in the respiratory tract of others in the vicinity,” according to Ville Vuorinen, assistant professor at Aalto University.

The concern is that aerosol particles could accumulate in public spaces by the virus hitching a ride on the droplets we expel when we cough, talk or breathe. Whether the virus can travel far enough, or linger long enough, to infect someone else through inhalation is as yet unknown.

Wearing face coverings ― such as homemade masks ― may reduce the number of droplets we expel, thereby reducing possible exposure in supermarkets and other crowded spaces. This is why some places now recommend face coverings in public settings.

So, can air conditioning spread the virus?

A study of an air-conditioned restaurant in Guangzhou, China, found coronavirus spread to three separate families eating in the restaurant ― all of them sitting in proximity of the air conditioning unit.

The direction of airflow seemed to correlate with the diners who contracted the virus, researchers said. Among the 83 customers who ate in the restaurant on the same floor as the infected person that day, 10 became ill with coronavirus. The other 73 were identified as close contacts and quarantined for 14 days.

The most likely cause of the outbreak was droplet transmission, researchers said. But larger respiratory droplets remain in the air for only a short time and travel short distances, generally less than 3 feet, they added. The distances between those already infected and people at other tables, who went on to contract the virus, were greater than 3 feet.

“Strong airflow from the air conditioner could have propagated droplets” between the tables, researchers suggested.

Hunter said the Chinese study is consistent with the “droplet spread” hypothesis and “does not, in any way, imply [Covid-19] is spread by air conditioning.” He also cites the case of the Diamond Princess cruise ship. It was suggested that air conditioning onboard the ship, where some 700 people were infected, spread the virus, but the evidence does not back this up.

One study awaiting peer review suggests most transmission appears to have occurred through close contact and infected surfaces and objects. Hunter agreed, adding that case reports show none of those who were infected appeared to have picked up the virus while quarantined in their rooms.

However, Professor Qingyan Chen, an expert in mechanical engineering at Purdue University, Indiana, believes air conditioning could potentially spread COVID-19, as the units aren’t designed to filter out particles as small as the coronavirus.

“The coronavirus could be airborne so the virus could be spread through air conditioning systems, although the virus concentration in air is very low,” he told HuffPost UK in an email.

If you visit a grocery store (or travel on a bus or a train) for a few minutes up to a few hours, the risk of being infected through airborne virus is low, he said. However, if you’re exposed to air with a very low virus concentration for a long time ― such as in an office ― it is “likely to be infected.”

While more research is needed, virologist Julian Tang from the University of Leicester said we should assume airborne transmission is possible unless experimental evidence rules it out ― not the other way around ― and that people should take precautions to protect themselves, he told the journal Nature.

Those precautions are particularly wise in crowded, indoor environments. Hunter said facilities managers should be turning down the volume of airflow in buildings with air conditioning to prevent droplets from spreading further.

Other studies have recommended increasing ventilation, not recirculating air and reducing the numbers of people in indoor environments to limit the spread.

Where ventilation is inadequate and density of people is high, some experts recommend the use of masks and respirators.