Masks, social distancing and hand hygiene have become widely accepted as ways to reduce the spread of coronavirus but some scientists are saying the ventilation of our homes and workplaces could be another tool in our prevention armoury.
As we learn more about COVID-19, more evidence has emerged that the virus spreads through aerosols as well as through larger droplets, especially in closed, crowded spaces.
But the good news, according to aerosol scientist Alex Huffman from Denver University, is we can do things to reduce the risks indoors.
"Ventilation is a really important piece of the puzzle," he told the ABC's AM program.
Keeping fresh air flowing has not yet been a high priority as public health officials have focused on the important work of increasing the adoption of social distancing and mask-wearing but now, said Dr Huffman, it was time to begin talking to the public more about our indoor environment.
Some solutions, he said, could be implemented by people in their own homes at little to no cost but making changes to offices and public buildings needed a bigger effort from politicians and business.
"It is a resource issue but I also think it's a leadership issue. If your leaders are convinced that this is a critical piece of the puzzle, then they will find a way to make it happen."
Airflow inside buildings and other enclosed areas like public transport has received less attention than other prevention strategies, in part because of debate within medical and scientific circles about the role of aerosols in the transmission of the virus, with some experts focusing on bigger droplet particles as being more important in transmission.
But in recent weeks, the World Health Organization, the American Centers for Disease Control, the European Commission and Canada have acknowledged airborne aerosol transmission has a significant role in the spread of coronavirus.
Australia's Infection Control Expert Group has acknowledged the potential for aerosol transmission in some clinical settings and has noted the risk may be higher in poorly ventilated indoor crowded environments.
The group's current prevention advice lists good hand hygiene, physical distancing, staying at home and masks.
It does not list airflow.
Dr Robyn Schofield, an atmospheric chemist from the School of Earth Sciences at Melbourne University, said the threat of aerosol transmission is particularly significant for the virus spreading indoors.
"If you have an inside space with a lot of people in it, a lot of people breathing, you will get a lot of aerosol building up and so the risk is higher.
"If the source person is wearing a mask, most of the respiratory aerosol has been caught by the mask so the risk is lower but if masks are off, people are eating and there is poor ventilation, you have a situation where the risks are higher."
UK-trained GP Dr David Berger has been advocating on behalf of frontline health workers during the pandemic.
He said there was a frustrating reticence to acknowledge the importance of airflow.
"It almost seems like there is a lot of face and ego in this and people aren't prepared to backtrack.
"As long as people believe that washing their hands and staying 1.5 metres apart is going to keep them safe, they are working on the wrong information. This is all about ventilation."
Occupational hygienist and engineer Kate Cole said: "Whether it's a hospital, an aged care facility, a school or an office — everyone should be looking at what can they do to increase the amount of fresh air, remove the amount of contaminated air and increase the volume of air going into their spaces.
"We want the information on ventilation to get out to the public because it all just helps reduce our risk."
Ventilation, aerosols and atmospheric chemistry can sound overwhelmingly complicated but Professor Lidia Morowska from the School of Earth & Atmospheric Sciences at QUT believes there are easy precautions that can be taken now.
"We have to do whatever we can do now, opening windows, increasing ventilation, turning up the buttons in offices or wherever we are."
"Building managers — pressuring them to increase ventilation and reduce re-circulation. All these things can be done now," she said.
Dr Schofield added that simple and cost-effective precautions are available to many of us.
"If you're in a building with a great HVAC [heating, ventilation and air conditioning] system, you want to try to bring as much outdoor air in [as you can]."
While broad social restrictions have helped combat the pandemic in Australia, COVID-19 infections are rising dramatically in many northern hemisphere countries as they head into winter.
Shelly Miller, Professor of Mechanical Engineering at the University of Colorado, believes ventilation will be important to reducing the spread there.
"It's going to be a really rough and terrible winter for all of us and it's heartbreaking."
"We have to wear a good-fitting mask, they have to be efficient and we have to social-distance. Then we need to look at ventilation and supplement that with air-cleaning.
"Putting all of these things together is a layered approach that can help us to make things much less risky."
All major buildings where people congregate should be assessed for ventilation, according to Professor Mary Louise McLaws, an expert in epidemiology, hospital infection and disease control and a member of the WHO's expert panel on COVID-19.
She said confined spaces need ventilation rates of 3 litres of air per second per person.
"It's [the ventilation message] not getting nearly enough traction because it costs money," she said.
"It will cost money for every single building to go back and ensure it can do 3 litres per person per second and that is an enormous undertaking.
"They can do it in a hospital and some of the older-style hospitals are now being required to improve their airflow: they're becoming woke to the importance of this."
The Victorian Health Department's Building Authority has commissioned engineering assessments of the HVAC systems within wards dedicated to suspected and conﬁrmed COVID-19 patients at 20 hospital sites over the coming weeks.
Some countries are already taking substantial action, with Germany recently investing 500 million euros to improve ventilation systems in public buildings.
But while assessing the ventilation of our built environment could be costly, Dr Berger said it could pay dividends beyond COVID-19.
"But what it does mean is that [if we do nothing about ventilation] we are at risk of this virus and we will not have a defence against the next."
"We are doing the equivalent of learning in the 19th century how important it was to wash your hands. It's that level of discovery".