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Covid-19: Is the coronavirus ‘airborne’ and how likely are you to catch it when indoors or outdoors?

SINGAPORE — When the Government started testing people living in public housing blocks recently after individuals infected by the coronavirus were found to be residents there, the question on most people’s minds was whether they could get infected by their neighbours and by what means.

Experts said that a coronavirus infection can happen outdoors but the risk is very much lower.

Experts said that a coronavirus infection can happen outdoors but the risk is very much lower.

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  • Evidence now suggests that airborne transmission may play a larger role in the spread of Covid-19 
  • WHO recently acknowledged that aerosols carrying the virus can “remain suspended in the air”
  • Still, the Sars-Cov-2 virus is far more likely to spread to someone who is close to an infectious person
  • Ventilation and rate of air exchange matter in reducing disease spread in indoor and outdoor settings
  • Experts give advice on how to improve ventilation, reduce the risk of infection indoors or out


SINGAPORE — When the Government started testing people living in public housing blocks recently after individuals infected by the coronavirus were found to be residents there, the question on most people’s minds was whether they could get infected by their neighbours and by what means.

This came at a time after the World Health Organization (WHO) gave an update last month in the Q&A section on its website that the coronavirus can be spread in poorly ventilated or crowded indoor settings where people tend to spend longer periods of time.

The reason was because “aerosols remain suspended in the air or travel farther than 1 metre”, it said.

It looked to be a new development because at the start of the Covid-19 pandemic last year, the health authorities including WHO believed that the disease was spread mainly by respiratory droplets that were inhaled or came into contact with the eyes, nose and mouth and via contaminated surfaces.

After a year though, evidence now suggests that airborne transmission may play a larger role in the disease spread than previously thought — something that WHO and the Centers for Disease Control and Prevention in the United States (US CDC) recently acknowledged.

Yet, the idea of airborne transmission of an infectious disease is not new.

Chickenpox and measles are examples of contagious diseases known to spread by way of aerosols — a mixture of particles small enough to be suspended in the air that can contain viruses.

TODAY asked the experts to help clear the air on what is the risk of getting infected if the virus is “airborne” and how people may protect themselves better at home, when taking public transport, taking lifts in apartment blocks, when exercising outdoors and so on.


Infectious diseases expert Hsu Li Yang said that before the Covid-19 pandemic, “airborne transmission” was thought to be caused by infectious particles smaller than 5 micrometres (or microns) that could remain suspended in the air for longer periods of time, infecting people further away from the infectious source.

On the other hand, “droplet” transmission was believed to occur with larger-sized particles that could not be suspended in the air for long and could not spread as far.

“The assumption was that only small particles below 5 microns could spread longer distances and remain infectious, and that ‘droplets’ larger than that can only infect those who are nearby (within 2m).  Thanks to the work of aerosol scientists, we now understand that this is incorrect,” he said.

Associate Professor Hsu is the vice-dean of global health and head of the infectious diseases programme with the Saw Swee Hock School of Public Health at the National University of Singapore (NUS).

He and other experts explained that a combination of large droplets and small aerosols are released into the air when an infected person breathes, coughs, sneezes or speaks.

This is similar for infected people with or without symptoms.   

Particles larger than 5 microns that may contain viruses can be suspended in the air and travel far. The width of a human hair is about 60 to 120 microns. Photo: Mohamed Hassan/Pixabay

Assoc Prof Hsu said that particles of different sizes, including those larger than 5 microns, can be suspended in the air and travel far, potentially reaching a distance beyond 2m and infecting people further away.

“The distance (the aerosols) travel is dependent on the environment and surrounding ventilation, as well as the frequency of the aerosol-generating action (such as talking or coughing).” 

Adding to this, Associate Professor Tham Kwok Wai from NUS’ School of Design and Environment said that all exhaled droplets rapidly undergo evaporation.

The larger droplets quickly evaporate as they fall under the influence of gravity and can become smaller droplets. They may also be propelled farther if the person coughs or sneezes, for example, and potentially infect someone.

Smaller droplets remain airborne and are likely to move with the airflow patterns created by winds or air movement in an indoor environment.

“Being lighter, the smaller droplets may travel very far as they remain airborne. The risk (of getting infected) is higher in indoor and enclosed environments, especially if they are poorly ventilated,” Assoc Prof Tham said.

He has done research on transmission of viruses such as influenza in indoor settings and spoke on behalf of the indoor air quality research unit at NUS’ department of building, which also includes Associate Professor David Cheong and Professor Chandra Sekhar.


Assoc Prof Hsu said that the key is not to get hung up about particle size or whether it is “droplet” or “airborne” transmission, but to focus more on environmental factors that can reduce aerosol spread.

He emphasised that one fact remains unchanged: The Sars-Cov-2 causing Covid-19 and other respiratory viruses are far more likely to infect those in close proximity to an infectious person.

The risk is generally higher indoors than outdoors because ventilation and air exchange plays a role in transmission.

The nearer the person and the more time that is spent close together, the higher the risk of transmission of the virus.

The risk of contracting the coronavirus gets higher when there is repeated exposure to an infected person over a prolonged period. Photo: Congerdesign/Pixabay

“Depending on other factors including ventilation, infections can take place a considerable distance from an infectious person as well, resulting in super-spreader events as well as possibly some of the unlinked infections where the source cannot be traced,” Assoc Prof Hsu said.

“For variants of concern that are more infectious, like the B1617 variant, the risk of super-spreader and distant-spreading events become somewhat greater, but not appreciably so such that the virus now behaves like measles, which is a very infectious virus, for example.”

Assoc Prof Tham pointed out that airborne transmission is just one way in which the virus can be transmitted and that the nature of Sars-Cov-2 is still being researched.

“More significantly, the virus has undergone mutations that fundamentally changed its infectivity. We are fighting an enemy which has been able to rapidly evolve.”

On a common concern about contracting the virus from eating food handled by infected persons, Assoc Prof Hsu said that there is presently no evidence of transmission through such means, partly because the cells that the Covid-19 coronavirus are able to attach to are in the respiratory tract.  


The experts said that an infection can happen outdoors but the risk is very much lower.

This is because ventilation and air exchange are much better than that in indoor environments.

Fresh, moving air disperses and dilutes the aerosols and the virus they carry.

However, even outdoors, there is still a risk of getting infected if ventilation and airflow is poor in an area or space, especially when people let down their guard and do not adhere to safety measures such as masking up properly and keeping a safe distance.

For example, when people crowd under a shelter at a park or bus stop on a still day with no air movement to disperse exhaled particles, Assoc Prof Tham said.

A man taking a walk along the Henderson Waves bridge in Telok Blangah. Photo: Ili Nadhirah Mansor/TODAY

He added that stairwells may also be poorly ventilated and contain stale air.

Referring to the Covid-19 cases from different households at Block 506 on Hougang Avenue 8, Assoc Prof Tham said that stairwells could be one possible mechanism of transmission within the same block although there is not enough disclosed data now to ascertain this.

“Maybe the (infected persons use or) share the same stairwell,” he said.

Four of the households there were found to be from the same column of flats on different floors. 

Assoc Prof Tham said that other probable routes of transmission when out and about in the neighbourhood would include being in close contact such as spending time talking, shopping and eating in poorly ventilated spaces.

Maintaining a safe distance will allow the air to have more opportunities to dilute the concentration of virus in exhaled aerosols from the nearest person.


Assoc Prof Tham also suggested avoiding inadequately ventilated places such as shops with no intake of outdoor air, for instance, those served only by split-unit air-conditioners.

These are air-conditioning systems that connects indoor aircon units to an external condensing unit.

Assoc Prof Tham explained that air within the air-conditioned space is recirculated, which may potentially transmit the virus when particles are expelled by an infected person.

For people who must visit enclosed places or shops with inadequate ventilation, he advised masking up properly and not spending a prolonged period of time there browsing and comparing prices, for example. 


Although research shows that a person who is exercising releases more droplets the more intense the exercise is, Assoc Prof Tham said that outdoor exercise is unlikely to be a significant contributor to infection in a neighbourhood.

“(In outdoor environments), most exhaled droplets will dissipate sufficiently such that its concentration is likely to be so low that it will not cause infection.”

Having said that, if someone is very close to the infected person, some of these droplets may be inhaled in enough quantities to result in infection, he added.

One high-risk situation is when an infected person coughs or sneezes right at another person.

Another example of a high-risk scenario would be running behind an infected person and breathing in expelled vapours — for a prolonged period of time.

“Although this is in an outdoor environment, a large proportion of what the infected person exhales is potentially in the air that you are inhaling.

"If you are behind him or her during a 10- to 15-minute jog, for example, you are exposing yourself (to the virus) for a rather long duration,” Assoc Prof Tham said. 

It is also possible to pick up the virus when one unknowingly walks into a floating suspension of virus-laden aerosols minutes after an infected person had coughed or sneezed in the same space, especially if the ventilation of the room is particularly poor.

However, if that happened outdoors, the suspension would have been dispersed within seconds, Assoc Prof Hsu said.


Another possible source of transmission are lifts, the experts said, especially when people talk or take off their masks in there.

It could also happen that someone coughed or sneezed in the lift right before exiting but the person entering may not know it.

Wear a mask when using a lift where space is usually tight. TODAY file photo

This is why it is important to continue to adhere to safety measures such as safe distancing and using masks with good filtration efficiency, meaning single-use or reusable masks with at least two layers of fabric and at least 95 per cent bacterial filtration capability.

“Although masks primarily stop an unsuspecting infected person from spreading viruses, masks with good filtration efficacy are also able to protect the wearer considerably,” Assoc Prof Hsu said.


The Agency for Science, Technology and Research (A*Star) has done studies and found that public transportation such as MRT trains and public buses are adequately ventilated.

For instance, air in MRT trains is exchanged every six minutes through ventilation systems and when doors open and close.

The ventilation in MRT train cabins uses a centralised air-conditioning system, which constantly filters the air in them and supplies them with fresh outdoor air. Photo: Raj Nadarajan/TODAY

Assoc Prof Tham said that most forms of public transportation would have complied with an advisory by the Land Transport Authority to increase ventilation, but protective measures may not be observed by private-hire transportation.

Besides ensuring that drivers and passengers mask up properly, the team from the indoor air quality research unit of NUS suggested that commuters ask private-hire car drivers to wind down windows, either fully or at least partially. This applies similarly when travelling in taxis, too.

“Opening windows on opposite sides, even partially, helps cross ventilation.”

If the driver or passenger is unwilling to wind down the windows, the team suggested that the car’s ventilation dampers be fully opened.

The mechanism of ventilation dampers allows air from outside to flow into the car. The lever or button to activate the dampers is usually controlled by the driver and the default is usually not to let in airflow from outside.

A study published in the journal Science Advances in December last year, which looked at airflow patterns inside a car to potentially reduce Covid-19 airborne transmission, found that blasting the car’s ventilation system did not circulate air as well as rolling down the windows.

Simulations showed that opening all four windows was the ideal.

However, the researchers found that opening car windows on the opposite side from where the person is seated — not the one right beside the person — also creates an airflow pattern that reduces airborne transmission between driver and passenger.


At home, improving air circulation and ventilation can help improve air quality.

This would mean opening windows and switching on fans, Assoc Prof Hsu said.

If there is an exhaust fan in the kitchen or bathroom, keep it running if you have visitors in your home. Keep the exhaust fans turned on for an hour after your visitors leave.

Another option is to place a fan close to an open window or door, facing outwards, to increase ventilation. Point the fans away from people. Pointing fans toward them may possibly cause contaminated air to circulate. Consider using ceiling fans to improve airflow.

These tips are adapted from the US CDC on improving ventilation at home as well as recent guidelines issued by Singapore's Building and Construction Authority, National Environment Agency and Ministry of Health on ventilating malls, offices, coffee shops and dormitories to prevent the spread of the coronavirus.


On whether air purifiers are useful in reducing transmission at home or an enclosed space such as a retail shop, Assoc Prof Hsu said that the majority of commercial devices sold to the public are inadequately tested.

These commercial products are different from air purifiers with high-efficiency particulate air (Hepa) filters used in hospitals to reduce risk of spread of viruses and bacteria in areas such as operating theatres and isolation rooms.

Hepa filters are able to remove more than 99 per cent of contaminants in the air, including particles comparable to the size of the coronavirus.

The US CDC said that these are the most efficient filters on the market for trapping particles that people exhale. It is important to select one with the right size for the room, for example, ensure that the Hepa fan system has a "clean air delivery rate" that meets or exceeds the square footage of the room in which it will be used.

Regardless of the number of air purifiers at home, Associate Prof Hsu said that if one family member is infected with Covid-19, the risk to other members of the household will still be great due to repeated exposure over a prolonged period.

Assoc Prof Tham emphasised that there is no single strategy to prevent Covid-19 spread. Getting vaccinated and other measures such as safe distancing and masking up provide added protection.

Related topics

Covid-19 coronavirus ventilation airborne transmission indoor air infectious disease

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