Is it necessary to mask up at home?

Of late, there has been an increased discussion on SARS-CoV-2 being ‘airborne’ and whether it is necessary to wear a mask even at home.

This discussion was triggered by an article in the journal The Lancet which strongly advocated that Covid-19 is primarily transmitted through airborne route, and suggested wearing a mask indoors. This was followed by an explicit statement by Dr V K Paul, head of India’s Covid-19 task force, that it was time people start wearing masks inside their homes as well.

This left people with many doubts and questions regarding wearing a mask at home and implications of airborne transmission. To make an informed decision on such issues, it is first important to understand the general scientific principles underlying the transmission of a virus like SARS-CoV-2 through air.

Droplet transmission

The primary source of Covid-19 spread (or any other respiratory disease) is the virus-laden droplets expelled from the nose/mouth of an infected person. This droplet transmission happens when a susceptible person comes in contact with infected droplets.

Droplet transmission can take place either through direct droplet or airborne routes. Whenever an infected person coughs or sneezes, a wide range of muco-salivary droplets are generated with diameters ranging between a few micrometres (microns) and about a millimetre (1,000 microns). Furthermore, such events generate a ‘jet’ of air as would be generated by blowing air through the mouth.

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The droplets greater than 100 microns (of the size of an average thickness of a human hair) are heavy and are not affected much by this jet flow or the background air circulation.

Such droplets settle down under the action of gravity but travel a distance of one to two metres away from the source before reaching the floor (or other surface) causing direct droplet transmission. This is the rationale behind the recommended physical distancing of one to two metres.

However, the droplets which are lesser than 100 microns in size get ‘trapped’ in the jet flow and travel longer than one to two metres if the cough/sneeze event is intense. Among these droplets, those that are typically lesser than five microns in size are termed as aerosols and are the main agents of airborne transmission.

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Due to their tiny size, aerosols remain suspended in the air and can respond easily to any ambient air movement in a confined environment (AC or a fan in a room). They can thus be carried away from an infected person over much longer distances (up to 8-10 metres which is the size of a large room) and stay afloat for much longer times.

The respiratory droplets also evaporate as they travel in air which reduces their size, provided the ambient air is not too humid.

Speech droplets

Aerosols are released not only during events such as coughing/ sneezing but also while speaking, singing or even breathing. Speaking is an integral part of our lives and speech droplets (0.5-10 microns) or aerosols are therefore believed to be a primary source of virus transmission by asymptomatic carriers of SARS-CoV-2. These droplets evaporate completely within a few seconds but the solid substances dissolved in them can continue to harbour the virus.

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Research has shown that SARS-CoV-2 can remain active in aerosols from tens of minutes to a few hours, thereby making speech aerosols and those generated by other respiratory events highly likely carriers of infection.

Wearing mask at home

The droplet and airborne modes of infection can be prevented by wearing a good-quality mask which not only traps the droplets/aerosols but at the same time diminishes the jet flow generated by a respiratory event. This explains the need to wear a mask to reduce the risk of infection.

However, there is no one-size-fits-all type of answer for whether masks have to be worn at home and it entirely depends upon the situation.

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When someone in your family has tested positive for Covid-19 or when an outsider visits your home, it is better to wear a mask as the aerosol transmission from such people is likely to infect the indoor air.

If a person in a household goes out (especially, vulnerable places) for work regularly, it may be desirable for the household members to wear masks at home.

If all the family members are largely confined within the household, it may not be necessary to wear a mask at home.

Open windows

Another frequently asked question by people is whether it is safe to keep windows of the house open as the virus transmits through air.

From the mechanism of airborne transmission discussed above, it is clear that the virus needs a carrier in the form of aerosols/droplets to get transmitted through air. Therefore, for any virus to enter house through a window, some source of aerosols needs to be present outside the window, such as a person talking or a group of people conversing. In the absence of any such source in the vicinity of the window, the probability of a virus entering the
house is minimal.

New variants

Another doubt puzzling people is whether the new variants of SARS-CoV-2 are more prone to be airborne than the earlier ones?

The degree to which a virus is airborne depends upon the transport of aerosols on which it resides which means that the new and old variants of the virus should undergo a similar airborne transmission.

However, it is possible that the concentration of virions expelled from the mouth, their sustenance time on aerosol particles and their infectivity are higher for the new variants of SARS-CoV-2. But these questions come under the purview of virology and not much related to the complexity of virus transmission.

As a general rule, therefore, it is safer to follow the recommendations of avoiding people gathering, wearing a mask, maintaining six feet distance and ensuring good cross-ventilation (along with proper hand hygiene) to minimise the risk of airborne transmission of Covid-19 in a household setting.

(The author is Assistant Professor at the Department of Aerospace Engineering, Indian Institute of Science, Bengaluru)