Updated: July 6, 2021 11:23:28 am
Written by Guruswamy Kumaraswamy, Krishnendu Sinha, Janani Murallidharan, Sourabh S. Diwan & Gautam Menon
Covid-19 spreads through the respiratory route. To minimise the risk of infection, we must pay attention to the flow of air in indoor spaces such as homes, classrooms, restaurants, offices, neighbourhood grocery stores, etc.
Virus in droplets
When a person infected with the coronavirus speaks, shouts, laughs, sneezes, or coughs, they emit droplets. These droplets range in size from the very small (referred to as aerosols) to large, but still less than a millimetre in size.
While large droplets fall to the ground under gravity within 1-2 metres of the person emitting them, smaller aerosols can stay suspended in the air and can be carried over large distances, over 8 metres away.
Masks trap droplets, dramatically decreasing the range over which they are carried. Thus, they are a very effective way to minimise the number and range of droplets emitted by people.
Well-fitted, high filtration efficiency masks such as the N-95 masks virtually eliminate the chance of droplets infecting the wearer. However, the quality of the fit is critical, and a perfect fit, with no gaps, is difficult to achieve in practice with a typical mask. Wearing two masks, one surgical and one cloth, reduces risk substantially.
It’s safer outdoors
For infectious airborne pathogens, the chances of infection are greatly reduced through good ventilation. When one is outdoors, normal draughts of wind are sufficient to disperse pathogens and greatly dilute their concentration. The risk of infection is thus very low when one is outdoors, and in situations that are not too crowded.
Dense crowds typical of urban train stations or markets in our country carry a higher risk, even if they are outdoors, since large numbers of people are in close proximity. Thus, in an outdoor setting that carries any risk of crowding, it is prudent to double-mask using well-fitting masks, and to minimise the time spent in proximity to other people.
As India opens up, physical distancing will not always be possible. There will be times when we will need to come into contact with people in closed rooms, in cafeterias, offices, or in social settings. An often-neglected fact is that even talking can spread Covid-19; our research shows that short unmasked conversations can carry a significant risk of infection.
In indoor settings, ventilation is usually poorer than in outdoor locations, and the risk of infection is correspondingly higher. There are well-studied methods in the engineering sciences that can be used to assess the risk of airborne infection indoors. These methods estimate the risk of contracting infection based on the number of people in a closed space, the time of exposure, and the ventilation in the room. The greater the number of infected people in a room, and the longer one spends in that room, the higher the possibility of getting infected.
A key aspect here is ventilation. The simplest measure of ventilation is the average air exchange rate. This tells us how often the air in a room is replaced with fresh air.
Consider an exhaust fan that vents the air from a room. An exhaust fan with a rating of 250 cubic feet per minute working at full capacity will, on average, exchange the air in a 15 ft x 10 ft x 10 ft room in about 6 minutes. In general, the higher the fan rating, the better the ventilation.
Pedestal fans placed near doors or windows can also be very effective. On the other hand, ceiling fans circulate the air in a room, rather than venting these out directly, and can improve ventilation only with open doors and windows.
Ventilating a room dilutes and removes potentially infectious aerosols, decreasing the risk of infection. Therefore, well ventilated airy rooms with open doors and windows reduce the risk of infection. An online calculator for estimating the safe period of time that you can be in a closed room, and yet limit the chance of infection has been developed by a group at MIT. and is available at: https://indoor-covid-safety.herokuapp.com/
One of the assumptions underlying these air exchange calculations is that the air in the room is well mixed and has the same, uniform concentration of pathogens everywhere. However, computer simulations of air-flows suggest that this is not the case. Specific parts of the room, such as corners, have pockets of air that form recirculating zones. Pathogens trapped in these zones are not easily vented out. Our simulations indicate that droplets in these regions can stay for 10 times longer than in well-ventilated parts of the room. A person sitting in a recirculating zone will be more exposed to infectious pathogen, thus increasing the risk of infection.
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A simple strategy to identify recirculating zones in a room is to use an agarbatti to gauge the direction of smoke drift. If the smoke rapidly drifts towards one of the openings, such as ducts, windows or doors, then this is a well-ventilated zone. If the smoke rises vertically upwards, or spirals in the same area, this indicates a recirculating zone.
These simple methods to study ventilation in indoor spaces can be tested and refined by combining them with computer simulations, as some of us at IIT Bombay are doing. We find that simply increasing the exhaust fan speed may not eliminate the recirculating zones. They can, however, be minimised by the suitable placement of pedestal fans. We have applied these methods to study how air flow can be improved in confined locations such as barber shops, by simply altering the placement of fans within them.
Small, closed, air-conditioned rooms carry enhanced risk. As India enters its monsoon season following the heat of summer, it may be feasible to reduce the use of air-conditioning, leaving doors and windows open, ensuring that these spaces sustain a through-flow of air and that zones of recirculation are avoided.
The role of ventilation in ensuring safety against the transmission of Covid-19 has not been emphasized sufficiently so far, but should be a crucial part of India’s future strategy.
The Experts: Dr Kumaraswamy is professor of chemical engineering at IIT-Bombay, Dr Sinha is professor of aerospace engineering at IIT-Bombay, Dr Murallidharan is assistant professor of mechanical engineering at IIT-Bombay, Dr Diwan is assistant professor of aerodynamics at Indian Institute of Science, Bengaluru, and Dr Menon is professor of physics and biology at Ashoka University, Sonepat. The authors are part of Fluid Mechanics Research for Covid-19 (FMRC), a group of researchers who came together last year to look at ways to utilise research in fluid mechanics to address some problems thrown up by the pandemic.
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