Covid and ventilation

The story: People love air-conditioned space. Sadly, the Covid virus does so, too. With the reopening of restaurants, gyms, malls, and offices, which

Covid and ventilation

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• The story: People love air-conditioned space. Sadly, the Covid virus does so, too. With the reopening of restaurants, gyms, malls, and offices, which may sometimes be poorly ventilated, the transmission of the coronavirus may be rapid.
• The questions: How to assess the risk in a closed space? How to identify recirculation zones? How can a simple intervention like a pedestal fan help?  Covid-19 spreads through the respiratory route. To minimise the risk of infection, people have to pay attention to the flow of air in indoor spaces such as homes, classrooms, restaurants, etc.
• Drops analysed: When a person infected with the coronavirus speaks, shouts, laughs or sneezes, 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.

1. 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. So masks help.
2. 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.
3. 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.

• Outdoors are best: 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 dilute their concentration. Dense crowds or markets carry a higher risk, even if they are outdoors, since large numbers of people are close by.
• How to calculate risk: In indoor settings, ventilation is normally poorer than in outdoor locations, and the risk of infection is higher. The risk of contracting infection is 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.
• Ventilation works best: 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.

1. 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.
2. Well ventilated airy rooms with open doors and windows reduce the risk of infection. An online calculator 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. But 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. A person sitting in a recirculating zone will be more exposed to infectious pathogen, thus increasing the risk of infection. A well-ventilated zone will have smoke moving quickly towards the "out zone". If the smoke rises vertically upwards, or spirals in the same area, this indicates a recirculating zone (bad for us).
• Summary: Small, closed, air-conditioned rooms carry greater risk. As India enters the monsoon season, it's best to leave doors and windows open, ensuring that these spaces sustain a through-flow of air and that zones of recirculation are avoided. 

 

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