Excellent study material for all civil services aspirants - begin learning - Kar ke dikhayenge!
SHARE:
Ventilation is key to battling SARS-CoV-2 and Covid
Read more on - Polity | Economy | Schemes | S&T | Environment
- The story: Scientists were surprised to find that when families co-incidentally were packed together inside a restaurant, one infected person spread the virus to many more. Initial analysis proposed that the infection had spread via respiratory “droplets”. But medical science believes that such droplets — defined as particles expelled while breathing that are more than five microns across — cannot travel more than a couple of metres after exhaling. But some of the infected were sitting far away. How could a single infected person transmit the virus to many others in just an hour when there had been no direct contact between them?
- Suprespreading is Covid's main weapon: "Superspreading" is loosely defined as being when a single person infects many others in a short space of time. More than 2,000 cases of it have now been recorded, in places as varied as slaughterhouses, megachurches, fitness centres and nightclubs, and many scientists argue that it is the main means by which Covid-19 is transmitted.
- Cracking the puzzle: In cracking the puzzle of superspreading, researchers had to re-evaluate their understanding of SARS-CoV-2’s transmission. Most documented superspreadings have happened indoors and involved large groups gathered in poorly ventilated spaces. That points to SARS-CoV-2 being a virus which travels easily through the air, contrary to the early belief that short-range encounters and infected surfaces were the main risks. This, in turn, suggests that paying attention to the need for good ventilation will be important in managing the next phase of the pandemic.
- Close contact: Social distancing and mask-wearing were recommended with the intention of cutting direct, close-range transmission by virus-carrying droplets of mucus or saliva breathed out by infected individuals. The main risk of spreading the illness indirectly was thought to come not from these droplets being carried long distances by air currents, but rather by their landing on nearby surfaces, on which viruses they were harbouring might survive for hours, or even days. Anyone who touched such an infected surface could then transfer those viruses, via their fingers, to their mouth, eyes or nose. Hence the advice to disinfect surfaces and wash hands frequently, as it was assumed the SARS-CoV-2 spread in the same way as influenza.
- Different particles, different behaviours: Doctors know that not all respiratory particles fall fast.
- Those smaller than five microns can become aerosols, staying aloft for hours and potentially travelling much farther than droplets, or simply accumulating in the air within a closed room. Anyone inhaling these aerosols could then become infected.
- But aerosols were thought to be relevant only in specialist medical settings, such as when patients are attached to a ventilator in an intensive-care unit. Intubation, as this process is known, does indeed create aerosols, as the breathing tube is forced down a patient’s trachea.
- The WHO played down the risks of aerosols, issuing guidance in March 2020 that the general public need not worry -- “FACT: #COVID19 is NOT airborne,” it had said.
- Non-medical researchers disagreed, as a superspreader case early in the pandemic in America proved. This was at a choir practice in Skagit Valley, Washington State, in March 2020. Of the 61 people present during a two-and-a-half-hour meeting, 53 became infected. Investigation showed that those infected were not the people closest to the index patients, which should be if transmission had been by droplet or surface contact.
- Medical experts got it wrong: Scientists soon showed that received medical wisdom was wrong! Because exhaled breath is a moist, hot, turbulent cloud of air, a five-micron-wide droplet released at a height of one and a half metres can be carried dozens of metres before settling. The generation of respiratory particles is not restricted to medical settings. Liquid drops of all sizes (incl. aerosols) are continuously shed while people are breathing, talking, sneezing or singing.
- WHO remained firm: The widespread assertion, still promulgated by the WHO, that droplets above five microns in diameter do not stay airborne, but rather settle close to their source, is a wrong foundation. Any particle less than 100 microns across can become airborne in the right circumstances. All of this matters because hand-washing and social distancing are hence not enough to stop an airborne virus spreading, especially indoors. Masks will help, by slowing down and partially filtering an infectious person’s exhalations. But the real weapon will be ventilation.
- Summary: How can the occupants of a room know whether it is well-ventilated? Just because a room feels spacious and an air conditioner is operating does not mean the air inside it is clean. Actually, CO2 concentrations can be a useful proxy for clean air. Outdoor air contains around 400 parts per million (ppm) of CO2, and people’s exhaled breath contains around 40,000 ppm. Exhaling into a room therefore gradually raises its CO2 concentration unless the ventilation is good enough to remove the excess. Anything below 500 ppm in a room means the ventilation is good. At 800 ppm, 1% of the air someone is breathing has already been exhaled recently by someone else. At 4,400 ppm, this rises to 10%, and would be dangerous. To keep the risk of covid-19 low, CO2 levels should be well below 700 ppm. In situations where it is not possible to reduce health risks by ventilation alone (nightclubs, or gyms) air filtration could be incorporated into ventilation systems. Air could also be disinfected, using germicidal ultraviolet lamps placed within air-conditioning systems or near ceilings in rooms.
* Content sourced from free internet sources (publications, PIB site, international sites, etc.). Take your
own subscriptions. Copyrights acknowledged.
COMMENTS