The challenge that the novel coronavirus (Covid-19) has thrown in front of the world is continuously evolving with each passing day ever since the pandemic broke out in the Chinese city of Wuhan in late 2019.
A latest study published by BMJ, the leading general medical journal, has added to the consternation by stating that the current rules on safe physical distancing are based on outdated science. Distribution of viral particles is affected by numerous factors, including air flow, argues a team of infectious-disease experts while asserting that evidence suggests Sars-CoV-2 may travel more than 2m through activities such as coughing and shouting. Rules on distancing should reflect the multiple factors that affect risk, including ventilation, occupancy, and exposure time, it has been suggested.
Physical distancing is an important part of measures to control Covid-19, but exactly how far away and for how long contact is safe in different contexts is unclear. Rules that stipulate a single specific physical distance (1 or 2m) between individuals to reduce transmission of Sars-CoV-2, the virus causing Covid-19, are based on an outdated, dichotomous notion of respiratory droplet size. This overlooks the physics of respiratory emissions, where droplets of all sizes are trapped and moved by the exhaled moist and hot turbulent gas cloud that keeps them concentrated as it carries them over metres in a few seconds. After the cloud slows sufficiently, ventilation, specific patterns of airflow, and type of activity become important. Viral load of the emitter, duration of exposure, and susceptibility of an individual to infection are also important.
Instead of single, fixed physical distance rules, the study has proposed graded recommendations that better reflect the multiple factors that combine to determine risk. This would provide greater protection in the highest risk settings but also greater freedom in lower risk settings, potentially enabling a return towards normality in some aspects of social and economic life.
The conventional wisdom behind 6ft separations originated from research by a German biologist, Carl Flügge, who in the late 1800s suggested that was as far as microbe-containing droplets could travel. His hypothesis missed farther-flung particles invisible to the naked eye – in particular, the tiny gobs of bodily fluid and virus that float on the air as aerosols. If the novel coronavirus can float in the air as a vapour, earlier assumptions of its range are inadequate. Airborne transmission is still not conclusively proved, but a growing number of experts see persuasive evidence in super-spreading events that have transmitted the virus to people several feet away from the infection source.
Specific airflow patterns, and not just average ventilation and air changes, within buildings are also important in determining risk of exposure and transmission. A case report from an outbreak at a restaurant in China described 10 people within three families infected over one hour, at distances of up to 4.6m and without direct physical contact. The pattern of transmission was consistent with the transient indoor localised ventilation airflow pattern. Few studies have examined how airflow patterns influence viral transmission; most studies report (if anything) only average indoor ventilation rates. Neglecting variation in localised air flow within a space oversimplifies and underestimates risk modelling. In homogeneous flow, patterns are known to emerge in occupied indoor spaces that depend on air conditioning, ventilation system or location, occupancy of the space, air recirculation, and filtration.
The study suggests that physical distancing should be seen as only one part of a wider public health approach to containing the Covid-19 pandemic. It needs to be implemented alongside combined strategies of people-air-surface-space management, including hand hygiene, cleaning, occupancy and indoor space and air managements, and appropriate protective equipment, such as masks, for the setting.
from Gulf Times https://ift.tt/3hKLuJ7
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