CHENNAI: With Covid cases figures rising steadily, and the World Health Organisation (WHO) earlier this month acknowledging the possibility of contagion through short range aerosol transmission, mindful people who up to this point followed all possible guidelines to ward off the virus, are in a quandary. How do you find if the air you breathe is clean?
Consultant pulmonologist at Kauvery Hospital, Dr Anantha Subramanian strongly suspects some of his patients contracted the infection through airborne transmission. “Most commonly, the virus transmits through droplets. But in some patients, it does not work. For instance, many frontline workers who took all precautions contracted it,” he said.
A senior government hospital doctor, on condition of anonymity, said airborne transmission is the only explanation why some frontline workers wearing complete protective equipment ended up infected. “The release of viral-loaded aerosols is very common in a patient who is put on or weaned off a ventilator. Coughs resulting from high pressure formation in lungs also lead to this.” The WHO, referring to several studies, observed that some outbreaks in crowded indoor spaces suggest the possibility of aerosol transmission. “Such situations have arisen during choir practices, in restaurants or in fitness classes,” the WHO had said. However, it was quick to add the caveat that airborne transmission is although “combined with droplet transmission”.
Droplet and aerosol transmission
To understand the conundrum better, let us visualise droplets as big drops and aerosol as small drops. Droplets coming from a sick person’s mouth or nose fall to the ground before they can get much farther than six feet; reason why the government insists on people to maintain over 1.5 metres distance socially. But aerosols are lighter and can travel long distances before landing.
Assistant professor of Physics at Indian Institute of Technology-Madras Sivarama Krishnan employs another example. “Imagine aerosol as a drunk person walking in a poorly-lit street at night. He takes uncoordinated steps, often back and forth, before finally converging slowly to a bench or a lamp-post. The smaller the particle the more intoxicated it is and more stochastic (unpredictable) its travel,” he said.
In a poorly ventilated room, aerosol particles circulate continuously until they find their ‘lamp-post’, which could be a wall, furniture or a person’s body, Krishnan added. The life time of a virus that lands on a surface could be anywhere between a few minutes to a few hours. A study published in the journal ‘Nature’ about two hospitals in Wuhan said, the most infected area in the hospital was a 1-meter-square mobile toilet in the patient area.
Senior epidemiologist and member of the State Special Task Force Against Covid-19 Dr P Kuganantham said, “I have suspected several cases of airborne transmission. The risk is the highest in AC rooms, particularly cabins or cage-like rooms. Which is why I am strongly against the opening of malls or cinemas. One asymptomatic person is enough to infect all people in an AC room.”
Open the windows and wear your masks
Spacious well-ventilated rooms, on the other hand, pose lower transmission risk. So, an outdoor vegetable market will be safer than a supermarket, which in-turn will be safer than a small departmental store; assuming people follow social distancing. So if you are indoors, open windows to ensure maximum ventilation.
Wearing masks too encumbers the aerosols’ long journey, states a study titled ‘Visualizing the effectiveness of face masks in obstructing respiratory jets’ by Siddhartha Verma and his team at Florida Atlantic University. The study suggests that current social-distancing guidelines may need to be updated to account for aerosol-based transmission.