Making a worrying observation, scientists have said that there is evidence that the novel Coronavirus in smaller particles in the air can infect people and have written to the World Health Organization (WHO) to revise recommendations, the New York Times reported on Sunday.

In an open letter to the agency, which the researchers plan to publish in a scientific journal next week, 239 scientists in 32 countries outlined the evidence showing smaller particles can infect people, Reuters quoted the NYT as saying.

Whether carried by large droplets that zoom through the air after a sneeze, or by much smaller exhaled droplets that may glide the length of a room, the coronavirus is borne through air and can infect people when inhaled, the scientists said.

However, the WHO said the evidence for the virus being airborne was not convincing, according to the media report.

“Especially in the last couple of months, we have been stating several times that we consider airborne transmission as possible but certainly not supported by solid or even clear evidence,” Dr. Benedetta Allegranzi, the WHO’s technical lead of infection prevention and control, was quoted as saying by the NYT.

So far, the World Health Organization (WHO) has been more cautious on the airborne threat of COVID-19 coronavirus.

The WHO, in a publication in March, had maintained that the novel Coronavirus is primarily transmitted through “respiratory droplets and close contacts” and does not seem to stay long in the air.

Respiratory infections can be transmitted through droplets of different sizes, according to the WHO, which recommended social distancing as droplet transmission occurs when a person has close contact (within one metre) with another person who has respiratory symptoms such as coughing or sneezing, which may spread these potentially infectious droplets, typically 5-10 microns in size, to your body.

Airborne transmission is different from droplet transmission, as it refers to the presence of microbes within droplet nuclei, which are generally considered to be the smaller particles of less than 5 microns in diameter, and which can remain in the air for long periods of time and be transmitted to others over distances greater than one metre, it said.

However, the US health authorities had adopted a more cautious line and urged people to cover their faces when out in public in case the virus can be transmitted through normal breathing and speaking.

Also, different publications came up claiming that the droplets of the virus can remain airborne for several hours, unlike cough or sneeze droplets that fall to the ground within seconds.

Aerosolization of the coronavirus is a contentious area for scientists who study it, because it is not clear how infectious the disease is in the tiny quantities found in ultrafine mist.

In June, a latest study, which assessed the progression of the pandemic in three major epicentres across the world, said airborne transmission of the novel Coronavirus is highly virulent, and could be the dominant route for the spread of COVID-19.

Scientists, including Mario J Molina — the recipient of the 1995 Nobel Prize in Chemistry — assessed the transmission pathways of COVID-19 by analysing the trend and mitigation measures used in the three epicentres of the disease — Wuhan in China, New York City in the US, and Italy.

The researchers, including those from the University of California San Diego in the US, expressed concern that the World Health Organisation (WHO) for a long time only emphasised the prevention of contact transmission, and largely ignored the importance of the airborne transmission route for the novel coronavirus.

Based on the study, published in the journal PNAS, they said airborne transmission, “particularly via nascent aerosols,” is highly virulent and represents the dominant route for the transmission of this disease.

According to the findings, “the difference with and without mandated face covering represents the determinant in shaping the trends of the pandemic.”

However, days later, more than 40 scientists signed an open letter calling for the retraction of the study which made “extraordinary claims” that airborne transmission could be the dominant mode of spread of COVID-19.

Scientists including Noah Haber from Stanford University in the US, said the PNAS study had methodological design flaws and made “easily falsifiable claims.”

Meanwhile, the NYT report comes as the overall number of global Coronavirus cases has increased to more than 11.4 million, while the deaths have soared to over 533,000, according to the Johns Hopkins University.

On Monday morning, the total number of cases stood at 11,409,805, while the fatalities rose to 533,684, the University’s Center for Systems Science and Engineering (CSSE) revealed in its latest update.

The US accounts for the world’s highest number of infections and fatalities with 2,880,130 and 129,906, respectively, according to the CSSE.

Brazil came in the second place with 1,603,055 infections and 64,867 deaths. In terms of cases, India ranks third (697,887) and is followed by Russia (680,283).

(With agency inputs)