On Thursday the World Health Organization (WHO) recognized that aerosolized droplets can lead to infection. According to the scientific brief "Transmission of SARS-CoV-2 can occur through direct, indirect, or close contact with infected people through infected secretions such as saliva and respiratory secretions or their respiratory droplets, which are expelled when an infected person coughs, sneezes, talks or sings."

The WHO says that airborne transmission can occur during medical procedures that generate aerosols and has been actively evaluating whether the virus may also spread through aerosols in indoor settings with poor ventilation.

Dr. Julian W. Tang, Associate Professor at the University of Leicester, United Kingdom, encourages the WHO to go a step further with its recommendations. "You cannot separate out droplet and fine aerosol emissions in everyday activities like talking, breathing and laughing.” Many scientists believe droplets and aerosols are on a continuum of sizes. “So if they accept that droplet transmission is happening they cannot exclude any contribution from aerosols."

On Monday, 239 medical experts wrote a letter to the WHO urging the body to react to the mounting evidence that aerosolized droplets floating in the air can cause infection. The WHO’s previous guidelines state that only larger droplets expelled from coughs and sneezes that fall to the floor can spread the disease, but Dr. Robert Glatter, emergency physician at Lenox Hill Hospital in New York, says that could be putting lives at risk.

"(The WHO) believe that aerosol transmission is not a significant contributor to infection,” said Glatter. “They have been very slow to acknowledge the potential for aerosol transmission, which in my mind can risk lives when crafting global public health messaging.”

“Ventilation is the key control point for an airborne virus,” says Dr Julian W Tang, one of the authors of the paper that lead to the change. “Based on multiple studies done by the authors, we believe that optimized ventilation is the way to move forward, removing the virus from the air before people inhale it. We think that’s one of the main ways it’s transmitted.”

Here's a story we published in March on how it's believed to move in a turbulent gas cloud:

A study published in the Journal of the American Medical Association found that under the right conditions, liquid droplets from sneezes, coughs and just exhaling can travel more than 26 feet and linger in the air for minutes.

“There is no virtual wall at this 3- to 6-feet distance” says Lydia Bourouiba, the study's author, who specializes in fluid dynamics and is an associate professor at the Massachusetts Institute of Technology. These findings suggest the greatest risk is for health care workers working with infected patients, she says.

As seen in this video, shot from different views and posted with Bourouiba's report, the invisible cloud can travel up to 26 feet:

The study focuses on a turbulent gas, the cloud emitted when someone coughs, sneezes or exhales. Liquid droplets of various sizes drop onto surfaces, while others can be trapped in a cloud that can swirl around a room with a payload, in theory, of pathogen-bearing droplets.

How cough and sneeze droplets travel

A lot goes into how far the cloud and its droplets travel: a person's physiology, the environment, humidity and temperature. “The cloud can reach up to 26 feet for sneezes and less than that for coughs — about 16 to 19 feet,” Bourouiba says.

According to a 2009 World Health Organization report, when someone coughs, they can spray up to 3,000 droplets. A sneeze could yield 40,000.

Virus-filled droplets smaller than a human hair

Bourouiba’s study did not look at gas clouds of patients infected with the SARS-CoV-2 virus but it hypothesized: "The rapid international spread of COVID-19 suggests that using arbitrary droplet size cutoffs may not accurately reflect what actually occurs with respiratory emissions" and lead to inadequate recommendations and more sick people.

These droplets can be very small — “ as small and invisible as the micron size to the ones that you can see that are on the order of the millimeter” says Bourouiba. A human hair is 60 to 120 microns thick.

Aerosols: The smallest droplets may carry COVID-19

Researchers don’t know how many virus-laden particles people infected with COVID-19 might expel in the average droplet, including the micron-size droplets — called aerosols — that linger in the air.

"Aerosols are different," says Dr. Stanley Deresinski, clinical professor of medicine and infectious diseases at Stanford University. "Very small particles may be suspended in the air for a long time, sometimes for hours. They're suspended by air currents."

Those floating airborne droplets — some shielded by turbulent gas clouds — can stay suspended long enough for someone to walk through and inhale the virus. Inside the gas cloud “the lifetime of a droplet could be considerably extended by a factor of up to 1000, from a fraction of a second to minutes,” Bourouiba's study says.

Droplets containing virus reach air circulation systems

Pathogens in the cloud could potentially reach air circulation systems inside buildings, says Bourouiba. “There was sampling done in air vents with positive detection of the virus.”

A separate JAMA Network study found that exhaust outlets tested positive for SARS-CoV-2: “small virus-laden droplets may be displaced by airflows and deposited on equipment such as vents,” the study says.

“Now, there are other questions about whether the detected virus particles are still live," says Bourouiba. "However, finding the virus in air vents is more compatible with that longer distance range that can be reached through the cloud.”

Fresh air can help rid coronavirus droplets

These findings heighten the dangers for those caring for COVID-19 patients. Without sufficient air circulation to disperse the cloud, its concentrated payload of droplets can linger in hospitals and homes.

“Drops are trapped in the cloud for quite some time and they can remain locally concentrated,” says Bourouiba.

The best defenses are the outdoors and open windows which dissipate the clouds or droplets.

“When one is outside, with air circulation or wind, the cloud and its payload is easily dispersed and less concentrated. Making sure that indoor spaces are aired frequently also reduces the concentration,” Bourouiba says.

Masks protect against floating droplets

Surgical masks are helpful at blocking large droplets, but unlike respirators they do not provide a reliable level of protection from inhaling smaller airborne particles, according to the CDC.

N95 respirators are tight-fitting and filter out at least 95% of airborne particles as small as 0.3 microns. They have a protection factor (APF) of 10, according to the CDC. That means the N95 reduces the aerosol concentration to 1/10 of that in the room — or blocking 90% of airborne particles.

An elastomeric respirator is a reusable device with exchangeable cartridge filters. It fits tight against the user's face and also has a APF of 10. Before reusing the mask, all its surfaces need to be wiped down with a disinfectant.

In addition to respirators, health care workers should wear personal protective equipment (PPE) to help limit exposure to the virus through their eyes or contaminated clothing.

Should you make a homemade face mask?

The CDC now recommends Americans wear cloth face coverings in public settings where it's difficult to stay at least 6 feet apart, such as grocery stores and pharmacies — especially in areas with high levels of community transmission.

When asked about the CDC recommendations for people the general public to wear masks or other improvised devices, Bourouiba offered a qualified response:

“The efficacy of those homemade solutions need to be quantified. Exhalations or violent exhalations such as coughs or sneezes would be deflected to the sides of these masks — as they are not perfectly sealed....

"It is important, therefore, to understand that such masks are not necessarily protective for the wearer in terms of preventing inhalation of the residual droplets in the air, which enter from the sides unfiltered, but they can provide a way to reduce the range of contamination from the droplets-laden cloud.”

The CDC press release says that cloth face coverings fashioned from household items or made at home from common materials at low cost can be used as an additional, voluntary public health measure. They recommend that critical supplies such as surgical masks or N-95 respirators continue to be reserved for health care workers and other medical first responders. Top trends on Google Friday were face mask patterns and bandanas.

The start of allergy season could also hasten new infections. Bourouiba warned that asymptomatic carriers could spread COVID-19 through any sneezing and coughing triggered by allergies.

Contributing: George Petras and Jim Sergent