The United States Centers for Disease Control and Prevention has updated its guidelines on the coronavirus to say that the infection can spread through aerosols, which are produced even when a person breathes.

“Through respiratory droplets or small particles, such as those in aerosols, produced when an infected person coughs, sneezes, sings, talks, or breathes,” the health institute said. “These particles can be inhaled into the nose, mouth, airways, and lungs and cause infection. This is thought to be the main way the virus spreads.”

Earlier, the CDC website, which was updated on Friday, said that coronavirus spreads mainly when in close contact – about 6 feet – and through respiratory droplets when a person with the virus “coughs, sneezes or talks”, according to CNN.

The health institute also said that there was “growing evidence” that droplets and airborne particles can remain suspended in the air and can travel beyond six feet. “In general, indoor environments without good ventilation increase this risk,” it said.

Airborne viruses are highly contagious, the CDC said, and added that the coronavirus can spread faster than influenza but slower than measles. “In general, the more closely a person with Covid-19 interacts with others and the longer that interaction, the higher the risk of Covid-19 spread,” it said.

Previously, the CDC had suggested maintaining a physical distance of six feet, disinfecting surfaces, regularly washing hands, and using masks as ways to curb the spread of the virus and prevent one from getting infected.

Now, the website says to stay at least six feet away from others whenever possible, while also maintaining social connections and taking care of their mental health. “The best way to prevent illness is to avoid being exposed to this virus,” it adds.

The CDC also changed its language on asymptomatic transmission. Earlier, the website said “people without symptoms may be able to spread the virus,” to now saying “people who are infected but do not show symptoms can spread the virus to others.”

Various scientists had earlier spoken about the viral transmission of the coronavirus. In July, 239 scientists wrote to the World Health Organization and other public health institutes, urging them to be more open about the possibility that people could get infected by droplets that were floating in the air.

“Studies by the signatories and other scientists have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure at distances beyond 1–2 m from an infected individual,” the scientists said.

After the scientists wrote to the global health body, it released a report on July 9 on airborne transmission and the spread of the virus.

“Some medical procedures can produce very small droplets [called aerosolised droplet nuclei or aerosols] that are able to stay suspended in the air for longer periods of time,” the WHO website says. “When such medical procedures are conducted on people infected with Covid-19 in health facilities, these aerosols can contain the Covid-19 virus.”

“There have been reported outbreaks of Covid-19 in some closed settings, such as restaurants, nightclubs, places of worship or places of work where people may be shouting, talking, or singing,” the global health body says. “In these outbreaks, aerosol transmission, particularly in these indoor locations where there are crowded and inadequately ventilated spaces where infected persons spend long periods of time with others, cannot be ruled out.”

Donald Milton, a professor of environmental health who studies how viruses spread and one of the 239 scientists that wrote the letter, said the CDC’s update was a major improvement and that he was encourage to see the health institute paying attention.

Globally, the coronavirus has infected more than 3.09 crore people and killed 9,59,295, according to Johns Hopkins University. The number of worldwide recoveries is more than 2.1 crore.