The US Centres for Disease Control and Prevention (CDC) updated its coronavirus guidance on Monday to say that the virus can spread from particles that can linger in the air for hours.
The CDC stressed that close contact with an infected person remains the most common way the SARS-CoV-2 virus that causes COVID-19 spreads but that it can also "sometimes" be spread by airborne transmission.
"Some infections can be spread by exposure to virus in small droplets and particles that can linger in the air for minutes or hours. These viruses may be able to infect people who are further than 6 feet away from the person who is infected or after that person has left the space," it said in its updated guidance.
Scientists have been calling on authorities to address the potential for airborne transmission for months.
In July, 239 experts from 32 countries wrote an open letter published in an the British Medical Journal that airborne transmission of COVID-19 was "being underestimated". The World Health Organisation subsequently said that airborne transmission in crowded and poorly ventilated areas "cannot be ruled out.
The CDC's update guidance came on the same day scientists from several US universities including the Harvard School of Public Health wrote in an open letter released in Science that "there is overwhelming evidence that inhalation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) represents a major transmission route for coronavirus disease 2019 (COVID-19)."
They flagged that although viruses in droplets typically fall to the grounds in seconds within two metres of the source — which makes physical distancing efficient in curbing the spread of the virus —, viruses in aerosols can remain suspended in the air "for many seconds to hours, like smoke, and be inhaled."
"Aerosols containing infectious virus can also travel more than 2 m and accumulate in poorly ventilated indoor air, leading to superspreading events," they said.
"There is an urgent need to harmonise discussions about modes of virus transmission across disciplines to ensure the most effective control strategies and provide clear and consistent guidance to the public," they added.
The CDC advises people to stay at least 6 feet away from others whenever possible, to cover their mouth and nose with a mask when around others, to avoid crowded indoor spaces and ensure indoor spaces are properly ventilated by bringing in outdoor air as much as possible.
More than one million people have died from COVID-19 globally, according to Johns Hopkins University, and a further 35.5 million have been infected with the virus.