(CNN) The US Centers for Disease Control and Prevention on Friday updated its explanations on how coronavirus is transmitted, stressing that inhalation is one of the main ways the virus is spread and placing less emphasis on the risk of picking it up from surfaces.
The changes to the CDC website are in line with a gradual shift in the agency's advice to stress that the virus spreads through the air -- either to be breathed in, to get into the eyes or to settle onto surfaces that people later touch.
"COVID-19 spreads when an infected person breathes out droplets and very small particles that contain the virus. These droplets and particles can be breathed in by other people or land on their eyes, noses, or mouth. In some circumstances, they may contaminate surfaces they touch. People who are closer than 6 feet from the infected person are most likely to get infected," the agency says on its updated website.
"How people get this virus can be boiled down to three very simple concepts," Dr. John Brooks, chief medical officer for the CDC's Covid-19 response, told CNN.
"Keep the air clean, avoid getting exposed on your mucus membranes and keep your hands clean."
People more commonly catch the virus when they are standing close to someone who is infected and particles fly out to either land on the face or to be breathed in, the new explainer stresses. Less commonly, people catch the virus from breathing air contaminated by people who are further away, or by touching a contaminated surface and then touching their own eyes, nose or mouth.
"If you and I are standing within a few feet of each other talking, we now know infectious particles are flying out, even if you are talking softly," Brooks said.
The new explanations do not change what people need to do, but might help the public better understand how the virus spreads, Brooks said. Guidance remains the same -- wear a mask when near other people or inside and sharing air with others, keep a distance from others when possible and wash hands frequently. And get vaccinated.
The CDC also updated its scientific brief on how the virus spreads.
"Modes of SARS-CoV-2 transmission are now categorized as inhalation of virus, deposition of virus on exposed mucous membranes, and touching mucous membranes with soiled hands contaminated with virus," the new guidance reads."The principal mode by which people are infected with SARS-CoV-2 (the virus that causes COVID-19) is through exposure to respiratory fluids carrying infectious virus," it adds.
CDC added to its science brief on mask use to counter fears about the safety of wearing masks. "Research supports that mask wearing has no significant adverse health effects for wearers," it said.
Masks don't just filter the air, Brooks pointed out. "Wearing a mask covers your mucus membranes. It is more difficult to touch your mouth when a mask is over it," he noted.
Scientists who had been lobbying for the changes had both praise and criticism.
"I'm really happy with a whole lot of this stuff. I think it's an important and major step forward," Dr. Donald Milton, who studies how viruses are transmitted at the University of Maryland, told CNN.
But Milton signed a letter along with six other experts on aerosols to say the CDC needs to do and say more.
"However, we are concerned that CDC's accompanying document, How COVID-19 Spreads, is misleading, and potentially harmful," they wrote.
"In that document CDC says that breathing in small droplets and particles (i.e., aerosols) that contain the virus when people are far apart or have been in the same enclosed space for more than a few minutes is UNCOMMON (our emphasis) This will lead people to continue to think that maintaining distance is sufficient to prevent transmission."
It's not, they said.
"We know that transmission at distances beyond 6 feet occurs because of superspreader events, careful studies of smaller outbreaks, and the physics of aerosols. It can easily happen indoors in a poorly ventilated environment, when people are not wearing masks."
They want CDC to push for better ventilation in places such as meatpacking facilities where air is recirculated, and to emphasize the importance of face masks known as respirators, including N95 respirators, in places where people are forced to breathe recycled air.
"If you are working in a meatpacking plant where they recirculate the air because they have got to keep it cold, and you are elbow to elbow with other people, you need that better respirator to protect you," Milton said.
Meat and poultry processing facilities have been sites for several Covid-19 superspreading events and workers in those industries have been especially likely to become infected.
"They are just putting up Plexiglass shields, which can make it worse," Milton complained. He and other aerosol experts have said plastic shields can help concentrate potentially infected air and do little to protect people from the virus, other than stopping large droplets from nearby coughs, sneezes and talking.
I don't disagree," Brooks said in response. "This is not new guidance. This is the beginning of how, perhaps, guidance will begin to evolve," he added.
"I would stay tuned. This is an issue we are concerned about."