The CDC issued new advice saying cloth masks protect the wearer from COVID-19 infection. It's not that simple.

• The CDC released new guidance on mask wearing Tuesday, suggesting cloth masks may provide a degree of "personal protection" to the wearer, by reducing how much coronavirus they inhale.
• Previously, the agency said masks are primarily worn to avoid spreading the virus away from infected people (who don't always show symptoms).
• Universal masking has helped prevent the spread of the coronavirus, but scientists are still hotly debating how much wearing masks really helps healthy people if infected people don't have them on too.
• One public health expert suggested on Twitter that the new individual-focused phrasing "is notable for what it says about our national character."

Wearing a mask is a simple, effective step we can all take right now to prevent the coronavirus from spreading.

For many months now, we've been told this is largely because wearing face masks helps people keep their germs to themselves, and avoid spreading the respiratory disease to new hosts.

But in new guidance posted to its website on Tuesday, the US Centers for Disease Control and Prevention (CDC) went a step further, in suggesting that wearing a mask "can also reduce wearers' exposure to infectious droplets through filtration."

In other words, the CDC now says wearing a mask isn't just about keeping sick people's germs away from healthy folks, it may also directly help protect each and every mask-wearer, by preventing them from picking up a new infection, too.

The agency even said that the combination of source control (from sick people) and "personal protection for the mask wearer" from everyone in a community wearing masks "is likely complementary, and possibly synergistic."

But health professionals, personal protection experts, and mask researchers have all stressed for months now that it's not so simple, and that the benefits of mask-wearing are generally greatest when infected people (including the ones who show no signs of illness) put them on.

The CDC cited studies — with caveats

This is quite a departure from what the CDC suggested in the early days of the pandemic, when the agency stressed that masks need only be worn by infected people, and the healthcare workers and others caring for them, in order to help "keep respiratory droplets contained."

In its new scientific brief, the CDC cites several recent studies, which all suggest that where more masks are worn, coronavirus infection risks are reduced.

But the agency also cautions that these studies are all based on observations made outside the laboratory, and often done retroactively. As such, it's impossible to really know how much of the benefits of mask wearing come from blocking virus transmission at the source, and how much of the protection comes from shielding new people from getting hit.

It may be true that heavy-duty respirators keep people healthy, but experts are not sure this benefit applies to cloth masks

There is better evidence that surgical masks (with their electrostatically charged fibers built to trap incoming dirt and viruses) and more heavy-duty full-seal face mask models (like N95s) can both prevent infectious particles from entering a person's breathing space.

Epidemiologist Saskia Popescu, a leading infection prevention and control expert, said on Twitter Wednesday morning that while it's true fabric masks can offer "some variable protection" to the person who's wearing them, there might be another big reason why the CDC is stressing these yet-untested benefits.

"The bigger issue is that the CDC has to reiterate this to get people to wear a mask," Popescu said in her tweet. "Meaning that we've had to move beyond source control, but now personal gain to get [people] to mask up."

Virologist Angela Rasmussen from Columbia University agreed.

"This is notable for what it says about our national character," she tweeted, adding "at the end of the day, I hope this encourages people to wear masks. I just wish we didn't have to encourage people to care for their communities by appealing to individual interests."