Mind the gaps.

That’s the conclusion of a U.S. Centers for Disease Control and Prevention study released Feb. 10. Considering the exponential spread of much more transmissible viral variants, it’s one we should act on.

The study found that it’s not just about the mask. It’s about the fit of the mask, and a good fit can make a huge difference in how well masks cut down on and protect from infectious aerosols and slow the transmission of COVID-19.

While there are many ways to improve fit – from mask fitters to pantyhose-type elastic sleeves – the CDC research team focused on two options.

The first was double masking: that is, wearing a three-ply cotton-cloth mask over a surgical mask (also called a medical-procedure mask). (The team did not test double-masking with two cloth masks.)

The second was knotting and tucking the surgical mask to change its fit and close the gaps. This short video shows how that’s done.

The CDC study: Wearing a double mask

The CDC study happened in a lab. The key experimental tools included two rubber versions of human heads, a 10-foot by 10-foot by seven-foot sealable chamber, and a sort of humidifier that emitted potassium chloride particles of the sizes considered most complicit in the aerosol spread of the coronavirus (0.1 to 0.7 microns across). Those aerosols were pushed through the mouth of one of the rubber heads for 15 minutes at a time over several runs.

The team tested 10 scenarios, some with the aerosol-emitting rubber head unmasked, some with the receiving head unmasked, and some with both heads masked in different ways. They tested simulated coughs; they tested simulated breathing. Key findings:

1. The unknotted surgical mask alone blocked 42% of the particles from a simulated cough; the cloth mask alone blocked 44% of those particles.
2. A cloth mask over a surgical mask (a double mask) blocked 92% of coughed particles.
3. Double-masking the emitter cut the exposure of an unmasked receiver by 82%; knotting and tucking the emitter’s surgical mask reduced it by 63%.
4. Leaving the emitter unmasked but double-masking the receiver cut the receiver’s exposure by 83%; when the receiver wore a knotted-and-tucked surgical mask alone, aerosol exposure fell by 64%.
5. When both emitter and receiver were double-masked, aerosol exposure fell by about 96%; the same held for when emitter and receiver both wore knotted and tucked surgical masks.

Points 1 and 5 point to the conclusion that the material of the mask – three-ply cotton or three-ply surgical – is a lot less important than making sure whatever mask one wears fits right.

“Fit matters. Wear a mask that covers your mouth and nose and doesn’t have big, gaping holes at the sides,” said Dr. Michelle Barron, UCHealth’s top infectious-disease specialist.

Barron says that, while the CDC’s scientific approach looks solid, the perceived recommendation of double-masking – one emphasized by breaking stories in the Washington Post, the New York Times, Fox News, CNN, and others – could lead to confusion and, potentially, unwarranted concern among the public.

First things first: Wear a mask correctly, physical distance, wash hands

These were laboratory conditions, not real-world scenarios, she noted. In the real world, we’re taking – or should be taking – many steps to reduce risk: avoiding close contacts among those outside our immediate circle, skirting crowds and enclosed spaces, washing hands, and, yes, masking up. Barron likened this portfolio of preventative steps to the one that helps us avoid getting hurt in car accidents.

“You drive the speed limit, you wear a seat belt, and the car has airbags,” Barron said. “One of those alone is unlikely to be the only thing that works. That’s why there are multiple safety features in place.”

Further, it has been hard enough to get people to wear a single mask properly, much less to double up, she said.

“The risk is that people will worry about spreading the coronavirus because they’ve only been wearing one mask,” Barron said. “That’s not the issue. The issue isn’t that people aren’t wearing two masks – it’s that they aren’t wearing a mask at all, or wearing it under their nose or under their chin at a Super Bowl party with 20 other people.”

The CDC study’s highlighting the importance of proper mask fit is more important than its focus on double-masking and knotting and tucking, Barron said. Even cookie-cutter surgical masks fit different people differently, and whether it’s a cloth mask or a surgical mask, finding the right fit is important.

“Masks are like shoes – one shoe isn’t going to fit everybody,” she said.

If doubling up or knotting and tucking works for you, go for it, Barron said. But ultimately, we might be better served to focus on properly wearing one good mask that actually fits.