So the CDC changed its masking guidance yesterday to recommend tighter-fitting masks as well as double masks. But they specifically warned against layering KN95s, which surprised me given how common that combo has become

This is how they explained it when I followed-up:

KN95s and N95s work very well on their own and adding masks on top or underneath could "affect how well they fit the face and decrease their effectiveness" and also make them hard to breathe through and uncomfortable, officials say.

"Masks that are not comfortable may not be worn by people as long as they are needed. It's important for consumers to find a mask that fits well but that also is comfortable enough that they can wear it for prolonged periods, if needed," reads a CDC statement sent to me.

So it seems like the CDC is worried that layering these masks could reduce the tightness of the seal (I could imagine that if you put a surgical mask under an N95, for example) and also that the combo is too uncomfortable

Given the large numbers of people who seem to be comfortably wearing that combo of masks, I don't know how real that last concern is. But I guess the take-away is to be aware of whether you're somehow making your (K)N95 fit worse when you add extra masks to it

Also here's some context on what I'm talking about! https://twitter.com/skarlamangla/status/1359554176334766083?s=20

And the full response from the CDC