I know that surgical masks [should ideally not be re-used too often](https://www.huffpost.com/entry/reuse-face-mask-coronavirus_l_5e78dbf9c5b63c3b6494ad80) (one figure I heard is that they should be disposed of after eight cumulative hours of use) – my question is, why is that exactly?
I get why this would be the case with something like a World War I-era gas mask, where there is an active filtering process going on and those filters can become saturated. But if I understand correctly, a surgical mask is a purely passive thing, and relies only on physically catching (most of) the droplets you exhale in the material. What exactly, if anything, makes the mask more permeable to those droplets after wearing it for a while?
Note that this is distinct from the answers to [this question](https://www.huffpost.com/entry/reuse-face-mask-coronavirus_l_5e78dbf9c5b63c3b6494ad80) – obviously, any kind of cloth face mask will become ineffective (and eventually rot away to nothing) over a period of years or decades, depending on storage conditions. I’m specifically asking about how a mask can become ineffective through *use* while still well *within* its shelf life.
(Related searches: [1](https://www.google.com/search?hl=en&q=site%3Awww.reddit.com%2Fr%2Fexplainlikeimfive%20surgical%20mask%20expiration), [2](https://www.google.com/search?hl=en&q=site%3Awww.reddit.com%2Fr%2Fexplainlikeimfive%20surgical%20mask%20limited%20uses) & [3](https://www.google.com/search?hl=en&q=site%3Awww.reddit.com%2Fr%2Fexplainlikeimfive%20surgical%20mask%20saturation))
In: Chemistry
They’re a barrier and help block particle and water inhalation/exhalation.
After enough time they will become saturated, and the particles and water will be breathed in *from* the mask itself.
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