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.
As you say, the surgical masks (like the ones in your question back ground) work as a physical barrier and filter. Over time the mask will be accumulating filtered pathogens. The longer a mask is used the more chance there will be pathogens on it, and the more likely there will be a contact breach (for example the user touching mask).
In addition these masks are not water proof. The will become moist from droplet contact contact both from the outside and from the users breath. Moisture will affect the masks ability to act as a physical shield, more easy for pathogens to be spread and is uncomfortable to use and held for long term contact on the user’s skin.
Most infection control advice would be “up to 4-8 hours unless visibly soiled or moist”. The number is somewhat arbitrary but based on a risk analysis of the above factors….as well as economic considerations unfortunately. In a health care setting, masks should probably be changed between patients but few institutions would recommend this as it would drive PPE consumption through the roof.
Shelf life measures shelf life of the mask, not how long they can be used for. If you take it off the shelf and use it, the equation changes.
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