As mentioned here, healthcare professionals receive training on how to ensure their facemasks form a good seal to their face. The process isn't trivial and shouldn't be taken lightly. Here's what that training looks like.
First off, regulations require a fit test for size )using equipment laypeople don't have). All of the steps below are worthless if the user's face doesn't correspond to the mask size.
Prior to donning, masks are inspected for damage and wear.
The user ensures proper nose and chin coverage and proper strap placement, then conforms the metal nosepiece.
Once the mask is on, the user conducts a careful seal check, with both positive and negative pressure (i.e. the mask should bulge slightly during exhalation and collapse slightly during inhalation). The trained user can also detect leaks by sensing if air is flowing across their face outside of the mask during breathing.
Here's what it all looks like:
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There is currently a shortage of N95 masks, which are desperately needed by hospitals and healthcare professionals. If this video is more widely circulated, it will hopefully convince laypeople that buying up available masks is worthless without the training, and that the masks should be made available to professionals first. Healthcare workers are the front line in the fight against COVID-19, and we have to get beyond our fear and selfish impulses in order to support them.