CDC homemade respiratory mask, published in the H5N1 ("bird flu") days. Not sure how much relevancy it has now, but it's interesting. Handmade and reusable.
More:
https://wwwnc.cdc.gov/eid/article/12/6/05-1468-f1
https://wwwnc.cdc.gov/eid/article/12/6/05-1468_article
Previously:
Newspaper welding mask - video
Commercial pilot oxygen mask - GIF and video
Augmented reality mask for firefighter - GIF
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