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Study:
https://www.nature.com/articles/s41598-022-15409-x

Summary below from:
https://www.coffeeandcovid.com/p/-coffee-and-covid-tuesday-july-19

The researchers surveyed 109 medical-school volunteers on their mask usage and lifestyles, and then cultured bacteria and fungi from the face-side and the outer-side of their masks. They found bacteria and fungi growing on the masks.

Bacterial colonies were found on 99% of the samples on the face-side, and 94% on the outer-side. The researchers found fungal colonies growing on 79% of the samples on the face-side, and 95% on the outer-side. They also found that longer mask usage significantly increased the fungal colony figures (but not the bacterial colony numbers).

Although most of the bacteria were non-pathogenic, they found several potentially pathogenic bugs: S. aureus (commensal bacterium; its overgrowth can cause various diseases); B. cereus (intestinal bacterium, causing food poisoning); Staphylococcus saprophyticus (urinary tract infection); and Pseudomonas luteola (an opportunistic pathogen).

The conclusion? The authors suggested “that immunocompromised people should avoid repeated use of masks to prevent microbial infection.”