This is true by definition, but misleading. What this statement gets wrong is that even masks that aren’t professionally fitted do an incredible job. For example, researchers ran fit tests with a mask that they had not previously fit tested on 22 volunteers. They found a median fit score of 99.3%! So basically, if you take one of these masks off the shelf, chances are it’s doing a fantastic job.
Click here to read more myths of the tiny particles: https://smartairfilters.com/en/blog/npr-got-masks-facts-wrong/