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This is commonly stated, but untrue. Here are a couple of studies on this topic. [1] [2] And the always critical third link. [3]

The nature study carried out specific tests and indeed surgical masks did surprisingly very well at blocking particulate matter all the way down to 1 micrometer, with a significant effect on much smaller particles as well. PM2.5 = 2.5 micrometer / 2500 nanometers. The NIH study compared the health effects of individuals with/without masks in everyday activity in Beijing.

I think the assumption is that if the particulate matter is smaller than the size of the 'gaps' within a mask then it would be ineffective. I always just visualize a simple experiment. Imagine shooting a stream of particles of a given size at a wall. Now interject a 'net' of practically any size. Now interject multiple 'nets' (as is the case in typical surgical masks). It becomes clear that even quite large large filters will have a non-zero effect on stopping matter far smaller than the max size that can make it through.

[1] - https://www.nature.com/articles/jes201642

[2] - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2662779/

[3] - https://sci-hub.tw/

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