Mask Facts - Published by the AAPS - Association of American Physicians and Surgeons, curated by Marilyn M. Singleton, M.D., J.D.

https://aapsonline.org/mask-facts/

A COVID-19 (SARS-CoV-2) particle is 0.125 micrometers (μm);
Influenza virus size is 0.08 – 0.12 μm;
A human hair is about 150 μm

*1 nm = 0.001 micron; 1000 nm = 1 micron; Micrometer (μm) is the preferred name for micron (an older term)

N95 – A properly fitted N95 will block 95% of tiny air particles down to 0.3 μm from reaching the wearer’s face.

The term fine particles, or particulate matter 2.5 (PM2.5), refers to tiny particles or droplets in the air that are two and one half microns or less in width.