A doctor I know ordered masks claiming N95, KN95, or similar certification from 7 different Chinese companies and fit-tested them (a process involving putting the mask on and spraying ~0.3μm particles of a bitter-tasting compound into the air); 6 models failed for all people tested while the last one failed for some people (indicating adequate filtration but poor fit).
A doctor I know ordered masks claiming N95, KN95, or similar certification from 7 different Chinese companies and fit-tested them (a process involving putting the mask on and spraying ~0.3μm particles of a bitter-tasting compound into the air); 6 models failed for all people tested while the last one failed for some people (indicating adequate filtration but poor fit).
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For example, aliexpress is widely used by hobbyists for cheap electronics components like sensors and microcontrollers.