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Because KN95 masks aren’t overseen and vetted by NIOSH, they aren’t considered as effective as the U.S.-certified N95 masks.

Since KN95 aren’t actually certified by NIOSH, they usually come at a lower financial cost than high-grade N95 masks.

Other authoritative bodies like the CDC have repeated the myth, and despite changing their warnings to reflect the fact that there is no evidence this can happen or has ever happened, the agency still maintains a rather misleading guide under NIOSH’s website, which largely deals in hypothetical situations that are very unlikely to ever happen.

Many children can fit in the so-called “NIOSH grid,” the profile of face shapes and face sizes that is used to develop respirators.

Emily Haas, a scientist at the C.D.C.’s National Institute for Occupational Safety and Health, or NIOSH, said researchers were still grappling with the need to regularly disinfect them and complaints about muffled communication, though some newer models make it easier for wearers to be heard.