N95 masks are the US standards for respirator masks; KN95 masks are the Chinese standards for masks.
More than likely, you are most concerned about how well the mask filters out contaminants. Both N95 and KN95 masks are rated to capture 95% of 0.3 micron particles, so they are equal on this concern. In fact, N95 and KN95 mask specifications are so nearly equivalent that most mask users would be quite bored making the comparison.
The 3 differences we think are most interesting are:
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To be certified as a KN95 mask, the Chinese government requires the manufacturer to run mask fit tests on real humans with ≤ 8% leakage. The N95 mask standard does not require manufacturers to run fit tests. It should be noted that many US hospitals require fit tests on their employees.
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N95 masks have slightly stricter requirements for pressure drop while inhaling. N95 is ≤343 Pa versus KN95 is ≤ 350 Pa.
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N95 masks have slightly stricter requirements for pressure drop while exhaling. N95 is ≤ 245 Pa versus KN95 is ≤ 250 Pa.