The heading explicitly excludes protective masks that have neither mechanical parts nor replaceable filters — disposable filtering facepiece respirators and basic dust masks fall into that excluded category and are classified elsewhere. Gas masks and breathing appliances here must incorporate mechanical components (valves, harnesses, demand regulators) or use replaceable filter canisters. When a product sits on the boundary, the presence or absence of a replaceable filter element is typically the decisive criterion.

NIOSH certification is not a customs-clearance requirement per se, but it is a prerequisite for lawful sale as occupational respiratory protection under OSHA's respiratory protection standard. Importers of uncertified units intended for industrial use risk product seizure or marketplace restrictions post-entry. Buyers should confirm NIOSH approval numbers with the foreign manufacturer before placing orders, and importers should retain certification documentation in their records.