1 . COMMON CUSTOMS TARIFF - TARIFF HEADINGS - INTERPRETATION - EXPLANATORY NOTES OF THE CUSTOMS COOPERATION COUNCIL - OPINIONS OF THE COMMITTEE ON COMMON CUSTOMS TARIFF NOMENCLATURE - AUTHORITY - LIMITS 2 . COMMON CUSTOMS TARIFF - TARIFF HEADINGS - ' ' BREATHING APPLIANCES ' ' WITHIN THE MEANING OF HEADING 90.18 - CONCEPT - FILTER MASKS - INCLUSION