The primary filter, also known as the "Primary Filter" or "Pre-filter" in English, serves as the first line of defense in an air filtration system. It is mainly used to capture larger particles such as dust, hair, and pollen, thereby protecting the subsequent medium and high-efficiency filters and extending their service life. The primary filter is also referred to as a preliminary filter or a new air filter. The filter material often uses high-quality polyester synthetic fiber PET (non-woven primary filter cotton), and the frame is commonly made of moisture-proof and water-resistant raw pulp wood fiber cardboard. The efficiency of the primary filter usually reaches a certain percentage in filtering dust particles larger than 5 microns, such as the G4 level primary filter which can achieve 95% efficiency. In addition, the structure of the primary filter is generally divided into dragon skeleton and frame skeleton. The dragon skeleton is characterized by its strength, even distribution of filter materials, but it is time-consuming and labor-intensive to produce. The frame skeleton is characterized by convenient filter material replacement, time-saving in production, but relatively weaker in strength. Primary filters are widely used in pre-filtration of central air conditioning and centralized ventilation systems, pre-filtration of large air compressors, clean air return systems, and pre-filtration of local high-efficiency filtration devices.