Pre-filters are used for panel filters in high dust conditions or when long exploitation period is required. Prefilters are additional filter media made of fabric and glued to the top of panel pleats. Prefilters are designed for preliminary air filtration. These filters are to trap bigger contaminants fractions allowing the main filter to remain unclogged. The application of prefilters significantly improve absorptivity, which means the ability of storing contaminants in the filter without exceeding the limit of air flow resistance.