Whatman Grade 4 filter paper has extremely fast filtering (20-25 µm) with excellent retention of coarse particles and gelatinous precipitates such as ferric hydroxide and aluminum hydroxide. Very useful as a rapid filter for routine cleanup of biological fluids or organic extracts during analysis. Used when high flow rates in air pollution monitoring are required and the collection of fine particles is not critical. Used in air pollution analysis. 100 circles/box.