These filter papers are the fastest Qualitative Grade. ASTM initial filtration speed is 12 seconds. Coarse particles and gelatinous precipitates are retained. Retention rating is 20-25 µm. Offers good gas filtration performance. Used in removal of dust in air, and air pollution analysis. 0.6 mg ash content per 12.5 cm circle. Surface is normally hard, and smooth. (For cost savings, see Ahlstrom equivalent, 446-300-40 Series)