Andersen Cascade Impactors draw air through six stacked stages at 28.3 L/min, each stage holding an agar plate beneath a perforated plate of 400 nozzles. As air speeds up stage by stage, particles deposit by size, from above 7 um on the first stage down to 0.65 um on the sixth, mirroring where they would settle in the human respiratory tract. Colonies grown from each plate give both a viable count and a size distribution of airborne bacteria and fungi. Stainless steel construction withstands autoclaving between runs. Typical uses include bioaerosol monitoring in cleanrooms, hospitals and food plants, inhaler testing for fine particle fraction and MMAD, and filter efficiency studies.