The lungs possess several mechanisms for cleansing themselves from locally produced biological debris and inhaled, deposited soluble and insoluble materials. Insoluble particles deposited on the conducting, ciliated airways will be primarily removed by mucociliary clearance, over a period of several hours, with cough acting as a back-up clearance mechanism. Insoluble particles deposited in the non-ciliated, alveolated regions of the lungs will be cleared primarily by the macrophages very slowly, with a biological half-life of several months. Soluble substances can pass through paracellular pathways and be cleared via the blood stream.