Inside the Human Body
The organs of the body are sealed against the outside by dense cell layers, the so-called epithelia. Particles cannot get into the body unless they overcome this barrier by e.g., disturbing the tight bond between the cells or by penetration into the cells. To achieve this, the particles must overcome the membranes that surround each body cell and seal it from the environment. This process is referred to as endocytosis. Since the cells continuously take up material from their environment (nutritional components, proteins and sugar, liquids, etc.) endocytosis is a permanent process in almost all body cells.
Particles in the direct vicinity of a cell can be taken up intentionally (for example by active phagocytosis through scavenger cells in the lung during its clearance) or unintentionally through inclusion along with other material. Once taken up, they can interact in the cell with different substances. Particles that have penetrated the body are normally discharged via the natural routes of excretion.
In the Environment
Particles released into the environment can change in many different ways and interact with other components of the environment. In the water they may be transported or bind to natural organic matter and certain contents of the particles may dissolve. In soil and air, they can form larger assemblies (agglomerates) e.g., with natural occurring nanoparticles or bind to other organic compounds. Agglomerates may reach a size and weight outside the nano-size and sink down in air and water. All these processes determine if and how environmental organisms get into contact with nanoparticles and whether they can be taken up.