Bioaccumulation begins when a chemical enters into an organism from the environment where both the contaminant and the organism co-exist. This first phase is typically referred to as the uptake of the contaminant, which is a complex process in itself. Scientists have learned that chemicals tend to diffuse passively from a place of high concentration to one of low concentration. The driving force for this passive transport is the natural tendency of molecules proceeding from order (e.g., highly-packed place) to chaos (e.g., loosely-packed).

A number of factors may facilitate the passive transport of chemicals. For example, certain chemicals do not mix well with water. They are called lipophilic (meaning fat loving), or hydrophobic (meaning water hating). In either case, they tend to move out of water and into the cells of an organism, as the latter typically offer a more lipophilic microenvironment.

The same factors facilitating the uptake of a chemical continue to operate inside an organism, thus minimizing the chemical’s opportunity of returning to the outer environment. Some chemicals are attracted to certain cellular sites, and are temporarily if not permanently stored when binding to proteins or dissolving in fats. If uptake proceeds slowly or is discontinued, or if the protein binding is weak, the chemical can eventually be excreted.

The uptake and the storage of chemicals are also influenced by their water solubility, which is a measure of their potential to dissolve in water. In general, chemicals that are highly water soluble do not readily enter the cells of an organism. Once inside the organism, water-soluble chemicals are easily removed unless the cells have a specific process to retain them.