According to an IUPAC definition (IUPAC 1997) solubility is the analytical composition of a saturated solution expressed as a proportion of a designated solute in a designated solvent. In text books of chemistry, a substance is said to be soluble if more than 0.1 g of that substance dissolves in 100 ml solvent. If less than 0.1 g dissolves in 100 ml solvent, the substance is said to be insoluble or, more exactly, sparingly soluble (Fundamentals of Chemistry; http://www.chem.wisc.edu/deptfiles/genchem/sstutorial/FunChem.htm). For the definition of “poorly soluble particles” in the framework of `lung overload` see also Section 1.3.
Inhaled particles are processed by biological systems upon deposition, and are exposed to complex mixtures of biogenic solvents. Thus, biosolubility means solubility in a biological system e.g. cell system, biological fluid (or its simulants), in an animal. Biosolubility may differ significantly from the solubility in water and varies depending on biological systems.