The case for fractional solubility profiles

Default values for the solubility of various compounds in the lung are provided in publications of the International Commission on Radiological Protection as absorption types to characterizes the potential uptake of radionuclides to blood. The default assignments are conservative and reflect compounds likely to be encountered in the workplace. In practice, solubility profiles for many compounds, both natural and man-made, are complex, with a fraction of the compound in each absorption type, denoted as F, M, or S. Only soluble compounds of tritium and iodine can be reasonably assumed to be of one absorption type. The assumption of a single absorption type for airborne distributions of solid particulate matter can introduce order of magnitude errors in internal dosimetry calculations. The problem is particularly acute for isotopes with dual toxicity (e.g. uranium which is both nephrotoxic and radiotoxic), and when a dose estimate must be derived with only a single bioassay measurement. For inhalation exposures during an accident, treatment decisions frequently must be made quickly to be effective. While much work has been done to develop rapid bioassay methods that will provide data in a clinically useable timeframe, little consideration has been given to the magnitude of the error in the dose estimate resulting from the assumption of the default solubility profiles.