Answer to Question #9832 Submitted to "Ask the Experts"

The assessment of risk resulting from intakes of radionuclides depends on the ultimate dose that accrues to the affected individuals as a consequence of the intakes. The impact of inhalation of radioactive materials can be reasonably assessed through the use of existing information. In particular, considerable work has been done by the International Commission on Radiological Protection (ICRP) on the dose implications associated with the intakes of a wide array of radionuclides. This work has invoked the best available models that describe, in a quantitative fashion, the initial deposition and subsequent distribution of the respective radionuclides following intake by inhalation or by ingestion so that it is generally not necessary for you to be concerned about the fate of the radionuclides within the body.

The major derived radiation protection quantity, obtained by application of appropriate mathematical models and restricted by a priori committed dose limits (committed dose refers to dose calculated over a period of 50 years following an intake of a radionuclide), is the annual limit on intake (ALI) of a particular radionuclide. This represents the quantity (Bq) of the radionuclide of interest that will result in the allowable committed effective dose to the individual or the allowable equivalent dose to any specific tissue. We should note that the current radiation protection limits specified by the U.S. Nuclear Regulatory Commission and Agreement States for inhalation of radionuclides are based on recommendations of the ICRP that date back more than 30 years (e.g., ICRP Publication 26, Recommendations of the International Commission on Radiological Protection and ICRP Publication 30, Limits of Radionuclides by Workers). More recent recommendations of the ICRP have been adopted by many countries, and some have been approved for use in the United States. These include ICRP Publication 60, 1990 Recommendations of the International Commission on Radiological Protection; ICRP Publication 103, Recommendations of the International Commission on Radiological Protection, 2007; ICRP Publication 66, Human Respiratory Tract for Radiological Protection; International Commission on Radiological Protection. ICRP Publication 67, Age-dependent Doses to Members of the Public from Intake of Radionuclides; ICRP Publication 71, Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 4 Inhalation Dose Coefficients, and others.

For inhalation, the mathematical models involved include a respiratory tract model that, in consideration of the nature of the physical and chemical characteristics of the inhaled material predicts the deposition pattern in the respiratory tract and the routes and kinetics of removal from the respiratory tract to systemic circulation and to excreta. The respiratory tract is mathematically coupled to a gastrointestinal tract model and systemic metabolic model that allows calculation of committed doses to the various internal organs/tissues. These doses are ultimately used to determine the equivalent dose to any specific tissue and the effective dose to the body resulting from unit intake of a specified radionuclide. The results are then used in conjunction with the appropriate dose limit (e.g., 50 mSv effective dose under U.S. regulations or 20 mSv effective dose used by many other countries) to determine the ALI for the radionuclide of interest.

There are some allowances in the ICRP models to modify results to reflect different particle size distributions when particulate aerosols are inhaled. The default particle sizes used in the earlier ICRP recommendations was 1 micrometer AMAD (activity median aerodynamic diameter) and 5 micrometers AMAD in later recommendations. Once the ALI for a given radionuclide is determined it is a simple matter to judge the significance of a given intake by dividing the intake quantity by the respective ALI. If several radionuclides are inhaled, the effective dose committed to the individual, E,  may be calculated simply from the following:

where the index i defines a particular radionuclide, and E_limit refers to the effective dose limit that has been established (e.g., 50 mSv per year – for radiation protection purposes, in the case of internal dose from a given acute intake the dose is calculated over the 50 years following intake, but it is assigned in the year in which the intake occurred). This dose may then be used in the risk assessment process that you are invoking. It may be important to keep in mind that if you are interested in accrued risk as of a particular point in time, all of the dose that might ultimately accrue over a lifetime may not have accrued as of the time of interest, and if the ALI is used in the fashion above to predict effective dose, the dose may require modification if sufficient time has not passed since intake to allow most of the internal dose to accrue. For example, in the ICRP Publication 30 distribution and retention data for thorium, it is noted that 70 percent of the material that enters the blood is transferred to bone where it is retained with a biological half-life of 8,000 days (22 years). Clearly, if one were interested in assessing the risk associate with an intake of ²³⁰Th that occurred five years prior, only about 15 percent of the ultimate bone dose would have accrued to that point in time. Such considerations may be important in certain situations when one is attempting to judge the importance of a given intake in inducing specific disease.

In summary, much of the work required to interpret doses and ultimate risks from intakes of radioactivity has already been done, and is available in the existing literature. I hope this is helpful to you.

George Chabot, PhD, CHP