REDCALdep Tool for Particle Deposition Calculations in the Human Respiratory Tract: Applications to Military Exposure Guidelines

Abstract

Introduction: Inhalation is a primary pathway for the intake of airborne particles, including radioactive materials, accounting for up to 73% of annual exposure to natural sources of radiation. Among workers, inhalation is the most common route of radionuclide intake, according to the European Commission's radiation protection technical report. The significance of inhalation exposure increases in military operational environments and occupations involving airborne radioactive particles, such as nuclear weapons testing, accident cleanups, and handling of radioactive materials. The likelihood of inhalation exposure and respiratory risks varies depending on environmental releases, occupational settings, and deployment-related activities. The deposition of inhaled aerosols depends on a range of factors broadly classified as environmental conditions and individual characteristics. The complex interactions among these factors necessitate theoretical models, supported by empirical data, to accurately predict deposition patterns. This study developed a customizable computational tool for predicting particle deposition in the human respiratory tract, optimizing accuracy, efficiency, and practicality. These attributes are particularly suited to military exposure scenarios, where timely and precise assessments are critical for evaluating exposure levels and implementing protective measures. The tool contributes to a broader initiative to safeguard lung health in active duty service members, veterans, military beneficiaries, and the general public exposed to radionuclide or toxic metal contamination from radiological or nuclear events.

Materials and methods: To predict aerosol deposition in the human respiratory tract, the algorithm of the International Commission on Radiological Protection (ICRP) Publication 66 human respiratory tract model was adopted and integrated into an in-house Python based computational tool, REDCALdep. This tool incorporates updates from ICRP Publication 130 and allows for enhanced customization, including physiological parameters tailored to military exposure guidelines (MEGs), individualized deposition computations, and sensitivity analyses.

Results: The deposition fractions (DFs) calculated using REDCALdep for a reference worker were benchmarked against ICRP-published values, demonstrating excellent agreement. Relative differences in total DFs were below 5% for both aerodynamic and thermodynamic lognormal particle size distribution, with most differences under 1%. Region specific differences were generally less than 3%, largely attributable to numerical precision and differences in rounding schemes. Additionally, REDCALdep uses a quantile range for particle sizes in the lognormal distribution, influencing the precision of the computed DF. Sex-weighted DFs were computed based on a 14-day MEG-defined activity schedule, encompassing sleeping, sedentary, light, and heavy activities. Compared to ICRP reference worker deposition values, the magnitude of the maximum relative differences in regional DFs were approximately 72%, with total deposition fractions differing by up to 49.44%.

Conclusion: These findings emphasize the importance of the newly reported MEG-specific deposition data. The MEG-informed DFs reported in this study represent a novel dataset that enhances internal dose assessments and risk evaluations specific to military scenarios. Through its customizable and precise deposition calculations, REDCALdep serves as a valuable tool for decision-making related to the inhalation of toxic and radioactive materials in occupational and military context, as well as for individualized dose assessments.