A Life-course screening-factor framework for radionuclides in drinking water: identifying when adult-based screening...

Adult-based drinking-water screening is practical for routine radiological assessment, but it reduces life-course exposure to a single adult reference condition. This simplification cannot determine whether the adult factor represents chronic lifetime-average exposure or whether the limiting annual dose occurs during infancy, childhood, adolescence, adulthood, or late life. A life-course screening-factor framework was developed for 17 radionuclides, with published reference results for Ra-226 and Ra-228 incorporated for comparative context. Chronic ingestion at 1 Bq L^-1 was modeled over 70 years using age-specific intake rates and age-dependent ingestion dose coefficients; screening performance was evaluated using R_peak = F_peak/F_adult and R_life = F_life/F_adult. Adult-based screening was neither uniformly conservative nor uniformly non-conservative. Lifetime-average underestimation (R_life > 1) occurred for Ra-224, Pb-210, Po-210, I-131, Sr-90, and Th-228 among the newly analyzed radionuclides; the published Ra-228 reference case showed the most pronounced bias overall (R_life ≈ 2.04). Uranium isotopes, Am-241, Cs-134, Cs-137, Pu-239, Th-230, and Th-232 were conservative for lifetime-average exposure, while C-14 and tritiated water were approximately adult-representative. Peak-dose blind spots were identified for Ra-224 (severe infant peak; R_peak ≈ 15.8), Th-228, Po-210, I-131 (young-child thyroid peak), and Sr-90 (two-peak skeletal-growth pattern). Uranium isotopes and Am-241 showed mixed behavior: conservative for lifetime average but with missed infant dose maxima. Pb-210 and Ra-228, despite identical adult-based screening factors, produced substantially different cumulative lifetime doses (44.4 mSv versus 71.4 mSv at 1 Bq L^-1), demonstrating that adult-based equivalence does not guarantee equivalent chronic exposure. These findings demonstrate that adult-based drinking-water screening performance is radionuclide-specific and metric-specific. True lifetime-average underestimation occurs when F_life exceeds F_adult, whereas F_peak identifies age-specific exposure windows hidden by adult-only screening. The proposed F_life and F_peak factors provide a practical life-course extension to adult-based screening for drinking-water radiological protection.