Identifying age-specific windows of vulnerability: a lifetime dose assessment of Ra-226 in drinking water

The World Health Organization recommends a reference level of 0.1 mSv y^-1for radiological exposure from drinking water, implemented through an adult-based screening framework that does not explicitly account for age-dependent water intake or radionuclide biokinetics. For bone-seeking radionuclides such as Ra-226, these factors can modify both the temporal distribution and cumulative burden of ingestion dose over the lifespan. In this study, an age-dependent lifetime ingestion-dose model was developed for Ra-226 by combining age-specific drinking-water intake rates with International Commission on Radiological Protection ingestion dose coefficients and time-resolved biokinetic dose functions. Annual and cumulative effective doses were calculated from birth to 70 years and compared with estimates obtained using the adult-only screening approach, assuming a constant drinking-water concentration of 1 Bq l^-1. The cumulative lifetime effective dose reached approximately 20 mSv, corresponding to a lifetime-averaged annual dose of ∼0.29 mSv y^-1, compared with ∼0.20 mSv y^-1derived using the adult-based method. Although the absolute magnitude of Ra-226 ingestion dose is lower than that reported for Ra-228, the age-dependent model revealed a clear early-life dominance. The highest annual dose occurred during the first year of life (∼1.2 mSv y^-1), driven by elevated infant ingestion dose coefficients. Annual dose declined rapidly through early childhood and then approached a near-constant adult value of approximately 0.2 mSv y^-1. In contrast to Ra-228, Ra-226 did not produce a comparable adolescent dose peak, reflecting faster dose saturation following intake and limited long-term contribution from short-lived progeny. These results demonstrate that adult-only screening does not capture the timing and distribution of Ra-226 ingestion dose and that age-dependent lifetime modelling provides a more realistic basis for assessing chronic exposure from drinking water.