Metabolism of ingested U and Ra.

Area of Science:

• Environmental Health
• Toxicology
• Radiological Protection

Background:

• Existing literature on uranium (U) and radium (Ra) metabolism in humans is reviewed to inform drinking water standards.
• While radium metabolism is well-understood, significant knowledge gaps exist for uranium metabolism.
• Current understanding necessitates an equilibrium model for deriving drinking water limits, based on established intake-organ burden relationships.

Purpose of the Study:

• To review and summarize human metabolism of U and Ra relevant to drinking water standards.
• To establish best estimates for metabolic parameters and organ burdens.
• To recommend a safe limit for U in drinking water based on toxicological considerations.

Main Methods:

• Literature review and synthesis of existing data on U and Ra metabolism.
• Application of an equilibrium model using best-estimate metabolic parameters.
• Analysis of human gastrointestinal (GI) absorption of U.
• Toxicological assessment focused on kidney effects.

Main Results:

• Skeletal burden estimates: 25 days for 226Ra, 10 days for 228Ra, 0.3 days for 224Ra.
• Estimated skeletal burden for long-lived U isotopes is 11 days (range 1-35 days).
• Average human GI absorption of U is estimated at 1-2%, independent of age or intake mass.
• Recommended U limit in drinking water is 100 micrograms/liter (equivalent to 67 pCi/l) to prevent kidney toxicity.

Conclusions:

• The recommended U limit of 100 micrograms/liter is based on kidney toxicity and a modified Spoor and Hursh metabolic model.
• Further research is needed on U distribution in the human body, pharmacokinetics, GI absorption from various sources, and chronic toxicity.
• Establishing drinking water standards requires robust metabolic data and toxicological assessments.