Agency for Toxic Substances and Disease Registry 
Case Studies in Environmental Medicine (CSEM) 
Uranium Toxicity
What Is the Biological Fate of Uranium in the Body?

Learning Objectives

Upon completion of this section, you will be able to

• identify where uranium is most likely to be retained in the body.

Introduction

Health effects related to natural and depleted uranium exposures are related to the chemical properties of uranium, while effects from highly enriched uranium include a radiological component, since it is much more radioactive. Overall, absorption of uranium in the body is low, regardless of the route of exposure.

Inhalation Exposure

Inhaled uranium deposits in the various portions of the respiratory tract and the lungs based on particle size (i.e., larger particle size deposited higher in respiratory tract). Most of the deposited uranium clears rapidly via mucociliary transport to the throat. Once there, the uranium is cleared via sputum or swallowing and primarily fecal excretion. Soluble uranium dissolves and is absorbed into the circulatory system more rapidly than insoluble forms. A portion of the uranium can reside in the lungs for years.

An analysis of uranium mill crushermen (workers in the dustiest section of the mill) found that 1-5% of inhaled uranium was absorbed systemically and excreted in urine and 95% was excreted in the feces [Fisher et al. 1983]. Other studies have suggested that only a small portion of inhaled uranium penetrates to the alveolar region, where it can remain for years [West and Scott 1969].

Ingestion Exposure

From 0.5-5% of ingested uranium is absorbed into the blood in adults, with solubility of the compound influencing the portion that is absorbed.

Fate of Ingested Uranium

Animal studies indicate that absorbed uranium moves into the blood, where it is distributed first through soft tissues. Nearly 70% of the uranium absorbed in blood is filtered through the kidneys and cleared within 24 hours in urine. The remaining portion of uranium is distributed to other organs.

After a few days, the majority of uranium exists in the kidneys and skeleton [Bhattacharyya et al. 1989]. A study of humans exposed to uranium showed the skeleton to be the primary long-term depot site for ingested uranium and all but the most insoluble forms of the element. Soft tissue sites of deposition include the liver and kidneys. For inhaled uranium, the pulmonary lymph nodes are the chief deposition site. Uranium deposition in bones first occurs on the surface, but the uranium eventually diffuses into the bone. Human exposure studies have shown that approximately 80-90% of bone deposition is cleared within 1-2 years [ATSDR 2008b].

Overall, most ingested uranium is excreted in feces (95%) with the remainder eliminated in urine [Wrenn et al. 1985].

Key Points

• Most inhaled uranium is cleared through mucociliary transport, swallowed, and excreted in the feces.
• A small portion of inhaled uranium is absorbed into the blood.
• An even smaller fraction of ingested uranium is absorbed into the blood.
• Ingested uranium is primarily excreted in feces.

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