Introduction

Exposure to uranium in the natural environment occurs most commonly via oral exposures. Uranium enters the body by eating contaminated food or drinking water that contains uranium.

Dermal exposures occur through skin contact with uranium powders or metals. Usually only those working with products or processes using uranium would be exposed in this way. Another possible route of exposure is from retained depleted uranium metal fragments (shrapnel) that embed in soft tissue. These fragments oxidize in situ and provide a source of ongoing systemic absorption.

Inhalation of uranium powder can also occur and is the primary exposure route for workers. As discussed later in this section, inhalation may be an important route of exposure for individuals in “at risk” communities.

Ingestion

Ingestion is the most common pathway of exposure to naturally occurring uranium for the general public. Exposures can occur through

• ingesting food or drinking water containing naturally occurring uranium and/or
• ingesting food or drinking water contaminated through uranium mining or waste activities.

Uranium is found in drinking water at an average concentration of 2.5 micrograms/Liter (µg/L) throughout United States [Drury 1981; EPA 2006a]. Uranium also adsorbs to plant roots and can be ingested during food intake. Overall, the daily estimated oral intake for both food and water is about 3.0 µg/day of natural uranium. In areas of the country where natural uranium concentrations are elevated, populations can be exposed to higher average doses of uranium. For information on the location of uranium deposits in the United States, please visit U.S. Geological Surveys website on Uranium Resources and Environmental Issues [USGS 2009a].

Inhalation

Inhalation of uranium powder is the primary exposure route for workers. However, inhalation of uranium is a very minor source of exposure for the general population. Exposure scenarios include inhalation of contaminated air and dust from

• communities where remediation activities are ongoing,
• enrichment, or recycling,
• mining and milling,
• site remediation activities,
• the combustion of coal (since it contains uranium), and
• worker's skin, hair, and clothing.

In areas where uranium enrichment processes occur, atmospheric concentrations of uranium have been measured at 200 times higher than normal background levels. Mining activities can increase air concentrations of uranium, exposing mine workers to elevated levels of uranium. Uranium enrichment processing also has potential to release additional uranium into the atmosphere. Both mining and processing of uranium have significantly decreased since the early 1990s, so that the highest exposures would have likely occurred prior to this. However, uranium mining for the purpose of either building or providing supplies for new nuclear power plants did increase some in the late 1990's as sources of alternative energy were sought. Many of the Department of Energy mining and milling facilities have undergone or are undergoing remediation activities, which can also lead to increases in airborne uranium concentrations. This is currently the most likely exposure scenario for inhalation exposures.

Dermal

Dermal contact with uranium is also a possible exposure pathway for naturally occurring uranium. Dermal exposure scenarios include

• workers exposed to uranium powders or metals and
• workers coming in contact with uranium wastes.

It is not likely that community members would be exposed to elevated levels of uranium via direct skin contact, though children playing in contaminated areas may be at risk for this type of exposure.

Progress Check