Introduction

This section will discuss

• how cadmium is taken into the body,
• its absorption,
• how it distributes throughout the body, and
• why it accumulates in the human body due to its metabolism and excretion

How Cadmium Is Absorbed

The principal factor determining how much cadmium is absorbed is the route of exposure.  Once exposed, how much cadmium is absorbed depends on many factors:

• age,
• gender,
• smoking, and
• nutritional status.

As a cumulative toxin, cadmium body burden increases with age. Women have been shown to have higher blood levels of cadmium than men. Typically women, with lower iron status, are believed to be at risk for greater absorption of cadmium after oral exposure (Olsson et al. 2002).

Inhalation

Once in the lungs, from 10% to 50% of an inhaled dose is absorbed, depending on particle size, solubility of the specific cadmium compound inhaled, and duration of exposure (Jarup 2002). Absorption is least for large (greater than 10 micrometers [µm]) and water-insoluble particles, and greatest for particles that are small (less than 0.1 µm) and water soluble. A high proportion of cadmium in cigarette smoke is absorbed because the cadmium particles found in that type of smoke are very small (ATSDR 1999).

Ingestion

Most orally ingested cadmium passes through the gastrointestinal tract unchanged as normal individuals absorb only about 6% of ingested cadmium, but up to 9% may be absorbed in those with iron deficiency (ATSDR 1999). Also, cadmium in water is more easily absorbed than cadmium in food (5% in water versus 2.5% in food) (IRIS 2006). The presence of elevated zinc or chromium in the diet decreases cadmium uptake.

Dermal

Absorption through the skin is not a significant route of cadmium entry; only about 0.5% of cadmium is absorbed by the skin (ATSDR 1999).

Excretion of Cadmium

Absorbed cadmium is eliminated from the body primarily in urine. The rate of excretion is low, probably because cadmium remains tightly bound to metallothionein, MTN, which is almost completely reabsorbed in the renal tubules.

Because excretion is slow, cadmium accumulation in the body can be significant. Cadmium concentration in blood reflects recent exposure; urinary cadmium concentration more closely reflects total body burden. However, when renal damage from cadmium exposure occurs, the excretion rate increases sharply, and urinary cadmium levels no longer reflect body burden.

Accumulation of Cadmium

The total cadmium body burden at birth is non-detectable (CDC 2005). It gradually increases with age to about 9.5 mg to 50 mg (ATSDR 1999). The kidneys and liver together contain about 50% of the body's accumulation of cadmium (HSDB 2006).

Cadmium Half-Life

The biologic half-life of cadmium in the kidney is estimated to be between 6 to 38 years; the half life of cadmium in the liver is between 4 and 19 years (ATSDR 1999). These long half-lives reflect the fact that humans do not have effective pathways for cadmium elimination. Cadmium has no known biologic function in humans. Bioaccumulation appears to be a by-product of increasing industrialization. Any excessive accumulation in the body should be regarded as potentially toxic.

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