Cadmium is a naturally occurring heavy metal found in the Earth’s crust and is a by-product of zinc, lead, and copper smelting. It has no known beneficial role in the human body. Once this toxic element enters the body, its unique kinetic behavior leads to an exceptionally long retention time. This means that even low-level exposure over a lifetime can lead to significant accumulation within internal organs.
Primary Exposure Routes
The two primary ways cadmium enters the human body are through inhalation and ingestion. For the general population, the most significant source of inhalation exposure is tobacco smoke, as tobacco leaves naturally accumulate cadmium. The small particle size of cadmium aerosols in cigarette smoke allows for substantial absorption into the lungs, with estimates suggesting between 5% and 50% of the inhaled cadmium can enter the bloodstream.
Occupational exposure is another major inhalation route, particularly for workers involved in smelting, electroplating, and battery production. Ingestion primarily occurs through the chronic consumption of contaminated food and water. While only 1% to 10% of ingested cadmium is absorbed through the digestive tract, this low-level exposure contributes significantly to the body burden over decades. Foods such as shellfish, organ meats like liver and kidney, and certain crops like peanuts and soybeans accumulate higher concentrations of the metal.
The Concept of Biological Half-Life
The biological half-life of a substance is the time required for the body to eliminate half of the total amount of that substance. Cadmium’s biological half-life in humans is extraordinarily long, typically estimated to be between 10 and 30 years.
This prolonged retention is directly due to cadmium’s strong affinity for metallothionein, a small, sulfur-rich protein. Once absorbed, cadmium induces the synthesis of metallothionein, which then tightly binds to the metal. This highly stable cadmium-metallothionein complex prevents the metal from being easily metabolized or excreted, effectively trapping it within tissues for a long period.
Storage and Accumulation Sites
After absorption, cadmium is initially transported in the blood, often bound to proteins like albumin. The first organ to receive the majority of this metal is the liver, where it stimulates the production of metallothionein and binds the absorbed cadmium. Over time, the cadmium-metallothionein complex is released from the liver and travels through the bloodstream to the kidneys.
In the kidneys, this complex is filtered through the glomerulus and reabsorbed almost completely by the cells of the proximal tubules. The kidneys, specifically the renal cortex, become the long-term storage site for cadmium. The liver and kidneys together can contain approximately 50% of the total accumulated body burden, and this progressive buildup eventually leads to chronic health concerns, such as kidney dysfunction.
The Slow Process of Elimination
The human body possesses no efficient mechanism to actively eliminate stored cadmium, which reinforces its long biological half-life. Excretion of the metal occurs through the urine and feces, with both routes contributing roughly equally to the minimal loss. The rate of elimination is extremely slow, amounting to less than 0.01% to 0.02% of the total body burden per day.
This inefficiency is largely linked to the metallothionein complex that carries the metal. The reabsorption mechanism in the renal tubules, designed to conserve beneficial proteins, inadvertently traps the bound cadmium within the kidney cells. Consequently, the amount of cadmium excreted daily is minuscule compared to the amount stored, allowing the metal to accumulate continuously throughout a person’s life.