Aluminum

Aluminum (chemical symbol Al) is a trace metal and a nonessential element which is widely distributed throughout the Earth’s crust.

Chemistry

Physical chemistry

Aluminum is an element with the atomic number 13, one of the post-transition metals in the boron group (Group 13, IIIA) of the periodic table in the second row. It has a molecular weight of 26.98 Da, a melting point of 660 degrees Celsius, and a boiling point of 2327 degrees Celsius ¹.   

Aluminum does not occur in nature in its elemental, metallic form; rather, it is typically found in various compounds which include ores, such as bauxite or gibbite, or gemstones such as sapphires or rubies In compounds, aluminum typically occurs in its +3 oxidation state ². Elemental aluminum is a silver to white ductile, malleable metal. Exposure to airborne moisture results in the formation of aluminum oxide.

Radiochemistry

Aluminum has nine isotopes, two of which, ²⁷Al and ²⁶Al, occur naturally. ²⁷Al is a stable isotope, whereas ²⁶Al is radioactive, decaying by positron and (to a lesser extent) gamma ray emission with a half-life of 720,000 years ³.  

Nutrition, absorption, transport and storage

Nutrition

Aluminum occurs naturally in both unprocessed foods, both plant and animal based, as well as drinking water by virtue of its prevalence throughout the earth’s crust ⁴.

Absorption

After ingestion absorption primarily occurs in the proximal small bowel primarily via passive paracellular diffusion, although active transport mechanisms are also described. Inhalational exposures to aluminum powders, fumes and dusts does result in systemic absorption which is greater with smaller particle size. Dermal absorption of aluminum compounds is likely negligible ¹.

Transport

After absorption aluminum partitions primarily into the plasma with the majority circulating bound to transferrin, with a smaller fraction found within erythrocytes. Approximately half is distributed to the bone, with the remainder distributing to the heart, liver, spleen, kidney and brain. Within the cerebrospinal fluid aluminum is primarily bound to citrate and preferentially partitions into the gray matter ².

Storage

The average total body aluminum burden in humans ranges between 30 and 50 mg, the largest fraction of which is found in bone followed by the lungs. The brain may, in health, contain up to 0.75 mg/kg of aluminum which increases with aging. Intracellularly aluminum is sequestered within lysosomes as well as within the nucleus, accumulating to a greater degree in cells with low turnover rates ¹.

Elimination

The vast majority of aluminum is excreted unchanged in the urine; impaired renal function results in a significantly prolonged elimination half-life. Biliary elimination is less than 2%.

Toxicity

Acute

Acute aluminum toxicity typically results after massive exposure to an aluminum salt, usually in the presence of renal impairment. Potential sources of exposure include bladder irrigation with alum (potassium or ammonium aluminum sulfate) and phosphate-binding agents. Clinical manifestations are primarily neurologic, appear after a delay of days to weeks, and include ²:

• confusion, delirium, coma

• seizures

• myoclonus

Chronic

The manifestations of chronic aluminum toxicity vary based upon the route of exposure and patient features; pulmonary symptoms predominant in occupational inhalational exposures whereas chronic exposure in patients with chronic kidney disease results in toxicity affecting hematopoiesis, cortical bone, and the brain. These effects include:

• pulmonary toxicity

  • occupational asthma

  • pulmonary fibrosis (“aluminosis”)

    • bilateral, upper lobe predominant diffuse interstitial fibrosis

    • subpleural bullae

    • small, rounded, ill-defined opacities

• hematologic toxicity

  • hypochromic, microcytic anemia

• skeletal toxicity

  • osteomalacia, pathologic fractures

• neurologic toxicity

  • dementia, myoclonus, disturbances of speech

  • seizures

Related pathology

• heavy metals

• pneumoconioses