Is cadmium worse than lead?

 Reprinted from Toxic. March 1994 with permission.

Cadmium is a heavy metal with wide use and distribution in industrial processes. Sources include mining and metallurgy; manufacturing and processing industries; metal finishing; sewage disposal; combustion of fuels and wastes; pesticides and fertilisers. Cadmium is a by-product of zinc and lead smelting. The toxicology of cadmium is comprehensively reviewed by Friberg et al 1986, and Robards and Worsfold 1991.

Along with lead and mercury it is a "heavy" metal of environmental significance because of documented adverse effects in animal studies at low doses and in human epidemiology - and because of widespread environmental distribution. 

Environmental sources
Cadmium is present in all mediums. Airborne cadmium in urban air is in the range up to 0.060 µg/m3. Meat, fish and fruit contain 1 to 50 µg/kg and grains from 10 to 150 µg/kg. The greatest concentration is in the liver and kidney of animals, and in shellfish such as mussels and oysters which concentrate cadmium up to levels of 100 to 1000 µg/kg. In developed countries the total daily cadmium intake from all mediums may be in the range 10 to 40µg/kg.

Cadmium is more readily taken up by plants than other metals such as lead. In this context, the cadmium content of phosphate fertiliser is a significant source. Cadmium content of Australian fertilisers is in the range 20­60 ppm, and is derived mainly from rock phosphate. Williams has reported the problems of trace metals in superphosphate. Tobacco. tomatoes and brassicas take up cadmium more efficiently from soils than some other crops. Studies have shown a slow but steady increase in the cadmium content of vegetables over the years, and consequent increase in body burdens. 

Toxicology
Cadmium is more efficiently absorbed from the lungs than the gastrointestinal tract. Calcium and iron deficiencies may enhance the uptake of cadmium. Cadmium, like zinc, has a predisposition for the male reproductive organs, particularly the prostate. Dietary zinc decreases cadmium absorption. Blood cadmium in non-occupationally exposed persons is usually less than 1 µg/dl (micrograms per decilitre).

Although high levels of cadmium can cause acute effects, the principal long-term effects of low-level exposure to cadmium are chronic lung disease,  emphysema and chronic kidney disease. Effects on the cardiovascular and the skeletal system have also been reported. People heavily exposed in Japan developed Itai-Itai disease which is characterised by bone pain, osteomalacia and osteoporosis.

Kidney and skeletal effects appear not to be reversible and may be progressive according to follow-up studies of exposed populations. As with lead, cadmium is associated with increased blood pressure in several studies of occupationally-exposed workers. Electrical and biochemical heart disturbances have also been reported in experimental animals.

The IARC rates cadmium as a probable human carcinogen, category 2A, based on animal studies and human occupational epidemiology. This evidence is much more developed than for lead. Human studies have found increases in lung cancer and to a lesser extent, cancer of the prostate.

Much recent interest centres on the spermatogenic effects of low levels of cadmium exposure. A recent paper by Kok-War Hew et al shows that a single dose of 1 mg/kg of cadmium chloride resulted in failure of spermiation in experimental rats. Poor sperm characteristics have been correlated with high cadmium blood levels (mean 1.35 µg/dl) in human studies. (Archives of Andrology 29/2, 177-183, 1992, abstract).

Cadmium is a toxic, persistent and bioaccumulating metal which is increasing in the environment and human tissue.