Are heavy metals still a problem today?
The toxic heavy metals best known by the public are lead, cadmium and mercury. Because of its continuing importance in many countries, mercury will be treated separately. Arsenic is often included, even though it is not a metal in the strict chemical sense; it is sometimes called a metalloid.
In historical terms, lead is a very interesting toxic metal. It has a known history stretching back about 8000 years. It was first smelted in Asia Minor, present-day Turkey, and several 6000 to 8000 year old necklaces, beads and other lead decorations have been found at a variety of sites in the Middle East. Lead water pipes were already used in Mesopotamia around 3000 B.C. Greenland ice core studies have revealed that the use of lead greatly increased by the time of Classical Greece and the Roman Empire possessed large lead mines in Greece, Gaul and England.
There is an interesting theory that because of the common use in beakers and other kitchenware, the Roman upper class suffered from chronic lead poisoning, and this was one of the reasons for their sometimes irrational behaviour and the poor reproductive success. Lead acetate was even used to sweeten wine. So it is possible that lead was one of the reasons contributing to the fall of Rome. Some evidence of high lead concentrations has indeed been found by analysing the bones of victims of the volcanic eruption of Vesuvius in A.D. 79. But surely there were other reasons for the decline of the empire, including the smallpox epidemic in A.D. 165, called the Antonine plague.
The use of lead in plumbing or lead smoltering of waterpipes was a common practice in many countries until recently, despite the clear indications already from ancient times that lead was toxic. In defence of the Romans, their emperor Augustus did try to prohibit lead use in water pipes. Nicander depicted lead-induced anaemia and colic already in 250 B.C., and the occupational poisonings of the lead miners were well known.
The modern-day massive use of lead started when it was added to petrol as an antiknocking compound, tetraethyl lead (and in some countries this is still the case). Even though the potential dangers of this practice were warned against already during the 1920s when automobiles were still not commonplace, the use continued in most countries until the 1980s. The turning point was the discovery of the subtle linkage between developmental retardation in cognitive development in children and higher blood lead levels. After lead-free petrol was introduced, blood lead levels rapidly decreased, highlighting just how an important source lead exposure had been. The antiknocking agents had polluted urban air directly and after sedimentation they had contaminated vegetables and fruit.
The main toxic effects of lead are neurotoxicity, first of the peripheral nerves and then the central nervous system, kidney toxicity, and anaemia. During development, the most sensitive target organ is the developing central nervous system, especially cognitive development. IQ deficits of 4 to 5 points have been detected in children with higher than average blood lead levels (about 10 micrograms per decilitre or 0.5 micromole per litre). There seems to be no threshold for this effect. This means that the whole distribution curve of IQ in a society with high lead levels is shifted to the left, towards lower intelligence. This has significant effects at both ends of the curve, there are fewer children with a high IQ (e.g. higher than 120), and an increased number of children with a low IQ (e.g lower than 80). This may have a profound effect on the success of a population in today’s global economy.
Since lead antiknocking additives were prohibited, blood lead levels of children have declined to clearly less than 5 micrograms per deciliter (0.2 micromoles/l). Dietary intake of lead in Western Europe seems to be about one fifth of the tolerable weekly intake, but variation between countries can be as much as tenfold. The remaining sources may be old plumbing in some countries, and old paints with lead pigments.
Poor children living in slums have been shown to chew chips of paint – this is the so-called "pica" syndrome caused by poor nutrition. Another source in some places could be contaminated soil. Occasionally children's toys are imported from countries with lax controls, which have been painted with lead-tainted paints. We should not tolerate lead in plumbing or in regular paints in any society in the world today.
Of the three major heavy metals, only cadmium is absorbed through plant roots into the plant. Therefore it is important to control the quality of phosphate fertilizers, some of which contain high levels of cadmium as an impurity. This also limits the use of sediments from sewage treatment plants being used as a fertilizer, because urban sewage usually contains cadmium. Cadmium has the longest half life in the human body of all the heavy metals, about 30 years. This means that once cadmium has entered your body, very little of it comes out during your lifetime. This is very problematic for any organism, if it is continuously exposed to even small amounts of cadmium.
In addition to occupational exposures, the most important sources of cadmium are food and tobacco smoke. Intoxications due to cadmium have occurred in Japan when rice paddies were irrigated with water contaminated with cadmium. Cadmium accumulates into the liver and kidneys, and over the years it may cause kidney damage. Therefore its concentrations may also be high in the kidneys of wild animals such as elk. The habit of making a stew from newly-slain elk kidneys and livers, a favourite of elk hunters, may increase their cadmium intake considerably.
Cadmium at higher doses also affects bone metabolism, and may cause osteomalacia or osteoporosis, softened or brittle bones. It has also been claimed that cadmium might be involved in the development of hypertension and therefore in the risk of suffering vascular diseases. Occupational exposure to cadmium fumes may cause cancer.
In most of Europe, cadmium intake is relatively close to tolerable intake levels, but differences between countries are remarkable. In children, dietary intake was estimated at 30% of tolerable weekly intake, but in some countries even the average exceeds the estimated tolerable levels. Because of its long half life, strict controls are needed.
Heavy metals may cause very detrimental effects at relatively low intake levels. They are fairly well characterized compounds, and the controls are strict in most developed countries. Safety margins are not very high, however, and the differences in the risk levels between different countries may be notable.
Notes and references
- See the chapter "Is mercury a quicksilver bullet or a slow poison?"
- See the chapter "Arsenic in the drinking water – reminiscent of the movie “Lavender and old lace”?"
- Ice cores are bored to different depths and sliced like bologna sausages to analyse various historical samples. The deeper the slice the older is the sample, because ice is continuously formed by snowing on top. Snow carries with it the typical pollutants of the period. Various markers can be used to determine the age of each slice. More than two kilometres long cores have been bored representing a period of 160,000 years.
- The elimination of most substances out of the body follows first order elimination kinetics, meaning that the excreted amount per time unit is directly dependent on the amount (concentration) in the body. When elimination proceeds, the levels decline, and elimination slows down. To describe this kind of elimination the term half life is used. It means the time required for the amount in the body to be halved. After another half life the amount is again halved, i.e. one fourth of the original amount, and so forth. Thus the half life is constant for a chemical regardless of the amount (within reasonable limits).
One level up: Is man defiled by what goes to his mouth?
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