Distribution and Effects of Lead

The following is an extract from the Workshop Manual for the Lead Paint Management training programme being offered in 1996 by Macquarie University Graduate School of the Environment in conjunction with CTI Consultants. The authors have granted permission for this extract to be reproduced.

After lead is absorbed from the gastrointestinal tract or the lungs, it enters the blood stream. At first, lead attaches to proteins in the blood that carry it to different tissues or organ systems in the body. Blood has a fluid portion, called plasma, and a cellular portion. The cellular portion is made up of red blood cells (or erythrocytes) and white blood cells. Most of the lead present in the blood is bound to the red blood cell. Doctors can tell how much lead a person has been exposed to by measuring the amount of lead in the blood. These amounts are reported as a quantity per unit of volume. (usually micrograms µg per decilitre.)

Lead is distributed to many tissues and organ systems of the body. It's important to remember that lead cannot be destroyed or changed to something else in the body. The amount of lead stored in the body has been described as the "body burden" of lead. Among adults over 95% of the total body stores of lead are found in bone. For children about 70% of lead is stored in bone. This lead is not simply stored away in bone forever, but moves in and out as the body functions normally. For example, as children grow their bones restructure to permit normal shapes as they develop.

The amount of lead in important organs such as the brain, the blood forming system and the kidney are signs of the damage produced by lead accumulation. Several factors must be looked at in order to find the harmful health effects produced by lead:

• How much lead is present in the organ system?
• How long has the lead been present?
• Is the organ system at a time in its development when it can be affected by lead?

Lead is a cumulative poison. Unlike acute poisons, such as chemicals that can kill quickly by attacking the lungs, lead poisoning happens slowly. The lead that is taken in daily, mounts up in the tissues, especially the bones. Blood lead levels mainly show recent exposure (for example, the past few months of exposure) however; lead that is removed from bone is also present in the blood. It is quite possible that a person can have higher amounts of lead in his or her body than looking at the blood lead level would tell us. Because bone is not easily available for measurement of lead, the usual way to tell how much lead exposure a person has had is by chemically measuring the level in the blood.

The body gets rid of lead in the urine and through the gastrointestinal tract. However, many people (and most occupationally exposed workers) are unable to get rid of as much lead as they take in. That is why the "body burden" of lead increases over the decades. Until late in life, most persons are steadily getting more and more lead in their tissues. Only among the elderly, for example those 70 to 80 years old, does the body lead burden begin to get less.

Sometimes bone releases its lead. This may be when the person has a disease, for example osteoporosis, or sometimes during pregnancy and lactation. During pregnancy lead is transferred from the mother to the developing infant. Because lead freely crosses the placenta, the mother's blood lead amounts determine how much lead reaches the foetus. The infant's blood lead at birth is about 85-90% as high as the mother's blood lead level. The tissues of the developing infant, including the brain, take in lead during gestation. The lead taken in during this time has special importance because the developing brain is extremely vulnerable to the harmful effects of lead.

Damage does not occur to one organ system (for example, the nervous system) while not harming other organs at the same time. In humans, the central nervous system, especially of developing infants and very young children, is affected by lower amounts of lead than are other organs such as the kidneys. For this reason much of the focus of recent studies on the effects of lead has been on the harmful neurological effects of lead.

Nervous System Effects of Lead

It has only been understood during the past decade just how much the nervous system is affected by lead. That means, earlier recommendations on "safe" amounts of lead in blood were dangerously close to levels now considered very likely to cause mental retardation in children. Because the past ten years has been a period of very rapid change in understanding of the toxicity of lead, much that has been written (either older pamphlets, medical articles, guidelines for occupational health, etc.) is out of date as to harmful effects that occur at low levels of lead exposure. In the 1960's blood lead levels >60 µg/dL concerned medical care providers. By the 1980's this level was lowered to 25 µg/dL. The Centers for Disease control has recently (October 1991) reduced the level at which interventions are recommended to 10 µg/dL. (See section on NHMRC).

In 1990, the US Public Health Service established the national goal of eliminating, by the year 2000, all occupational exposures that result in worker blood lead levels greater than 25 µg/dL (DHHS, 1990). The mean blood lead level for males in the United States during the period from 1976 - 1980 was 16 µg/dL and this has now decreased in 1988 - 1991 to 4 µg/dL in the latest National Health and Nutrition Examination Survey (NHANES III). In addition, the American Conference of Governmental Industrial Hygienists has proposed that worker blood lead levels be controlled to 20 µg/dL.

Effects in Adults

At very high lead exposures adults also can develop what is called "acute lead encephalopathy". This can occur suddenly. Warning signs include irritability, headaches and hallucinations, and dullness. With very high exposures the person could go into convulsions, paralysis and even die. Blood lead levels that cause these effects are well above 150 µg/dL among adults. A more typical picture of nervous system damage in the adult shows harmful effects of lead on various nerves such as the motor nerves. This damage, in advanced cases, results in "wrist drop" or "foot drop" (the inability to maintain the hand or foot in a normal position due to weakness of muscle tone because of nerve damage).

At lower exposures asymptomatic (without symptoms) effects on the peripheral nerves occur. This means that changes are present that are detectable only by special diagnostic techniques. Workers having blood lead levels lower than 70 µg/dL have been found to have slowed movement of nerve impulses. In adults exposed to lower amounts of lead, some changes typically reported are increased occurrence of fatigue and short-term memory loss, decreased functioning of the nervous system for activities that depend on visual intelligence, and visual-motor co-ordination.

Effects in Infants and Children

The clinically evident effects of lead on the nervous system differ for children and adults. For children blood lead concentrations of about 100 µg/dL to 160 µg/dL and higher are associated with severe damage (encephalopathy). When this happens there is swelling of the brain. This increased pressure severely limits the brain's functioning. Before chelation therapy (administration by injection of organic acids that bind or chelate lead, so that it can be eliminated) was begun in the 1960s, lead poisoning this severe resulted in about a 65% rate of death for children. In current practice, these cases are rare. When properly diagnosed and appropriate chelation therapy used, the death rate is considered to be about 1 or 2%. Although lead exposures this high are rare in the United States today, they are encountered in industrialising countries that have not tried to control lead exposures. Children surviving an episode of lead encephalopathy frequently have permanent brain damage, including retardation and severe behavioural disorders.

Effects on the Blood-forming System

Lead impairs the synthesis (formation) of a substance called "heme" which is extremely important to human life because it carries oxygen to tissues of the body. Lead interferes with the production of this substance at several different steps. Lead exposed persons can develop anaemia. In adults, anaemia is usually seen in severe chronic lead poisoning and blood lead levels of 70 µg/dL and higher are usually found.

Lead has a more severe effect on the blood- forming system in iron deficient people. Generally young children and women of child bearing age are much more likely to be iron deficient than are adult men. Because the combination of iron deficiency and lead exposure cause more severe effects on the blood forming system than either condition alone, women and children tend to show more severe effects. These occur at lower blood lead levels in women and children than in men.

Effects on the Kidney

High exposures to lead that produce acute lead poisoning can damage the kidney in both adults and children. One of the functions of the kidney is to absorb certain substances which are filtered through the kidney. Lead interferes with these functions by altering the metabolism of the kidney. After lead levels are reduced the kidney is able to again do these functions. However, if the lead exposures in childhood continue for a long time and are high amounts, children may show kidney disease later in life as adults. Chronic nephropathy (kidney disease) in lead workers is now recognised as a separate disease. Chronically lead poisoned workers can show elevated blood urea nitrogen. So far there is relatively little information on the renal (kidney) effects of exposure to relatively low levels of lead among either children or adults.

Hypertension

Long-term, high exposures to lead have been reported to be linked with high blood pressure and stroke. One researcher has followed two groups of workers occupationally exposed to lead (4,519 battery plant workers and 2,300 lead production workers from smelters) for a number of years. Both groups of workers have significantly more deaths than would be expected by hypertensive disease and chronic renal disease.

Reproductive Effects

Female workers with high lead exposures and the wives of male lead workers have a higher rate of miscarriages. Male workers with elevated lead exposures (e.g., blood lead levels of 50 µg/dL) have more abnormal sperm cells and lower sperm counts

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