distribution of lead

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 this is micrograms [m/dL] per decaliter [dL]. (Older information may also refer to micrograms per 100 grams of blood.) These units of measurement are about the same.

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 amounts 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:

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 exposures (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 of lead 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 or 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 (breast feeding). 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 fetus. 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.

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 health effects that occur at low levels of lead exposure. In the 1960's blood lead levels of 60 m/dl or more concerned medical care providers. By the 1980's this level was lowered to 25 mpg/dI. The Centers for Disease Control has recently (October, 1991) reduced the level of concern to 10 m/dI.

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 m/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).