How does lead harm living things?
The human body does not metabolize lead, but rather absorbs it directly into the bloodstream. If lead gets into an adult body through the skin or digestive tract, the amount of lead absorbed into the blood is generally about 20 percent of the amount ingested. If inhaled into the lungs, where exhaling is the only method of excreting waste, a body can absorb up to 100 percent into the bloodstream.
Once absorbed into the body, lead may be present for weeks to months before being excreted as waste. A small fraction remains in the body indefinitely, mostly in the bones and teeth. This stored lead may stay put, but it can also be released back into the bloodstream during times of stress (pregnancy, illness, etc), in effect repeating the lead exposure.
Once lead gets into the bloodstream, it circulates through body with red blood cells and plasma. It is the small amount of lead in blood plasma that gets transferred into soft (brain, liver) and mineralizing (bone, teeth) tissues. And it is this small amount which appears to do the damage associated with lead poisoning.
Lead finds a home in bones and teeth because the body treats this mineral much as it would calcium. In fact, some molecules in our bodies incorporate lead as easily as they incorporate calcium. Replacing calcium with lead results in a calcium deficiency for the body, possibly leading to osteoporosis. The main danger, however, lies in the possibility of later lead release.
The consequences of lead contamination in soft tissues can be severe because lead interferes with enzymes that help the brain and other cells work. Its primary affects are on the peripheral and central nervous system, kidney function, blood cells, and the metabolism of Vitamin D and calcium. But lead can also cause hypertension, reproductive toxicity, and developmental effects.
In children, high-level lead contamination of the nervous system can cause encephalopathy, a degenerative disease of the brain associated with hyperirritability, loss of muscle control, convulsions, stupor, and coma or death. At lower levels, lead’s interference with this system can cause Attention Deficit and Hyperactivity Disorder, hearing impairment, impaired balance and peripheral nerve function, and learning disabilities. Adults are not at risk for developmental disorders, but encephalopathy and/or impaired motor control and nerve function can result from extremely high blood lead levels. These conditions result from lead poisoning-associated brain swellings and lesions on the brain called cerebella calcifications.
Acute, high doses of lead appear to have reversible affects on the kidneys. Lead at these doses impairs the ability of the kidneys to filter wastes from the bloodstream and control blood levels of certain chemicals leading to symptoms such as increased levels of sugar, phosphates, and amino acids in the urine. Chronic exposures, however, can stress the kidneys to the point of irreversible damage. Interstitial nephritis is a permanent condition associated with chronic lead poisoning that usually ends in kidney failure.
Lead interferes with the body’s ability to make hemoglobin, the filling for red blood cells. It does this by inhibiting two processes necessary to make heme, the part of hemoglobin that feeds oxygen to cells. There are several types of anemia associated with diminishing amounts of hemoglobin. Depending on the ingestion levels and time span over which lead is absorbed, the resultant anemia can cause fatigue, dizziness, weakness, irritability, pallor, rapid heartbeat, and shortness of breath. There are also neural, kidney, hormonal and liver functions that depend on the same processes as those used to make heme.
Cardiovascular diseases can have many causes and risk factors. Lead exposure may contribute to the onset and development of the disease, though there is little hard evidence to suggest the role it plays. Several studies show elevations of blood pressure in those who have been exposed to lead. The cumulative estimate for rise in systolic blood pressure lies around 1-2mm with each doubling of blood lead, or a 1-2 percent variance in blood pressure. Additionally, cardiovascular diseases seem to be exacerbated by kidney diseases (and vice versa) in a way that makes the two conditions difficult to link separately to lead poisonings. While reducing lead exposure may curb the effects on a patient’s kidneys, it may not save them from developing cardiovascular problems. One study found that adults who had been exposed to lead as children had a significantly higher risk of hypertension 50 years later.