Lead in environmental health

(Second of three parts)

Part 2: Bioavailability and toxicity of lead

The death of the great German composer Ludwig van Beethoven in 1827 at age 57 in Vienna, Austria was largely attributed to lead poisoning. The autopsy indicated that he suffered from cirrhosis of the liver — an indication that he had excessive alcohol intake, apparently from addictive wine drinking. It was confirmed recently by scientists at the US Department of Energy’s Argonne National Laboratory near Chicago, Illinois that indeed he succumbed to lead poisoning as large amounts of the metal were found in his skull bone fragments, confirming the long-term accumulation of lead in his body.

For any chemical substance to be of any consequence to human health, it needs to be bioavailable when taken internally by oral ingestion or inhalation — the most common exposure routes. However, before becoming bioavailable, it will have to cross a physiological membrane (or barrier) and be circulated systemically. This physiological filter, so to speak, is represented by the digestive or gastro-intestinal (GI) tract. Once absorbed (signifying bioavailability), lead is distributed primarily among three body compartments — blood, soft tissue (kidney, liver, lungs, spleen, heart, muscles, and brain), and mineralizing tissue (bones and teeth). Mineralizing tissues contain about 94 percent of the total body burden of lead in adults. Lead has a biological half-life in the blood of about 25 days; about 40 days in soft tissue; and much longer at about 25 years in bones. Thus BLL may return to normal after a single exposure. The body accumulates lead over a lifetime and releases it slowly, so that even small doses over time can still cause lead poisoning.

The amount of lead from soil, dust, and paint that can reach the GI tract depends to a large extent on the gastro-geochemistry of these materials in the stomach, which in turn depends on the chemical form of the lead. Our stomach has acidic pH which favors dissolution of lead minerals found in soil, dust, and paint. An empty stomach (say after fasting) may have a pH below 2 that favors absorption, rising to above 4 after eating.  However, the pH rises again close to neutrality along the small intestine that may shift the chemical dynamics, causing possible reformation of lead minerals.

The solubility and eventual bioavailability of lead largely depends on its chemical form (chemical species is more technically correct). Lead from petrol used in internal combustion engines, like tetraethyl lead, is present as lead halides (e.g., PbBrCl) in fresh exhaust, but alters to lead carbonates, oxides, and sulfates upon atmospheric transport, mediated by photochemical reactions and depositing in soil, mostly in these forms.

The most common forms of lead in paints are red lead (oxide form), white lead (also known as basic lead carbonate), basic lead sulfate, and lead chromate. The lead halides (like the chlorides, bromide-chlorides) and hydroxides are the most bioavailable forms, with the carbonates, sulfates, chromates and the oxides being less bioavailable, and with the lead sulfide being the least.

In adults, only about 10 to 30 percent of ingested water-soluble lead salts on the average are absorbed by the GI tract. Absorption is higher in infants and pregnant women — as much as 50 percent of the total intake. The bioavailability of lead could be enhanced by malnutrition like deficiencies of calcium, iron, zinc, and vitamin C — not uncommon among the poor.

The extent of lead bioavailability in humans is easily assessed by measuring the amount of the metal in blood (blood lead level or BLL). The current threshold level of lead in the blood set by the WHO is 10 micrograms per deciliter, or 100 parts per billion.

Lead can damage any organ in the body but it is particularly damaging to the central nervous system (CNS) (the brain and spinal cord) and red blood cells (with hemoglobin carrying oxygen). Lead is toxic to both adults and children. It is particularly dangerous for children younger than seven years because they are still growing and their nervous system is still developing. Childhood (or pediatric) lead poisoning causes irreversible health effects — reduction in intelligence, loss of short-term memory, hearing loss, learning disabilities, problems with coordination, and behavioral problems like aggression and violence. Prenatal exposure can cause reduced birth weight and immune suppression, which could explain why some children could develop asthma and allergies. Adults may also experience CNS effects even at relatively low BLL, manifested by subtle behavioral changes, fatigue, and impaired concentration.

Lead poisoning costs society an enormous financial burden. Children affected are likely to drop out of school and more prone to get involved in smoking, drug abuse, crime, and possess lower earning potential than normal kids. Dovetail the sacrifice and financial drain from the parents to the health care that they need and the amounts become staggering. In the US alone, childhood lead poisoning was estimated to cost some $43 billion a year until the year 2000.

Lead poisoning has been declared by the Center of Disease Control and Prevention as the most prevalent but preventable environmental disease in children. While the number of children (one to five years old) in the US having elevated BLL has precipitously dropped to only 310,000, today after limited use of leaded petrol in 1989, the number in some developing countries is still alarming due to continued exposure to diverse anthropogenic sources (e.g., battery recycling, flour mill, leaded ceramics, soldered can food, etc.), lack of comprehensive environmental regulations and their enforcement, poor public awareness, lack of systematic blood monitoring, and unaffordable health care.

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Domy Adriano is a professor emeritus at the University of Georgia’s College of Agricultural and Environmental Sciences, and the Savannah River Ecology Lab. He has had academic stints at Kansas State University, University of California, Riverside, and Michigan State University. As complement, see Adriano (2001), Trace Elements in Terrestrial Environments: Biogeochemistry, Bioavailability and Risks of Metals. Springer, N.Y., 866 p. (cited over 900 times according to Google Scholar). E-mail him at [email protected].