Lead in environmental health

 (First of three parts)

The recent massive recall of toys made in China by Mattel and other toy retailers was compelled partly by leaded paints used in them. The main concern was the potential for lead poisoning in children. Since China manufactures about 80 percent of the world’s toys that are widely distributed, this article is timely and aims to promote public awareness of the possible health consequences this toxic metal could cause in humans. This article is serialized in three parts.

Part 1: Exposure to lead

Lead is a heavy metal that occurs naturally in the earth’s crust but is generally insoluble and immobile. However, it is found in soil, plants and water in trace amounts. It has no known biological importance in any organism; rather, it is a toxin. Because society needs this metal for many reasons, its ores are mined, smelted, refined and manufactured for a myriad of applications. It is these operations that cause widespread contamination of the environment. Lead is viewed as the most important environmental metal.

Aerial deposition of this metal from mining and smelting operations is by far the most important contributor to global contamination. It is emitted to the atmosphere in the form of aerosol or particulate matter that remains suspended for long periods of time until deposition on the earth’s surface. In an extensive survey of farms in England and Wales in the late 1990s, atmospheric deposition accounted for more than 70 percent of the total lead input, followed by sewage sludge with only 18 percent. Thus lead is ubiquitous in nature because of atmospheric transport.

Lead has probably been used by mankind for at least 6,000 years. Lead mining is believed to predate the Bronze Age, with the earliest recorded lead mine in Turkey about 6,500 BC. It has several attractive properties: low melting point, easy workability, corrosion-resistant, forms alloys easily, holds pigments well, rather inexpensive, easily recyclable, etc. The earliest lead mining and smelting operations were done by the Romans for centuries, unsurpassed till the Industrial Revolution.

The Romans, being the earliest heavy users of lead, were also the first widely affected by lead toxicity. The affluent class of Roman society suffered from gout, then believed to be due to excessive lead intake. This lead poisoning (known also as plumbism from the Latin plumbum for lead) was induced by their use of leaded cooking utensils and pots, wine urns and decanters, plumbing, vessels to store grape juice, cosmetics and medicines, etc. Wines stored in these vessels were shipped all over the Roman Empire. Presumably, the acidity and organic acids in the wine promoted dissolution of the lead from the vessel. Summed up from all the sources, the Romans were consuming up to two orders of magnitude of lead per day than did an average American in the 1980s. (Some scientists believe this high lead exposure adversely affected the intelligence and well-being of many Romans, thus contributing to the fall of their empire.)

In early civilizations, lead poisoning commonly occurred in upper classes of society due to their more luxurious lifestyle. In contrast, this disease is more prevalent among the poor in modern times. For example, in the US, minorities often live in poor inner city neighborhoods with older housing (built before 1950) having lead pipes and paints.

Today, lead intake by humans comes from two major anthropogenic sources: environmental and occupational exposure. The former is more important for the general public, while the latter is confined to work settings. For the general population, the exposure, measured as dose, comes from the soil, water, air, food, and skin contact. The total dose is measured as milligrams (of lead) per kilogram body weight per day. The soil represents orally ingested soil contaminated with lead from leaded petrol emission and/or leaded paint chips and dust; water represents drinking water from leaded plumbing; air represents inhaled aerosol/particulate matter enriched in lead; food represents ingestible materials tainted with lead (dusty vegetables, canned food, etc.), and skin contact from occupational chores.

With children — the most sensitive segment of the population, especially those younger than seven years — oral ingestion and inhalation are the most important exposure routes. Children younger than three years are more exposed through their normal hand-to-mouth activity. Inhalation and skin contact are the most important routes for workers.

The most likely troublesome exposure scenario is in inner city neighborhoods due to leaded paint dust and chips. Health experts believe that deteriorated lead-based paints in older homes is the primary exposure source for young children as lead dust settles on their toys and the floor. Dust brought into the house from footwear is also a major source. Until the 1990s, lead exposure represented an epidemic in Baltimore, Maryland, striking more than 7,000 children a year and was believed to be a contributing factor in the city’s crisis of violent crime, failing schools, and disintegrating neighborhoods.

In developing countries, lead businesses are often interspersed within neighborhoods. The case of lead smelters to recycle batteries in Metro Manila is a classic example of heavy exposure of workers and their families to the toxic metal — resulting in blood lead levels (BLLs) of over 50 micrograms per deciliter, higher than those for radiator workers.

Because leaded petrol is the single most important source of lead exposure, its use in transportation was limited in the 1990s in most developed countries (in 1989 in the US). In the Philippines, a presidential decree to ban the use of leaded petrol was issued in 1985. According to the US Center of Disease Control and Prevention, children aged one to five with elevated levels of lead in their blood (threshold level set by the WHO is 10 micrograms per deciliter) are estimated at 310,000 today, down from three million children in 1970 when BLLs became alarmingly high.

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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]