Lead Content of Soil, Plants, Foods, Air,
and Chinese Herb Formulas
by Subhuti Dharmananda, Ph.D., Director, Institute for Traditional Medicine, Portland, Oregon
Lead is the heaviest of the non-radioactive metals (atomic number 82; atomic weight 207) that naturally occur in substantial quantities in the earth's surface. Lead is present in all soils, rivers, lakes, and seawater. Despite its weight, lead is also in the air, a component of dust and of sea spray. Lead is present in the proportion of 16 parts per million (ppm) in igneous rocks, the most common ancient rock on the surface, and at an average of about 10 ppm in common soils that are far from sites of contamination; natural soils usually have less than 50 ppm of lead but are never lead free.
The most serious worldwide lead contamination has been due to the introduction of lead (in the form of tetraethyl and tetramethyl lead) into gasoline, starting in 1923, with billions of tons of lead released into the atmosphere in the vehicle exhaust. Most of the discharged lead lands on the soil, in the water, and on living organisms, particularly the plants that grow along the roadways. Although lead was phased out of gasoline in the U.S. in 1971, leaded gasoline is still being used in many countries, most notably in Africa. In Europe, the lead content of gasoline has been gradually reduced, but not eliminated. Leaded gasoline has been phased out in Austria, Denmark, Finland, Sweden and Switzerland, and restricted to a maximum 0.15 grams/liter (about 150 ppm) in most countries, compared to earlier levels that were up to five times that amount (750 ppm, with reductions to 320 ppm in the mid-1990s). China announced its intention to eliminate lead from gasoline at the end of 1999, limiting the average lead content of gasoline to 0.005 grams/liter (5 ppm, similar to the content of unleaded gasoline). The 5 ppm standard has been adopted recently by several other countries as well.
Even when lead additives are removed from gasoline, smaller amounts of lead continue to be emitted from automobiles, trucks, and other vehicles. In addition to some natural lead in gasoline, the gradual loss of fine particles of metal, rubber, and other components that contain lead contributes to pollution, particularly in urban areas with high traffic density.
Lead first enters the food supply in significant quantities by contamination of the soil and by lead dust accumulating on the plants. The lead is taken up by plants through roots and leaves. The plant materials are eaten by humans, and animals that are eaten by humans gain lead from the plants they eat. Although proponents of vegetarian diets claim that meat is particularly contaminated by heavy metals, plant foods tend to have higher lead levels than animal foods because they can absorb and retain more lead than most animals. Animals have a low absorption rate of ingested lead and have physiological mechanisms to discharge the ingested lead; much is sequestered into the bones, which are usually not consumed by humans. By contrast plants are efficient at absorbing soil lead and retain the lead that has been taken up. About 7% of the lead in soil is taken up by plants (excessive lead will kill off the plants).
Lead from the atmosphere that lands on soils has low mobility and tends to stay in the top inch of soil. Therefore, shallow-rooted plants, such as grasses and common vegetables, are particularly vulnerable to picking up lead contamination that originated in the atmosphere. One of the main dietary sources of lead is from grains (a type of grass), and whole grains are particularly high since the fibrous seed coat retains minerals; the second main dietary source of lead is vegetables; meats are usually third.
The moral of the story, eat more meat.
