a 1980 report to the National Academy of Sciences, geochemist Clair C. Patterson
wrote an astonishing sentence: Sometime in the near future it probably
will be shown that the older urban areas of the United States have been rendered
more or less uninhabitable by the millions of tons of poisonous industrial lead
residues that have accumulated in cities during the past century.
Fourteen percent of all children in New Orleans and 25 percent of children in its inner-city areas are exposed to potentially dangerous levels of lead in the soil. The children either directly inhale the lead particles or are exposed from hand-to-mouth contact in play areas. Projects in New Orleans and elsewhere are trying to reduce and eliminate this risk. All images courtesy of Howard Mielke.
That grim prediction is proving true, with lead poisoning now endemic in inner-city areas of major U.S. cities. In inner-city New Orleans, for example, a quarter of all children are excessively exposed to lead, according to recent studies.
Although the United States has seen a significant decline in lead poisoning as a result of bans on lead in paint and especially as an additive in gasoline, leads legacy remains in the soils of our cities. Fortunately, geoscientists and others are coming up with ways to combat this health problem and close the book on lead poisoning in the environment.
From mines to soils
While many industrial products contain lead, two products stand out: lead-based paint and lead additives to gasoline. In the United States, the gross weight of white lead used as pigment in paint from the late 19th century to 1978, 6.6 million tons or 13.2 billion pounds, equaled the lead used in tetraethyl and tetramethyl additives to gasoline between 1929 and 1986.
The peak use for lead-based paint occurred in the 1920s, when the U.S. economy was shifting from agrarian to industrial. At that time, rail transportation was the main means for moving goods and providing services, and lead-based paints were applied to homes across the nation. The legacy of lead-based paints still remains as thin layers on older painted buildings, which, when intact, do not readily transfer exposure. But deterioration or haphazard removal of the paint, such as from power sanding, contributes tiny particles of dust that contaminate the local environment.
By 1970, the U.S. urban economy relied on motor vehicles and highways for transportation, and a second and higher peak of lead appeared in the environment. In contrast to lead-based paint, combustion of lead gasoline emitted tiny particles of dust that are extremely potent: Children either directly inhale them or are exposed by hand-to-mouth contact when the particles accumulate in a childs play area.
The automobile became a delivery system of lead into the urban environment because of two features of the modern city. First, the ground transportation system evolved to become dominated by private automobiles fueled by gasoline with lead additives. Second, the societal relocation to the suburbs combined with a daily suburban-to-inner-city commute by motorists distributed the metal contained in the fuel along the traffic routes into the cores of U.S. cities. By these processes, the emission of fine aerosol lead particles became concentrated in the most densely populated areas of U.S. cities.
Although lead additives to gasoline were banned for highway use on Jan. 1, 1986, the lead emitted for over 50 years still remains in the soil. If undisturbed, the metal accumulated in soil is expected to remain for hundreds of years. It will be a permanent feature of cities unless something is done about it.
Now recognized as a major public health achievement, the U.S. rapid phase-out of leaded gasoline resulted in a remarkable reduction in childhood lead exposure. The Centers for Disease Control and Prevention set the health guideline for lead levels in the blood at a maximum of 10 micrograms per deciliter. Prior to the rapid reduction, around 15 million children annually 9 out of 11 U.S. children had blood-lead concentrations that reached the maximum or greater. After the ban, less than 1 million children 1 out of 11 U.S. children had blood-lead concentrations of 10 micrograms of lead per deciliter or more, and the number of children above that threshold has been declining steadily. Still, the fact remains that lead poisoning is rampant in U.S. cities where children of poor, African-American and other minority families live. The legacy of accumulated lead continues to poison a significant portion of the U.S. population.
The U.S. Environmental Protection Agency states that a hazard exists when lead is greater than 400 parts per million (ppm) in bare soil for childrens play areas, or 1,200 ppm average for bare soil in the rest of the yard. In Europe, the lead standards for residential soils range from 40 to 150 ppm, and in Canada, the soil guideline for children is 140 ppm. This difference in standards begs the question about margins of safety for accumulated lead and whether or not U.S. children are being adequately protected from the range of health effects of lead poisoning.
These effects include increased blood pressure, kidney dysfunction, diabetes and cataracts. Adverse health effects, such as impaired hearing acuity and interference with vitamin D metabolism, have also been observed in blood at levels of 10 micrograms of lead per deciliter or less.
In the general population, the highest risk of lead poisoning occurs among the developing fetus and children because of their immature physiological development and developmental stage. Blood-lead concentrations as small as 2 to 5 micrograms of lead per deciliter have been associated with neurological impairment of children, and larger blood-lead concentrations are inversely correlated with performance on standardized intelligence tests. Chronic childhood lead exposures of 10 to 15 micrograms of lead per deciliter can be significant in decreasing learning ability (shown in low performance scores) and increasing delinquent behavior in later life (see sidebar, page 25).
A sink and a source
Air-deposited lead and other metals are integrated into soil, where they remain
a relatively large and active reservoir for exposure. Studies in Maryland, Minnesota,
Indiana, New York and Louisiana demonstrate a consistent pattern of soil-lead
concentrations in urban environments that are related to city size and community
location. Soil-lead concentrations diminish with distance from the city center.
For Baltimore, Md., Syracuse, N.Y., Indianapolis, Ind., Minneapolis-St. Paul,
Minn., and New Orleans, La., the largest contamination (around 500 to 1,000
ppm) is clustered in residential neighborhoods within the inner center; less
contamination occurs in suburban areas (50 to 200 ppm); and the smallest concentrations
occur in rural areas (10 to 25 ppm). The probability that the inner-city clustering
of lead could be due to chance is extremely small. Soils in old communities
of large cities have lead concentrations that are 10 to 100 times higher than
similarly aged communities in small cities.
At Xavier University in New Orleans, we have set out to evaluate the inhabitability of urban areas containing 500 to 1,000 ppm or more lead. We concur with results from other studies that one of the most common routes of exposure to lead is through hand contact with contaminated dust and soil and subsequent ingestion by hand-to-mouth activities.
In 1996, we studied lead concentration on childrens hands at daycare centers in various parts of New Orleans. In the inner city, an average of approximately 30 micrograms of lead was measured on childrens hands after outdoor play, or six times more lead than they picked up while playing indoors. These childrens hands held five times more lead than the 6 micrograms per day total considered to be the tolerable daily intake of lead for children younger than six years old. In mid-city New Orleans, smaller amounts were picked up both indoors as well as outside. The amounts of lead on childrens hands after play were directly related to the amounts of lead measured in the outdoor soil.
In New Orleans, we also identified the significant association between soil lead and blood lead. A soil lead map is a better gauge for predicting childhood lead exposure than a map of house ages, because old paint is only one source of the lead problem. About 25 percent of the children younger than six years old living in inner-city New Orleans exhibit blood-lead levels 10 micrograms of lead per deciliter and higher. Exposure is not evenly distributed throughout the city. Exposure is endemic to old neighborhoods that have a long history of abundant traffic flows and also have houses with lead paint that are often renovated by power sanding, releasing fine particles of lead dust into the urban air.
In collaboration with researchers from Syracuse, N.Y., and Indianapolis, Ind., we have also found that the seasonal exposure to lead is strongly and inversely associated with soil moisture. The lower the soil moisture, the more lead dust is picked up by children from the soil and the higher the childrens blood lead. Few cities have been mapped for lead and other metals to the extent that New Orleans has, and therefore healthcare providers elsewhere have limited information about the geochemical conditions that confront them. Our study in New Orleans, however, has identified problems and yielded promising solutions.
An alluvial alternative
New Orleans is geologically intriguing because of its location on the Mississippi River Delta. Its geology is directly tied to the river. Near-surface bedrock does not exist in New Orleans and the city is located on a deep mantle of alluvial sediments. The Mississippi is the third largest river on Earth and drains 2,914,527 square kilometers (1,125,300 square miles), or about 41 percent of the United States and parts of three Canadian provinces. The quality of fresh sediments provided by the Mississippi is the basis for establishing the current natural metal concentrations of the alluvial soils of New Orleans. We determined the level of background metals by sampling fresh alluvium from within a flood protection structure, the Bonnet Carré Spillway, which was opened during the flood of 1997. The soils of New Orleans should be similar to the parent materials from which they are derived. Inner-city soils have a median lead content of 481 ppm. By comparison, modern alluvium contains exceptionally small quantities of metals (median of 4.7 ppm lead).
Researchers are depositing soils from the Bonnet Carré Spillway, upstream of New Orleans, on inner-city contaminated areas, to bury lead that lies in the top few inches of the soil.
The fresh alluvium provides critical information about the amount of lead that has accumulated in the urban core since the early 18th century, when New Orleans was first settled. It shows that the redistributed and accumulated lead in New Orleans reflects anthropogenic activities and not what is expected from naturally occurring geological processes.
The sediments themselves also provide a partial solution to the problem, by covering and diluting contaminated community soils, and providing a clean soil surface that would assist in preventing exposure of children to toxins in their current environment. Thus, we undertook a soil project called Recover New Orleans, focusing on three of the 10 communities where median soil lead exceeds 1,000 ppm. Fifteen properties with houses and 10 vacant properties that were scheduled for housing construction were enrolled in the project.
We measured the soil lead at the beginning and end of the project. The median soil-lead content prior to remedial work was 1,216 ppm. One hundred truckloads of soil were hauled in from the Bonnet Carré Spillway to spread on the properties, with the soil at least 6 inches (about 15 centimeters) deep. In the end, we hauled in 1,710 cubic yards, weighing about 750 tons equal to 2.5 minutes worth of sediment transported by the Mississippi through New Orleans.
After the project was completed, the median soil-lead level on all 25 properties was 6 ppm (with a narrow range of 3 to 18 ppm). These decreases in soil lead were accompanied by significant reductions of indoor floor lead and, in one child, resulted in a decline of blood lead from 17 to 9 micrograms per deciliter.
The next phase of the project will be to measure soil-lead changes over the next year to evaluate the long-term success of the pilot project. I suspect that when soils become dry in New Orleans there will be increased atmospheric transport of lead dust from the massive reservoir of lead in soil in the surrounding area, which in turn will recontaminate the new soil. If that occurs, then it means that much larger areas of the city will need to be remediated with low-lead soil in order to render the city habitable again.
Evidence in support of Clair Pattersons prediction of urban inhabitability comes from cities around the world. In Nordic countries and Germany, earth science agencies have begun collecting, analyzing and mapping the geochemistry of urban environments. Urban geochemical maps provide information to health agencies that can be used for identifying problem areas and developing primary prevention strategies to improve the quality of urban life.
Such management strategies have been tested in trials in Minnesota and British Columbia. The pilot project conducted in Minneapolis-St. Paul evaluated the hypothesis that soil was a major reservoir for exposing children to lead. Researchers found a seasonal increase in lead in childrens blood during summer months and a corresponding decrease during winter months. Childrens blood lead was measured at the beginning and end of the project. The communities were selected where soils contained 500 to 1,000 ppm of lead. In the target community, interventions occurred to reduce childrens exposure to bare soil, including establishing plant cover, topping soil with wood chips, placing paving stones on barren parking areas and providing sandboxes with clean sand. A control community of children only had blood-lead measurements at the beginning and end of the project, without interventions.
At the end of the summer, the children in the target community did not experience the expected seasonal blood-lead increase, while children living in the control community experienced a substantial summertime blood-lead increase. These results indicate that accumulated lead in soil is a reasonable explanation for the seasonality of childhood lead exposure.
Another well-documented example of remedial activities is reported from Trail, British Columbia, the site of the largest lead-zinc smelter in North America. Many neighborhoods in Trail contain 500 to 1,000 ppm soil lead (similar to the quantity of soil lead found in U.S. inner cities). In the 1980s, many Trail children exhibited elevated blood-lead levels.
The Trail lead-prevention program advanced the Minnesota pilot project model by using a stakeholders approach to solve the community lead problem. In addition to measures like those undertaken in Minnesota, extensive efforts focused on educating parents and childcare providers about washing hands, establishing clean outdoor play areas, and reducing the smelter lead emissions. The outcome of the last measure was particularly significant in reducing the blood-lead content of the children of Trail.
If existing knowledge and skills were systematically applied to managing environmental contamination, lead poisoning in the United States could be substantially reduced. In addition, understanding the anthropogenic processes associated with lead is instructive in preventing urban contamination by other toxic substances, to ensure that Earth is suitable for healthy habitation by future generations.
For several decades, the medical and scientific community has accepted
that elevated levels of lead in blood can cause adverse health and developmental
issues in children, such as anemia, hearing problems, lowered intelligence
and slowed growth. Research over the past few years, however, has revealed
another problem associated with lead: The soft metal may be one of the
most significant causes of violent criminal behavior in young people.