In
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,
lead’s 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 child’s 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.
Health effects
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 children’s
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 children’s 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 children’s hands after outdoor play,
or six times more lead than they picked up while playing indoors. These children’s
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 children’s 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 children’s 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.
Global treatments
Evidence in support of Clair Patterson’s 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 children’s blood during summer months
and a corresponding decrease during winter months. Children’s 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 children’s 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.
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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. Megan Sever |
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