Lead in the Inner Cities by Howard Mielke

1. Introduction / 2. Sources of Lead / 3. Health Consequences / 4. Re-evaluations / 5. Children at Risk / 6. Prevention

4.  Re-evaluations  [1,400 words]

Until the 1970s in the United States, the health guidelines for lead exposures were essentially the same for children as for adults. In 1979 guidelines for both the amount of lead and the differences in vulnerability between children and adults were revised when Herbert Needleman at Harvard Medical School reported that children’s cognitive abilities were affected by much lower lead exposures than previously recognized.

At about the same time that Needleman was doing his work, my colleague Rufus Chaney and I were conducting a study at the U.S. Department of Agriculture in Beltsville, Maryland, of the lead content of garden soils of Baltimore, Maryland. While we were performing the field studies, supervisors and other people frequently reminded me that the major source of lead in soil is lead-based paint. When we pored over the results of our analytical determinations, we came up with a problem of statistics. The distribution of the numerical results did not follow a normal curve. The statisticians I initially worked with wanted to truncate the results so that they conformed to the normal distribution.

In essence, the highest numbers were being removed from the database to satisfy the requirements of the parametric statistical models being used at that time. My brother, Paul Mielke, is a professor of statistics at Colorado State University, and I called on him for help. Paul and Ken Berry, also at Colorado State University, had recently completed their work on a non-parametric statistical test, Multi-Response Permutation Procedures, which provided a method for testing the kind of data that I had.

Paul had an appropriate nonparametric statistical model but no data base to analyze; I had the data base with no model to evaluate it. Serendipity played a role in our initial realization about the extraordinary accumulation of lead in urban environments. The research results were surprising. The pattern of lead did not match what we expected to find if paint alone was the major source of lead in Baltimore soils.

Our data did not support the lead-based paint hypothesis. Our observation was that in the inner city, where the soil-lead concentrations are highest, Baltimore had mostly unpainted brick buildings, since its inner city underwent a huge fire in 1904. On reconstruction, the new building codes required the use of fireproof (hence, brick) construction materials.

The sites of old housing constructed with painted wood siding are located in outlying parts of the city, where the lead content of garden soils is lowest. The highest lead-containing garden soils appeared to be associated with the inner-city location, not the presence of painted wood structures. The geographic pattern was extremely strong. Because of this simple observation, I sought an alternative source of lead.

Leaded gasoline fit the requirements for that alternative. We learned that gasoline was a huge source of lead, and we predicted that it was exhausted in a pattern that corresponded to the flow and congestion of traffic. The environment of the city was undergoing lead accumulations because of the daily commuter traffic flows that strongly characterize the industrial way of urban life. We suspected that the lead particles released with gasoline exhaust could travel through the air until they hit a barrier, such as the side of a house or apartment building. We also imagined that these lead particles would be washed down the sides of the buildings and into the soil anytime it rained. Based on this scenario, we would expect the greatest accumulations of lead to be found in the soil surrounding the foundations of buildings. We also thought it was possible that lead accumulated in soil was a major contributor to the childhood lead problem of Baltimore.

Our findings were published in 1983. Did the automobile act as a toxic-substance delivery system in all large cities? Critics of our publication argued that Baltimore is an unusual city because of all of the heavy industries there. They asserted that industrial emissions, and not leaded gasoline, accounted for the soil-lead accumulation problem in Baltimore. In 1979 I moved to Macalester College in St. Paul, Minnesota. I received a small grant to study St. Paul soils. The family of one of my students, Handy Wade of Boston, funded the purchase of analytical equipment to determine the lead content of soil samples. By 1984, the results were available.

We found that St. Paul and Minneapolis, where, in contrast to Baltimore, there is no heavy industry, had the same inner-city soil-lead accumulation as first observed in Baltimore. Even the quantities were similar. We also studied small towns and large cities and found that city size was a more important factor than age of city.

The soil-lead concentration in old communities of large cities is 10 to 100 times greater than comparable neighborhoods of smaller cities. In addition, soil-lead concentrations diminish with distance from a city center.

For example, in Baltimore, the highest garden-soil contamination is so tightly clustered around the city center that the probability that this distribution could be due to chance is 1 in 1023. The concentration of lead in the soil of the Twin Cities (Minneapolis and St. Paul) is 10 times greater than that in adjacent suburbs that have old housing, where lead-based paint concentrations are as high. In 1988, I joined the faculty of Xavier University in New Orleans. I repeated the soil-lead studies conducted in Minnesota and obtained similar results. Soil-lead concentrations were higher in congested high-traffic, inner-city regions of New Orleans, as compared with its older suburbs and with smaller towns. Although the age of the houses in these communities is similar, the traffic flows vary widely.
In New Orleans, with a population of about 500,000 people, the daily traffic count at major intersections in the inner-city averaged about 100,000 vehicles in 1970. In contrast, in a small town, such as Thibodaux, with its population of about 14,000, the busiest intersections averaged only about 10,000 vehicles a day in 1970. These data strongly suggested to us that proximity to a high-traffic route is a better predictor of soil-lead concentrations than is the age of the buildings in the area or the amount of lead-based paint in the buildings. We further speculated that the soil of larger cities is more concentrated in lead than is the soil of smaller cities because of the greater volume of traffic in larger cities and because each vehicle remains inside the larger city longer than it would inside a smaller city.

In New Orleans, we tested our hypothesis that soil-lead accumulations do indeed follow traffic volumes with a comparison of large versus small cities. To do this, my colleagues and I calculated the quantities of lead emitted within one-half mile of major intersections of the inner-city regions of New Orleans and Thibodaux using the average daily vehicle traffic records during the peak lead-use years. Both of these communities have older housing with lead-based paint located around the community core, but their traffic flows differ by a factor of 10.

Next, we estimated the lead emissions in these two communities, basing our calculations on available records of vehicular use of gasoline during the years in which lead gasoline was in use, which we applied to the traffic volumes in the two communities. From our calculations, we determined that during the peak years, 5.15 metric tons of lead was emitted annually in New Orleans, as opposed to 0.45 metric ton in Thibodaux -- less than one-tenth of the New Orleans concentrations. The distinctive pattern of lead that remains in cities has created “urban metal islands.” A consequence of the accumulation of lead in urban communities is that a widespread environmental health hazard is now imposed on a large segment of society.

The fact that the lead is not distributed uniformly throughout the city provides strong support for our hypothesis. Within the cities we studied in Maryland, Minnesota and Louisiana, the greatest amount of lead can be found in the central part of the city where the highway networks concentrate traffic. In effect, urban highways can be considered inadvertent dispersal systems of lead in densely populated areas surrounding the city center. Some people may argue that since lead has been removed from gasoline, it no longer poses a threat to children’s health. But the clays and organic matter in soil weakly bind lead, and our research findings give us reason to believe that lead remains in the soil of the inner cities for a long time. Our results refute the commonly held notion that lead concentrations are highest in all areas with older housing.

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Lead Unit Intro