Lead may be out of paint and gasoline. But it’s still in the soil.

The City of Flint Water Plant is illuminated by moonlight. A federal state of emergency has been declared in Flint due to dangerous levels of contamination in the water supply.
The City of Flint Water Plant is illuminated by moonlight. A federal state of emergency has been declared in Flint due to dangerous levels of contamination in the water supply. Getty Images

Every late summer, potentially toxic lead reaches peak concentrations in the blood streams of many Americans. Millions of blood samples demonstrate this distressing seasonal pattern. While historic data show that our parents and grandparents who lived in cities had higher lead in their blood than we do, the seasonality of summer-lead highs and winter-lead lows continues.

Despite a general decrease in lead exposure, a significant fraction of urban Americans continue to be poisoned and impaired by exposure to lead. A vast medical literature describes contemporary lead toxicity and resulting cognitive, behavioral, and physical dysfunctions that often originate during childhood.

While the city of Flint, Mich., brought attention to lead in a public drinking water system, lead exposure in America is more an urban soil and soil-air problem, not a water problem. Between the 1920s and 1980s, lead-based paints and leaded gasoline added enormous amounts of lead into the human environment. By the 1980s, growing concern about lead toxicity resulted in most lead being removed from paint and gasoline. At that time, little was appreciated about how soils would retain lead in urban environments.

Before the 1980s, lead-based paint and leaded gasoline greatly elevated lead in urban air and soil. Today’s paint and gasoline are virtually lead-free, but urban soils contain much of the lead formerly used in paint and gasoline. Because lead is tightly bound to soil particles, lead toxicity is a legacy problem.

Medical research shows that lead exposures are from direct contact with high-lead urban soil and urban soil dust, especially when summer days are windy and soils are dry. Thus, researchers explain the seasonal lead cycle of summer highs and winter lows in the human blood stream.

In a number of American cities, a person’s blood lead concentrations are correlated with lead concentrations in the soil of a person’s neighborhood. Lead exposure also occurs from household dust, which originates in part from foot traffic from neighborhood soil.

Children less than 2 years of age are particularly at risk. Crawling, inadvertent ingestion, vulnerable physiology, and small body size make young children the age group with highest blood lead concentrations.

The good news is that time is passing and soil legacies of lead-based paint and leaded gasoline are becoming a more distant memory. But while average lead concentrations in blood are slowly decreasing, the bad news is that soils have long memories and a number of city neighborhoods and people continue to be exposed. In the case of lead, urban soils effectively preserve the past.

A practical step to urban lead contamination is within the financial reach of most cities. Cities including Detroit, New Orleans, and Indianapolis all have maps of levels of soil lead. Maps identify hotspots where problems can be addressed, whether in neighborhoods with aged housing or neighborhoods undergoing new construction.

Whether a city’s soil is high or low in lead, where maps are made, people and cities benefit.

City management is guided by hundreds of maps — of floodplains, emergency routes, commercial zones, greenways, and bike routes. Soil lead maps can help address this soil and health crisis.

Every city needs a map of its urban soil lead, for when it comes to soil, as William Faulkner wrote, “The past is never dead. It’s not even past.”

Daniel Richter is professor of soils at Duke University’s Nicholas School of the Environment and Anna Wade is a Duke Graduate School student who studies the chemistry and mobility of lead in soils.