Lead’s use may be – but we still have a lead problem, especially in urban areas.
The first extraction of the metal lead from ores was ancient – around 7,000 BCE. In the millennia since, Egyptians have used it in cosmetics, Romans in their pipes, the British in their ammunition, and now every society in lead-acid batteries.
And, if it were not so toxic to humans, the use of lead would still be widespread in our daily lives.
Lead is a “heavy” metal, meaning it’s a dense element. Lead is also soft, malleable, corrosion resistant and distinguished by a low melting point. That’s what gives it its useful characteristics.
Yet lead is a highly poisonous metal. Its presence disrupts almost every organ in the body if inhaled or swallowed. Lead displaces other metals in the body, such as calcium and iron, disrupting chemical reactions. The most problematic effects are on children. By mimicking calcium, lead can enter a child’s developing brain and disrupt the functioning of mitochondria.
Currently, there is no known “safe” level of blood lead concentration in children. Since 1960, the Center for Disease Control’s advisory level for blood lead has dropped from 60 micrograms per deciliter to 5 micrograms per dL.
In the United States, risk of lead exposure is becoming more of a thing of the past. In the 1970s, lead-based paint was common, as it increased the paints’ durability and sped drying. It was also used in “leaded” gasoline, which made car engines of that generation (and prior) work more smoothly.
Unfortunately, the lead from gasoline was also sent into the atmosphere through car exhaust. It landed in the soil everywhere. The higher the concentration of cars – like cities and highways – the more lead.
Lead paint is a big risk for children. Medical studies showed lead exposure affected learning abilities and other health issues – especially in children. Lead paint that peeled off – or created dust – could be ingested or inhaled. This is especially true close to exteriors of older homes, around windows and other locations where lead paint was used.
Luckily, the U.S. banned manufacture of lead-based paint in the late 1970s. The EPA mandated a nearly 100% reduction of lead in gasoline by 1986. Between 1980 and 1991, mean blood lead levels of children ages 1 to 5 dropped by 77%.
Despite the phase out of lead in paint and gasoline, urban soils remain recognized as a leading source of lead exposure. That also goes back to lead being a heavy metal. That character means it tends to accumulate in soils and remains bioavailable for long periods of time.
Once deposited, lead remains strongly bound to clay and organic matter in the topsoil. Lead is not taken up in substantial amounts by plants, nor does it easily leach or migrate further down in the soil. Instead, this lead remains as part of the reservoir of urban soil and dust, susceptible to resuspension during dry periods. This resuspension is why children’s blood lead levels are believed to peak during the summers and reach a minimum in the winter.
There is some good news. A recent study in New Orleans reported an approximate 45% decline in soil lead over the span of 15 years.
Currently, a group of students and faculty at Duke University are mapping soil lead concentrations in Durham, North Carolina. We’ve collected street-side soils along 40 km of roadways in the city, and we’re currently sampling over 60 houses throughout the city. The results show there is still widespread lead contamination from before regulations went into effect.
Your average soil will naturally contain about 25 parts per million (ppm) of lead. Street-side soils in Durham currently have 245 ppm of lead on average. Sites that are across from old gas stations, fire hydrants, and older buildings have lead levels up to 3,000 ppm. Our only point of comparison is a 1976 study, which found 2850 ppm of lead in street-side soils.
This means that in some parts of Durham, lead levels may have dropped 90% from their peak values. Our results suggest urban soil lead is on average declining in Durham, but hotspots of contamination persist.
While the primary sources of lead emissions are in the past – leaded gasoline and lead-based paint – urban soil lead contamination is not. By mapping urban soil lead levels, we have a greater chance of making childhood lead exposure become ancient history.
Answered by Anna Wade, Duke University
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