The most common urban legacy contaminant is lead (Pb) due to its ubiquitous historical use in gasoline, paint products and industrial activities for most of the 20th Century. A legacy of Pb pollution in the soil continues to impact soil and human health as Pb does not break down and remains in soil for millenia. All soils contain background levels of Pb at relatively low concentrations due to naturally occurring soil lead minerals. Average U.S. background soil Pb 22 mg kg-1 Pb . In central Ohio the maximum background Pb concentration is around 41 mg kg-1, and soils with Pb concentrations above this are contaminated. Even though Pb content is elevated, a contaminated soil may or may not pose a risk to human health. When soil Pb concentrations and exposure are sufficient to pose a health risk, the soil is polluted.
Potential health risks of soil Pb exposure
Toxic to all organ systems, Pb is particularly damaging to the kidneys, cardiovascular system, and nervous system. Childhood exposure to Pb is of particular concern, as exposure during key developmental periods can result in permanent neurological impairment. Though the phaseout of leaded paint, gasoline, and solder greatly reduced the incidence of childhood lead poisoning, lead exposure is an ongoing concern for children today. Approximately 500,000 children in the US had elevated blood lead (>5ug/dL) in 2018 . Several studies recognize that exposure to polluted soil is associated with elevated blood lead in children . Exposure to soil Pb occurs primarily from incidental soil ingestion of dust particles transferred from hands or cleared from the airway. While a majority of urban soils do not have heavy metal concentrations that pose a significant risk to human health, Pb-polluted soils tend to be more widespread in older, larger cities compared younger, smaller cities or rural and suburban areas due to denser traffic, industry, and housing. . Lead poisoning rates are well above the national average in many cities .
Gardening in Pb-contaminated soils
Gardeners face additional exposure to soil Pb through incidental ingestion of dust while working in the garden, hand-to-mouth transfer, and through consumption of contaminated produce. Plants take up very small amounts of Pb through their roots and Pb-contaminated soil sticks to produce surfaces . However, soil with low levels of Pb contamination can be used without concern, and moderately contaminated soils can be managed or treated to reduce risk from incidental ingestion and plant uptake (See Managing and Treating Lead-Contaminated Soil)
 Smith, D. B., Cannon, W. F., Woodruff, L. G., Solano, F., Kilburn, J. E., & Fey, D. L. (n.d.). USGS Data Series 801: Geochemical and Mineralogical Data for Soils of the Conterminous United States. Retrieved from https://pubs.usgs.gov/ds/801/
 Ohio Environmental Protection Agency. 2013. Evaluation of Background Metal Soil Concentratiosn in Franklin County- Columbus Area. Retrieved from https://epa.ohio.gov/portals/30/vap/docs/Columbus%20Background%20Summar…;
 Centers for Disease Control and Prevention. Lead. (2018, October 10). Retrieved from https://www.cdc.gov/nceh/lead/default.htm
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