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Risk and Contaminated Soil

This page provides an overview of contaminant exposure pathways and describes factors that we can modify to reduce risk. Although this page focuses on lead (Pb) contamination, the dose equation and approaches are similar for most heavy metal and metalloid contaminants. See Managing and Treating Contaminated Soil for specific guidelines. 

Dose calculation

Soil metal exposure can be calculated with the above equation. This helps explain why children are the most vulnerable to lead poisoning. Children ingest higher rates of soil (SOIL INGESTION RATE). They also absorb more of the lead that they ingest (BIOAVAILABILITY). And because they are smaller than adults, they receive more lead relative to their size (BODY WEIGHT). All these factors contribute to children receiving a higher dose of lead from contaminated soils. In order to reduce the risk of a lead-contaminated soil to children, we can change some factors in the dose equation. A management strategy that addresses multiple pathways and exposure dependents will be more effective than focusing on fewer pathways. 

CONTAMINANT CONCENTRATION in a soil can be reduced by replacing or capping it with a clean, tested soil; deeply tilling soil to dilute contaminated surface soil with uncontaminated subsoil; and diluting soil by tilling in large amounts of organic materials, tested clean soil, or tested clean soil-like materials such as dredge. 

SOIL INGESTION rate can be reduced by tackling both intentional and incidental consumption of soil. Children may intentionally ingest soil (pica) while playing in soil. Children may also incidentally ingest soil through hand-to-mouth behaviors, especially toddlers. Observe children's play behaviors. Alternative play surfaces (sand boxes, outdoor carpet, rubber mats...) can reduce soil ingestion. Incidental ingestion primarily occurs in adults when the body filters soil dust from the respiratory tract and swallowed. Keeping soil moist and covered with plants, mulch, or other surface materials can reduce dust. Soil adhered to food crops can also contribute to soil ingestion. More soil adheres to leafy vegetables and root crops than fruiting vegetable or fruit crops. In any crop, mulching and windbreaks can reduce soil adherence. Finally, plants may take up contaminants through their roots. Leafy vegetables and root crops appear to accumulate more contaminants than fruiting vegetables or fruit crops. 

BIOAVAILABILITY: Bioavailability is the portion of soil contaminant that humans, animals, and plants absorb when they are exposed to contaminated soil. If a child ingested contaminated soil, highly bioavailable contaminants will be absorbed into the bloodstream, while poorly bioavailable contaminants would largely pass through the body. The availability of soil contaminants to organisms like humans, animals and plants depends upon the soil makeup and lead source material. However, soil lead can be made less bioavailable through the addition of amendments (see recommendations for Managing and Treating Contaminated Soil). Additionally, a well-balanced diet high in minerals like phosphorus, iron, and calcium and low in fat can reduce children's contaminant absorption.