In a new study, researchers assessed chickens in 55 residential homes across urban Sydney. Lead levels were investigated in the chickens, and their eggs. Contaminants were also measured in drinking water, chicken feed and soil, for every location. The average level of lead in eggs from the backyard chickens in the study was 301 µg/kg, and it was 7.2 µg/kg in the nine commercial free-range eggs analyzed. The researchers noted, that there are no global standards for safe levels of trace metals in eggs, the findings revealed eggs from backyard hens contained up to 40 times more lead than what can normally be found in commercial products.

The new research also revealed that contaminated soil was the main source of lead exposure for the backyard chickens. Lead concentrations in chicken blood and eggs could be directly linked with lead level in surrounding soil.

A previous study looking at egg lead levels from chickens in New York City community gardens indicated 100 µg/kg is probably a good threshold for safe human consumption. More than half of the eggs analyzed in the new study exceeded this threshold.

Thus, to keep eggs below the safe 100 µg/kg threshold, the researchers estimate lead in soil needs to be under 117 mg/kg. This is around four times lower than the current US Environmental Protection Agency (EPA) guidelines for safe lead levels in residential homes. However, the EPA does recommend significantly lower soil lead levels for gardening or food-related cultivation.

Deeper analysis of the data showed older homes were much more likely to have high lead levels across soils, chickens and their eggs, since they are most at risk of legacy contamination from the former use of lead-based paints, leaded petrol and lead pipes that has been already confirmed by other studies.