The more than 400,000 Australians who keep backyard chickens will tell the truth: there’s nothing better than the fresh eggs you get from your hens. It’s not always just the freshness or flavor of their eggs that set them apart from those sold in shops.

In our recently published research*, we found that backyard hens eggs contained, on average, more than 40 times as much lead as commercially produced eggs. In our Sydney study, almost one-half of the hens had blood levels of information that were significant. About half of the eggs tested contained levels of information that could be harmful to consumers.

Even at low levels, lead exposure is considered harmful to human health. This includes, among other effects, heart disease as well as decreased IQ and renal function. In fact, the World Health Organization has that there is no safe amount of lead exposure.

How can you tell if this is an issue with the eggs that you get from your backyard hens or not? Lead levels in soil vary from city to city. Here are maps showing areas with high and low risks for hens, eggs, and lead in Sydney, Melbourne, and Brisbane.

The research reveals lead poisoning in backyard chickens and what it means for urban farming and food production. Contaminated soils in older homes near city centers can increase lead exposure through backyard hens’ eggs.

Chickens enjoy scratching and pecking at the soil. Lead from the ground can get into chickens through scratching and pecking. Shutterstock

What was the result of the study?

The majority of lead is absorbed by the hens when they scratch the soil and eat food.

We tested for trace metal contamination of backyard chickens, their eggs, and garden soils in 55 Sydney homes. We also investigated other possible sources of contaminants, such as animal drinking water and chicken feed.

The data confirms what we expected from the analysis of over 25,000 garden samples collected from Australia via the program. It is the contamination that causes the most concern.

Lead concentrations in the blood and eggs of chickens were significantly correlated with the amount of soil lead. In some samples, we found contamination that could have come from commercial feed and drinking water. However, this is not an important source of exposure.

Unlike for humans, there are no guidelines for blood lead levels for chickens or other birds. Veterinary assessments and research indicate levels of 20 micrograms per decilitre (µg/dL) or more may harm their health. Our analysis of 69 backyard chickens across the 55 participants’ homes showed that 45% had blood lead levels above 20µg/dL.

We tested eggs from the same birds. In Australia or worldwide, there are no standards for the trace metals found in eggs. In the 19th Australian Total Diet Study, a small amount of eggs purchased at a shop had lead levels less than 5ug/kg.

In our study, the average lead level in eggs was 301ug/kg. Comparatively, the level of lead in nine commercial free-range eggs that we tested was 7.2ug/kg.

According to international research, eating an egg per day that has a lead content of less than 100ug/kg will result in a blood lead concentration of less than 1mg/dL for children. This is about the same level as found in Australian children who do not live in areas where lead mines and smelters are located. The group used by Australia to investigate exposure sources is five ug/dL.

The threshold for “food safety,” 100ug/kg, was exceeded by 51% of the eggs that we analyzed. Our modeling of the relationship between soil lead, chickens, and eggs revealed that soil lead must be below 117mg/kg to keep egg lead under 100mg/kg. This is much less than the Australian guideline of 300mg/kg for residential soils.

For the protection of chicken health and to keep blood lead levels below 20ug/kg in chickens, soil concentrations must be less than 166mg/kg. This is also much lower than the recommended guideline.

How can we map the risk across cities?

Our garden soil trace metal data (more than 7,500 homes and 25,000 samples) was used to map which areas in Sydney, Brisbane, and Melbourne were most at risk of high lead levels.

Levels of lead risk for backyard chickens across Sydney. Dark green dots indicate areas with safe lead levels. Light green and yellow dots are areas over the safe lead level. Orange and red dots indicate areas with high grades. Map: Max M. Gillings, Mark Patrick Taylor, Author provided

Map of Melbourne showing levels of lead risk for backyard chickens. Dark green dots indicate areas with safe lead levels. Light green and yellow dots are areas over the safe lead level. Orange and red dots indicate areas with high grades. Map: Max M. Gillings, Mark Patrick Taylor, Author provided

Map of Brisbane showing levels of lead risk for backyard chickens. Dark green dots indicate areas with safe lead levels. Light green and yellow dots are areas over the safe lead level. Orange and red dots indicate areas with high grades. Map: Max M. Gillings, Mark Patrick Taylor, Author provided

A deeper analysis of the data showed older homes were much more likely to have high lead levels across soils, chickens, and their eggs. This finding matches other studies that found older homes are most at risk of legacy contamination from the former use of lead-based paints, leaded petrol, and lead pipes.

What can backyard producers do?

Many people who are involved in backyard food production will be shocked by these findings. It has been growing over the last decade, fueled recently due to soaring food prices.

People are also turning to homegrown produce because of other reasons. People want to know the origins of their food, feel secure knowing that they are producing food without chemicals, and have a closer relationship with nature.

Urban gardening is an important activity that should be encouraged. However, studies on soil contamination in Australian gardens, as well as trace metals uptake by plants, have shown it must be done with caution.

Over the years, contaminants have built up on soils. These contaminants can enter the food chain through vegetables, honeybees, or poultry.

Typically, urban gardening exposure risks are focused on fruits and vegetables. Backyard chickens have received little attention. Many previous studies were unable to sample and find participants because of the difficulty in finding participants.