Eggs from backyard hens contain on average 40 times more lead than store eggs, study finds
There’s nothing like fresh eggs from your own chickens, over 400,000 Aussies who keep garden chooks will tell you that. Unfortunately, it’s often not just freshness and flavor that sets their eggs apart from store-bought ones.
Our recently published research found eggs from backyard hens contain, on average, more than 40 times the lead levels of commercially produced eggs. Almost one in two hens in our Sydney study had significant blood lead levels. Similarly, about half of the eggs analyzed contained lead at levels that could pose a health concern for consumers.
Even low levels of lead exposure are considered harmful to human healthincluding among other effects heart disease and decreased IQ and kidney function. In fact, the World Health Organization has declared there is no safe level of exposure to lead.
So how do you know if this is a likely problem in the eggs you get from backyard hens? It depends on the lead levels in your soil, which vary between our cities. We’ve mapped the high and low risk areas for chickens and their eggs in our biggest cities – Sydney, Melbourne and Brisbane – and feature those maps here.
Our research details lead poisoning of backyard chickens and what it means for urban gardening and food production. In older homes near city centers, contaminated soil can significantly increase people’s exposure to lead from eating eggs from backyard chickens.
What did the study find?
Most lead enters hens when they scratch and peck food on the floor.
We assessed trace metal contamination in backyard chickens and their eggs from garden soils in 55 houses in Sydney. We also explored other possible sources of contamination such as animal drinking water and chicken feed.
The amount of lead in soil was significantly associated with lead concentrations in blood and chicken eggs. We found potential contamination from drinking water and commercial food in some samples, but this is not a significant source of exposure.
Contrary to humansthere are no guidelines for blood lead levels in chickens or other birds. Veterinary assessments and research indicates that levels of 20 micrograms per deciliter (µg/dL) or more can harm their health. Our analysis of 69 backyard chickens in the homes of the 55 participants showed that 45% had blood lead levels above 20 µg/dL.
We analyzed eggs from the same birds. There are no dietary standards for trace metals in eggs in Australia or worldwide. However, in the 19th Australian Total Diet Studylead levels were below 5 µg/kg in a small sample of store-bought eggs.
The average level of lead in the eggs of the backyard chickens in our study was 301 µg/kg. By comparison, it was 7.2 µg/kg in the nine commercial free-range eggs we analyzed.
International research indicates that consumption of one egg per day with a lead level of less than 100 µg/kg would cause an estimated increase in blood lead levels of less than 1 µg/dL in children. It’s around the level found in Australian children not living in areas affected by lead mines or smelters. The level of concern used in Australia for the study of sources of exposure is 5µg/dL.
Some 51% of the eggs analyzed exceeded the “food safety” threshold of 100 µg/kg. To keep lead in eggs below 100 μg/kg, our modeling of the relationship between lead in soil, chickens and eggs showed that lead in soil must be below 117 mg/kg. This is well below the Australian residential recommendation for floors of 300mg/kg.
To protect the health of chickens and maintain blood lead levels below 20 µg/kg, soil concentrations should be below 166 mg/kg. Again, this is much less than the guideline.
How did we map risks in cities?
We used our database of trace metals in garden soils (over 7,000 homes and 25,000 samples) to map the locations in Sydney, Brisbane and Melbourne most at risk of high lead levels.
Further analysis of the data showed that older homes were much more likely to have high levels of lead in floors, chickens and their eggs. This finding aligns with other studies that have found that older homes are most at risk of contamination inherited from the ancient use of lead-based paints, leaded gasoline and lead pipes.
What can backyard growers do about it?
These findings will come as a shock to many people who have turned to backyard food production. It has increased over the past decade, recently stimulated by soaring grocery prices.
People are turn to local products for other reasons as well. They want to know where their food comes from, enjoy the safety of producing food without added chemicals, and feel a closer connection to nature.
While urban gardening is an extremely important activity and should be encouraged, previous studies on Soil contamination in Australian vegetable gardens and uptake of trace metals in plants show that it must be undertaken with caution.
Risks of exposure to urban gardening have generally focused on vegetables and fruits. Limited attention has been given to backyard chickens. The challenge of sampling and finding participants meant that many previous studies were smaller and did not always analyze all possible routes of exposure.
Mapping the risks of soil contamination allows home gardeners and chicken farmers to think about what the results may mean for them.
Especially in older places in the city center it would be prudent to have their floors tested. People can do it at VegeSafe or through a commercial laboratory. Floors identified as problematic can be replaced and chickens kept in areas of known clean floor.