There’s nothing like the fresh eggs from your own hens, the more than 400,000 Australians who keep backyard cooks will tell you. Unfortunately, it is often not just freshness and taste that separates their eggs from those in the shops.
Our recently published research found that eggs from backyard chickens 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 problem for consumers.
Even low levels of lead exposure are considered harmful to human health, including cardiovascular disease and reduced IQ and kidney function. In fact, the World Health Organization has stated that there is no safe level of lead exposure.
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 mapped the high and low risk areas for hens and their eggs in our biggest cities – Sydney, Melbourne and Brisbane – and present these maps here.
Our research details the poisoning of backyard chickens and explains what this means for urban gardening and food production. In older homes near the city center, contaminated soil can greatly increase people’s exposure to lead through eating eggs from backyard chickens.
What did the study find?
Most lead enters the hens when they scratch in the dirt and pick food from the ground.
We assessed trace metal contamination in backyard chickens and their eggs from garden soil in 55 homes in Sydney. We also investigated other possible sources of contamination such as animal drinking water and chicken feed.
Our data confirmed what we had expected from our analysis of more than 25,000 garden samples from gardens in Australia collected through the VegeSafe program. Lead is the pollutant of most concern.
The amount of lead in the soil was significantly associated with lead concentrations in chicken blood and eggs. We found potential contamination from drinking water and commercial feed supplies in some samples, but it is not a significant source of exposure.
Unlike for humans, there are no guidelines for blood lead levels for chickens or other birds. Veterinary reviews and research indicate that levels of 20 micrograms per deciliter (µg/dL) or more can harm their health. Our analysis of 69 backyard chickens in the 55 participants’ homes showed that 45% had blood lead levels above 20 µg/dL.
We analyzed eggs from the same birds. There are no food standards for trace metals in eggs in Australia or globally. However, in the 19th Australian Total Diet Study, lead levels were less than 5 µg/kg in a small sample of store-bought eggs.
The average lead level in eggs from backyard hens in our study was 301 µg/kg. In comparison, it was 7.2 µg/kg in the nine commercial free-range eggs we analysed.
International research indicates that eating one egg a day with a lead level of less than 100 µg/kg will result in an estimated blood lead increase of less than 1 µg/dL in children. That’s around the level found in Australian children who don’t live in areas affected by lead mines or smelters. The level of concern used in Australia to investigate sources of exposure is 5 µg/dL.
About 51% of the eggs we analyzed exceeded the threshold value of 100 µg/kg “food safety”. To keep egg lead below 100 μg/kg, our modeling of the relationship between lead in soil, chickens and eggs showed that soil lead must be below 117 mg/kg. This is much lower than the Australian residential soil guideline of 300 mg/kg.
To protect chicken health and keep blood lead below 20 µg/kg, the soil concentration must be below 166 mg/kg. Again, this is much lower than the guideline.
How did we map the risks across cities?
We used our garden soil trace metal database (more than 7,000 homes and 25,000 samples) to map the locations in Sydney, Brisbane and Melbourne most at risk of high lead levels.
A deeper analysis of the data showed that older homes were much more likely to have high lead levels across their soil, chickens and eggs. This finding is consistent with other studies that found older homes are most susceptible to contamination from past use of lead-based paint, leaded gasoline, and lead pipes.
What can backyard producers do about it?
These findings will come as a shock to many people who have turned to backyard food production. It has been on the rise for the past decade, spurred recently by skyrocketing grocery prices.
People are turning to home-grown products for other reasons as well. They want to know where the food came from, enjoy the security of producing food without added chemicals, and feel the closer connection to nature.
While urban gardening is a hugely important activity and should be encouraged, previous studies of Australian garden soil contamination and trace metal uptake by plants show that it must be undertaken with caution.
Pollution has built up in the soil during the many years of the cities’ history. These ancient pollutants can enter our food chain via vegetables, honey bees and chickens.
Exposure risks for urban gardening have typically focused on vegetables and fruits. Limited attention has been paid to backyard chickens. The challenge of taking samples and finding participants meant that many previous studies have been smaller and have not always analyzed all possible exposure routes.
Mapping the risk of contamination in soil enables garden owners and chicken keepers to assess what the findings may mean for them.
Especially in older, inner-city areas, it would be wise to have your soil tested. People can do this at VegeSafe or through a commercial laboratory. Soil identified as a problem can be replaced and chickens kept in areas of known clean soil.
Mark Patrick Taylor, Chief Environmental Scientist, EPA Victoria; Honorary Professor, Macquarie University; Dorrit E. Jacob, Professor, Research School of Geosciences, Australian National Universityand Vladimir Strezov, Professor, School of Natural Sciences, Macquarie University
This article is republished from The Conversation under a Creative Commons license. Read the original article.