Emerging Issues: Social Sustainability of Egg Production Symposium
The Impact Of Different Housing Systems On Egg Safety And Quality, Poultry Science, 2011
The persistent organic pollutants, certain historic pesticides, and heavy metals present a food safety concern because these chemicals bioaccumulate through the food chain and are not readily cleared from the body. Therefore chronic exposures can lead to body burdens that may result in adverse health effects, and these exposures need to be minimized.The smaller the flock size, the more time hens spent outdoors, and the larger the pasture, the greater the egg contamination. This is primarily a result of environmental pollution which contaminates soil - everywhere, even in rural areas - which persists (that is, does not break down), and which bioaccumulates.
Exposure to several of these chemicals could potentially be greater in free-range systems than in other systems because free-range hens come in direct contact with the outdoor environment and ingest soil or organisms in the soil. Contamination is also possible when hens in indoor cage-free housing come in contact with litter or barn posts and walls. In addition, free-range or litter-raised hens may be given additional veterinary drugs or chemicals to control diseases or parasitic infestations because of their exposure to wild birds or parasites, substances that could contaminate the eggs. The white paper “A comparison of hen welfare in relation to multiple housing systems” reviews several studies that have shown increased bacterial infections and ectoparasite infestations in litter based and free range systems compared with conventional cages.
The most widely reported chemical contaminations of eggs associated with free-range flocks are increased levels of dioxin-like compounds. In a California study, eggs from hens raised on soil contaminated from a nearby pentachlorophenol wood treatment plant had elevated levels of dioxins ("up to 100 times higher") compared with eggs from conventional cage-reared hens.
In Europe, food survey data show that free-range eggs have higher PCDD/F and polychlorinated biphenyl levels than conventional cage eggs. Almost 10% of free-range eggs exceeded the EU maximum residue limit (MRL) for PCDD/F in eggs; however, eggs from caged hens were well below this limit. There have been reports of elevated dioxin-like compounds in eggs from free-range hens in numerous European countries, including eggs not just from hens in municipal-industrial areas but also those in rural areas where soil dioxin levels were considered low.
In Brazil, free-range hens in an area historically treated with dichlorodiphenyltrichloroethane (DDT) for vector control had DDT levels twice the MRL recommended by the Food and Agriculture Organization of the United Nations even though DDT had not reportedly been used in the preceding 9 yr. The residue level was 1,000 times the level of DDT found in commercial eggs purchased at a local market.
In free-range eggs from backyard flocks in Belgium, the heavy metals lead, mercury, cobalt, and thallium had median concentrations 2 to 6 times higher than those of commercial eggs, presumably due to soil contamination.
Given the bioaccumulative nature of DDT and heavy metals, these elevated chronic exposures cause a concern that increasing body burdens of these chemicals may result in adverse health effects.
The use of contaminated litter and building materials can also introduce chemical residues to eggs produced in indoor noncage systems. In one instance, pentachlorophenol-treated wood shavings were used as litter, and eggs from hens raised on the litter had PCDD/F levels almost 30 times the EU MRL. Presumably, hens ingested contaminated wood through pecking, which led to the transfer of PCDD/F into the eggs and resulted in elevated concentrations."
This part about nutritional quality was surprising:
"Although claims made in the popular press extol the virtue of one housing system over another to enhance egg vitamin and omega-3 fatty acid content, few controlled studies have been undertaken to justify such assertions. ... Information dealing with the impacts of different housing systems on the nutritional quality of eggs is minimal worldwide and virtually nonexistent in the United States."