The dioxin scandal in European eggs in early 2011 is being followed by new dioxin-like PCB tainted eggs found in Germany. The source of the 2011 poison was tracked back to fatty acids of the production of bioDiesel which were illegally added to the feed of hens. However, no source is known which might have caused the 2012 PCBs which poisoned organic eggs of North Rhine-Westphalia/Germany and conventional free range farms in Lower Saxony/Germany. PCBs levels in eggs were found to be six times the permitted limit.
For the time being all samples of feed, water and local soil were found not to be the source of the contamination. Dioxins and PCBs are of industrial origin and may be found in eggs and meat oll over the world.
Dioxin in free-range eggs 
Lin, Hsu and Liao 2012 report that the levels of dl-PCBs in the free-range samples were 5.4 times higher than those in caged eggs. The dl-PCB levels in the free-range eggs were highly correlated with elevated levels of 17 polychlorinated dibenzo-p-dioxins and dibenzofurans, indicating a coexposure scenario in free-range hens. As the consumption of free-range eggs is becoming more popular worldwide, risk of cancer caused by dioxins and PCBs will increase.
The bioavailability of PCBs in contaminated soil in vicinity of a former fire involving treated wood was assessed by Fournier et al. 2012. The sandy soil contained 709μg indicator PCBskg(-1) dry matter. The concentration of indicator PCBs in yolk in abdominal fat and liver linearly increased with the amount of indicator PCB ingested, confirming the bioavailability of these compounds. 
Combustion of fossil fuels and biofuels increase environmental poisoning, being spread as dust and dioxin particulates. Bio fuels of first, second and third generation are based on carbon combustion producing dioxins and PCBs. The energy strategy for the next century should therefore be based on solar and wind energy which is carbon combustion free.
Schröter- Kermani et al. 2005 report that seabirds are top predators which accumulate persistent chemicals, such as PCDDs, PCDFs, and dioxin-like PCBs including several POPs like DDT, HCH, and HCB which can be found in high concentrations in their eggs. In the framework of the German Environmental Specimen Bank eggs of herring gulls are collected since 1988 from two North Sea islands and since 1993 from one Baltic Sea island, to monitore the industrials and agrochemicals cenvironmental contamination. 
Influence of dioxin in soil and their accumulation in egg yolk of organic eggs 
De Vries, Kwakkelz and Kijlstra 2006 reviewed studies related to dioxin in organic eggs in the Netherlands, Germany and other EU countries. The authors found that eggs are responsible for about 4 percent of the intake of dioxin in humans, organic eggs contain more dioxin than conventional eggs, and the organic eggs of a significant number of farms exceed the EU standards. The authors analysed the influence of dioxin in soil and their accumulation in egg yolk. Aside of feed as source of dioxin in eggs, the authors stress the importance of ingestion of worms, insects, grass, herbs and soil by hens. Hens on conventional and free-range farms spend less time in the outdoor run, compared with hens of organic farms.
Flock Size and measures to reduce dioxin in eggs 
De Vries and colleagues stress also that mall flocks were found to be outside most of the time whereas large flocks tend to remain inside. This explains why almost none of the larger layinghen farms (more than 1500 laying hens) in the Netherlands have problems maintaining their egg dioxin level below the EU standard of 3 pg TEQ.
Shortening the time hens spend ranging in the outdoor run, reduce the size of the outdoor run and covering the soil in the outdoor run are being proposed to reduce dioxin intake. Improving the general health status may prevent the hens from ingesting soil.
The maximum residue limit for dioxins in chicken eggs is set at 3 picograms WHO-PCDD/F-TEQ/g fat. 
Brominated flame retardant in eggs of free-range hens 
According to Fournier et al 2012, high concentrations of hexabromocyclododecane (HBCD), a brominated flame retardant, are sometimes recorded in eggs of free-range hens. The source of the contamination is the ingestion of soil. The γ-HBCD isomer is the most prevalent in soil. It is rapidly biotransformed and eliminated, and partly isomerized into the more persistent α-HBCD. The carry-over rate of ingested γ-HBCD to egg yolk is 1.2% and its half-life is 2.9 days in egg yolk, 13 days in abdominal fat, and 0,41 days in liver.
Three main stereoisomers, alpha, beta, and gamma, comprise roughly 10, 10, and 80% of the mixture, respectively. The major stereoisomer found in wildlife and human tissues including breast milk and blood of humans is the α-HBCD isomer, despite its low percentage of the fire retardant mixtures, suggesting the bioaccumulation of this isomer. 
 Lin C, Hsu JF, Liao PC: Coexposure of dioxin-like polychlorinated biphenyls and polychlorinated dibenzo-p-dioxins and dibenzofurans in free-range hens and implications derived from congener profile analysis. J Agric Food Chem. 2012 Feb 29;60(8):1963-72.
 Fournier A, Feidt C, Travel A, Bizec BL, Venisseau A, Marchand P, Jondreville C: Relative bioavailability to laying hens of indicator polychlorobiphenyls present in soil. Chemosphere. 2012 Mar 2.
 Schröter- Kermani C, Schmolz E, Herrmann T, Päpke O: PCDD, PCDF, and Dioxin- like PCB in Herring Gull Eggs from the North Sea and Baltic Sea: Levels, Patterns and, Temporal Trends. Organohalogen compounds 67 (2005), 1295-1299.
 De Vries M, Kwakkelz RP, Kijlstra A: Dioxin in Organic Eggs. A review. Received 19 July 2006; accepted 9 August 2006 NJAS 54-2, 2006
 Frequently Asked Questions on dioxins in food. The Federal Institute for Risk Assessment (BfR) FAQ, 10 January 2011.
 Fournier A, Feidt C, Marchand P, Vénisseau A, Le Bizec B, Sellier N, Engel E, Ratel J, Travel A, Jondreville C: Kinetic study of γ-hexabromocyclododecane orally given to laying hens (Gallus domesticus). "Transfer of HBCD in laying hens”. Environ Sci Pollut Res Int. 2012 Feb;19(2):440-7.
 Szabo DT, Diliberto JJ, Hakk H, Huwe JK, Birnbaum LS: Toxicokinetics of the flame retardant hexabromocyclododecane alpha: effect of dose, timing, route, repeated exposure, and metabolism. Toxicol Sci. 2011 Jun;121(2):234-44.