In a comparative study conducted in Taiwan, scientists demonstrated significant levels of polychlorinated dibenzo-b-dioxins and dibenzofurans (dioxins) in eggs collected from free-range flocks. An average level of 1.79 ppb was record in the free-range eggs with levels ranging from 1.8 to 5.5 ppb. In contrast a level of 0.3 ppb was determined in eggs derived from caged flocks with minimal variation suggesting that this level may in fact be a threshold.

Free-range hens are susceptible to ingesting toxins in soil. Dioxins which are released into the air from incineration of municipal garbage are all the probability the source of contamination. In contrast, caged hens which are housed in environmentally controlled building are protected from this source of contamination. The published results* contradict the perception that free-range eggs are more wholesome than commercial eggs derived from confined flocks.  

*Hsu, J-F. Chen, C. and Liao, P-C. Elevated PCDD/F levels and distinctive PCDD/F congener profiles in free range eggs (2010) J. Agric. Foods Chem. 58:7708-7714.