"Because free-range hens spend most of their lives in an outside environment, they have a better chance of being exposed to contaminants from the environment," wrote researchers led by Pao Chi Liao of the Environmental and Occupational Health department at National Cheng Kung University in Tainan, Taiwan.

Mr Liao and colleagues examined eggs from free-range and caged hens and hunted specifically for the dioxins - polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) - which they said were especially prevalent in Taiwan.

"Taiwan ... is a heavily populated, industrialised island and many municipal incinerators release PCDDs and PCDFs," they wrote. Dioxin levels in the free-range eggs were on average 5.7 times higher than those in caged hens, they found.

Seventeen per cent of free-range eggs were also found to exceed dioxin limits set by the European Community, reports The Straits Times, citing Reuters.

Dioxins build up in fatty tissues over time and have been linked to cancer. Experts say its other, non-cancerous effects on the immune system and reproductive and sexual development pose even greater threats to human health. Their findings were published in the Journal of Agricultural and Food Chemistry.