Eggs from free range chickens contain nearly six times more of the toxic chemical dioxin than eggs hatched by caged hens, a Taiwanese study has found.
Dioxin, a by-product of garbage burning and industrial activities, can cause a range of illnesses in humans including cancer and miscarriages.
The study, which analysed egg samples from six free range areas and 12 caged areas, was carried out by environmental scientists at National Cheng Kung University and published by the American Chemical Society earlier this month.
It confirmed previous research showing that chickens roaming in the wild had higher chances of being exposed to dioxin, experts said.
Taiwanese authorities last year slaughtered 9,000 ducks contaminated with dioxin from a farm polluted by a nearby illegal furnace slag dump.