How do foods and animals become contaminated with dioxin and related compounds?

Deposition of airborne dioxins onto plant and soil surfaces, and subsequent ingestion of  this contaminated vegetation and  soil by food animals, is considered the primary pathway by which dioxins enter the  food chain. The  levels produced in this way are considered background  levels.  Fish become  contaminated due  to  airborne dioxin deposition into the water and also from contaminated soil or industrial waste washed into rivers and  lakes, leading to high sediment concentrations. Inhalation and water pathways are not considered  significant sources of exposure for terrestrial animals. contamination above background  levels can occur if  food animals are fed products which are adulterated, either by natural or unnatural means, with high concentrations of dioxins.