The common term dioxins is often used for a number of chemically related lipophilic compounds (polychlorinated dibenzo-para dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), including certain dioxin-like polychlorinated biphenyls (PCBs) with similar toxic properties. However, the denomination of dioxin is given to 2,3,7,8-tetrachloro-dibenzo-para-dioxin (TCDD). Of the identified 419 types of dioxin-related compounds, which have a dioxin-like chemical structure, only about 30 are considered to have significant toxicity, with TCDD being the most toxic. Although dioxins and dioxin-like PCBs (for convenience herein after referred to as "dioxins") show similarities in their toxicological and chemical behaviour, their sources may be different.