Structure, Toxicity, and Sources Dioxins and furans are not single chemicals, but rather families of related compounds that differ in the number and position of their chlorine atoms. Each different compound is called a congener. Congeners that have the same number of chlorine atoms form a congener class. Toxicity varies among congeners.
Toxicity decreases as chlorine atoms are added or removed.
However, regardless of the number of chlorine atoms, the most toxic congeners are those that have chlorine atoms in at least the 2,3,7, and 8 positions. Because toxicity varies among congeners, concentrations of dioxins and furans can be expressed as equivalent concentrations of 2,3,7,8-TCDD in order to provide an indication of potential total toxicity. Toxicity of dioxins and furans also varies widely among animal species. Dioxins and furans are not deliberately manufactured. Rather, small quantities of these compounds are inadvertently produced as by-products of a variety of industrial processes.