2        3       Dioxins contain one to eight
    \      /        chlorine atoms covalently 
     C----C         bonded at any of the numbered
    /  __  \        locations in the model to the
1--C  (benzene ring)  C--4    left.  Closely related to
    \      /        dioxins are furans, in which
     C----C         one ether bond (C-O-C) is 
    /      \        replaced by a direct C-C bond.
   O        O       Also related are PCBs, which 
    \      /        would be two benzene rings
     C----C         attached by a single C-C bond
    /  __  \        and chlorinated with one to 
9--C  (__)  C--6    ten chlorine atoms.  Dioxins
    \      /        with Cl atoms at the 2, 3, 7,
     C----C         and 8 positions are much more 
    /      \        toxic than less chlorinated
   8        7       species.

Studies done by Bruce N. Ames of Berkeley University suggest that exposure to low doses of certain dioxins reduces the cancer risk from subsequent exposure to other carcinogens. Ironically, the same doses when administered after carcinogen exposure increase cancer risk.