) ▁are ▁a ▁group ▁of ▁comp ounds ▁compr ising ▁poly cyc lic ▁a rom atic ▁hydro car b ons ▁with ▁two ▁or ▁more ▁a rom atic ▁rings ▁and ▁one ▁or ▁more ▁ch lor ine ▁atoms ▁attached ▁to ▁the ▁ring ▁system . ▁Cl - PA H s ▁can ▁be ▁divided ▁into ▁two ▁groups : ▁ch lor o - subst it uted ▁PA H s , ▁which ▁have ▁one ▁or ▁more ▁hydro gen ▁atoms ▁subst it uted ▁by ▁a ▁ch lor ine ▁atom , ▁and ▁ch lor o - added ▁Cl - PA H s , ▁which ▁have ▁two ▁or ▁more ▁ch lor ine ▁atoms ▁added ▁to ▁the ▁mole c ule . ▁They ▁are ▁products ▁of ▁incomplete ▁comb ust ion ▁of ▁organ ic ▁materials . ▁They ▁have ▁many ▁con gen ers , ▁and ▁the ▁occurr ences ▁and ▁to xic ities ▁of ▁the ▁con gen ers ▁differ . ▁Cl - PA H s ▁are ▁hydro ph ob ic ▁comp ounds ▁and ▁their ▁pers istence ▁within ▁e cos ystem s ▁is ▁due ▁to ▁their ▁low ▁water ▁sol ub ility . ▁They ▁are ▁struct ur ally ▁similar ▁to ▁other ▁hal ogen ated ▁hydro car b ons ▁such ▁as ▁pol ych lor in ated ▁di ben zo - p - d io x ins ▁( P CD D s ), ▁di ben z of ur ans ▁( PC DF s ), ▁and ▁pol ych lor in ated ▁bi phen yl s ▁( PC B s ). ▁Cl - PA H s ▁in ▁the ▁environment ▁are ▁strongly ▁sus cept ible