Many routes exist for the synthesis of substituted furans. The asymmetric hydrogenation of furans and benzofurans has so far proven challenging. TEFs exist for seven congeners of dioxins, ten furans and twelve PCBs. The emission behaviour of PCB is comparable to that of dioxins and furans. Compared to benzene, furans, thiophenes, and pyrroles are more susceptible to electrophilic attack. Other persistent toxins are PCBs, DDT, pesticides, furans, dioxins and phenols. Workers often inhale lead, cadmium, dioxins, furans, phthalates and brominated flame retardants. Production of dioxins and furans is also a documented risk factor. These noxious fumes include dioxins and furans. This is useful for sulfonylation reactions, especially in the sulfonylation of furans. It is also used in the Feist-Benary synthesis of furans. Examples of persistent toxins are PCBs, DDT, TBT, pesticides, furans, dioxins, phenols, and radioactive waste. Ma and coworkers developed an alternative method for accessing furans utilizing the Stetter reaction. Fillet portions were analyzed for pesticides, polychlorinated biphenyls (PCBs), dioxins, and furans. Gold catalyses the alkylation of certain aromatic rings and a conversion of furans to phenols.