Biphenyl

Biphenyl occurs naturally in coal tar and crude oil and was historically recovered from coal tar distillates. It was first synthesized in 1869 by the Fittig reaction (coupling of bromobenzene with sodium). [1] Industrially, biphenyl serves multiple roles: as a high-temperature heat transfer fluid (often mixed with diphenyl ether as 'Dowtherm A') in chemical process equipment operating at temperatures up to 400°C; as an antimicrobial fungicide applied to the wrapping paper and wax coatings of citrus fruits to prevent mold growth during shipping; as an intermediate in the synthesis of polychlorinated biphenyls (PCBs), flame retardants, and pharmaceuticals; and as a component in liquid crystal displays. The citrus use — where oranges and lemons are dipped in or wrapped in biphenyl-treated material — means that consumers have a dietary exposure pathway through consuming citrus peel or through migration of biphenyl into the fruit flesh. [2] European regulators have set strict maximum residue limits for citrus. NTP carcinogenicity bioassays found evidence of liver and other tumors in rodents, and the compound is classified as a probable human carcinogen by EPA. It is listed as a HAP and appears on the TRI. [3]

Industrial workers in heat transfer system maintenance, biphenyl/PCT (polychloroterphenyl) synthesis, and PCB manufacturing historically had the highest exposures. Citrus fruit handlers and wax coating workers encounter biphenyl from fungicide applications. The general public is exposed primarily through dietary residues on citrus peel and from contaminated food processing equipment. Environmental releases from chemical plants create localized air and water contamination near facilities.

The probable carcinogen classification is based on liver tumors in rodents, with additional tumor findings in other organs. Acute biphenyl toxicity includes nausea, headache, and liver effects. Chronic occupational exposure has been associated with central and peripheral nervous system effects. [2] As a structural component of PCB molecules, biphenyl is intrinsically linked to one of the most consequential class of environmental pollutants. Environmental releases from heat transfer systems that leak are a contamination concern for soil and water.

Industrial heat transfer system workers, chemical plant workers producing biphenyl derivatives, and citrus packing house workers represent occupational risk groups. Consumers who eat citrus peel (including in marmalade, zest in baked goods, or candied peel) have measurable dietary exposures from treated citrus.

1. Industrial workers using Dowtherm or other biphenyl/diphenyl ether heat transfer fluids should prevent system leaks, use appropriate PPE for maintenance work, and work in ventilated areas. 2. Consumers who regularly use citrus peel should wash citrus thoroughly before zesting or candying; organic citrus is less likely to be treated with biphenyl fungicide. 3. Produce buyers can check for biphenyl treatment disclosure on packaged citrus. 4. Chemical workers should use closed-loop handling and vapor monitoring.