Sodium o-phenylphenoxide (SOPP)

O-phenylphenol (OPP) and its sodium salt (SOPP) were developed in the 1930s as surface disinfectants and post-harvest fungicides for citrus and other fruits [1]. The compound is applied as a dip or spray to citrus after harvest to prevent Penicillium mold during transport and storage [2]. SOPP is also used in wood preservation, textile treatment, and as a general biocide in hospitals and food processing facilities [1]. The cancer concerns arose from a Japanese bioassay in the 1980s showing bladder tumors in rats fed high doses of SOPP — this triggered intensive regulatory review in the United States and Europe, ultimately resolving as a rat-specific cytotoxicity effect at doses exceeding typical human dietary exposure [2].

Dietary exposure from residues on treated citrus peel — SOPP is applied to the surface of citrus fruit [1]. The residue is primarily in the peel, with minimal transfer to the flesh [2]. Workers applying SOPP dips in packinghouses face dermal and inhalation exposure [1]. Consumers who eat citrus peel, use it in cooking (zest), or make marmalade receive higher intake [2].

SOPP is metabolized to phenylhydroquinone and related quinone intermediates that are concentrated in the kidney, causing cytotoxic nephropathy at high doses in rats [1]. The bladder tumors in rats are now understood to be secondary to cytotoxicity-induced regenerative cell proliferation rather than direct genotoxicity — at typical dietary exposure levels, this mechanism is unlikely to operate in humans [2]. EPA classifies SOPP as a Group C possible carcinogen [1].

Citrus packinghouse workers [1]. Consumers who eat large amounts of citrus peel from conventionally treated fruit [2].

1. Wash citrus thoroughly with water and scrubbing before eating or zesting the peel [1]. 2. Choose unwaxed or organic citrus when using zest in cooking — organic production prohibits SOPP use [2]. 3. Peel citrus before eating if you are concerned about surface fungicide residues [1].