Advanced chemistry is not one of my fields of expertise, but I have access to reports that others may not have. At any rate, this:

> The presence of polychlorinated dibenzo-p-dioxin and furan (PCDD/F) impurities has been demonstrated repeatedly for chlorophenols. This includes wood treatment fungicides pentachlorophenol. 

> PCDD/Fs have also been detected in many currently used pesticides, including pentachloronitrobenzene (PCNB) and 2,4-dichlorophenoxyacetic acid (2,4-D) 

> PCDD/F concentrations also increased in the 2,4-D formulation by 3000% during 96min of sunlight exposure (from 4.5 to 140 lg kg-1)

> The present study demonstrates that substantial formation of PCDD/Fs can occur when currently used pesticide formulations (containing PCDD/F precursors) are exposed to sunlight. 

► Exposure to sunlight results in formation of dioxins in PCNB and 2,4-D pesticide formulations. 
► TEQ increased by up to 9-fold via photolysis. 
► High concentrations of dioxins may thus be formed via this pathway after pesticide use. 
► Dioxin fingerprints in pesticides change considerably during sunlight exposure. 
► The manufacture of commonly used pesticides can result in dioxin precursor formation.

Chemosphere 88 (2012) 364–370
Formation of dioxins during exposure of pesticide formulations to sunlight

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