Dairy Livestock Exposure to Persistent Organic Pollutants and Their Transfer to Milk: A Review

Abstract

The main emission sources of Persistent Organic Pollutants (POPs) should be attributed to Human activities although certain natural events may also enhance their production. After emission, these compounds can be potentially transferred to the food chain via interactions with livestock systems. POPs are characterized by some volatility, strong persistence in the environment, and a high lipophilicity, which leads to their accumulation in fat tissues. These molecules have raised concern about the risk of transfer through the food chain via the animal product. POPs are listed in several international conventions dealing with their potential toxicity for humans and the environment. This paper synthesizes current information on dairy ruminant exposure to POPs and the risk of their transfer to milk. Five major families of POPs have been considered: the two groups of dioxins and furans (PCDD/Fs), the Poly Chloro Biphenyls (PCBs), the Polycyclic Aromatic Hydrocarbons (PAHs) and the emerging family of Poly-Bromo-Diphenyl-Ethers (PBDE). Dairy ruminants are mainly exposed to theses POPs by oral ingestion, other contamination ways are considered as minor. The contamination of roughage and soil by these compounds is observed when they are exposed to emission sources (steelworks, cement works, waste incinerators or motorways) compared to remote areas. Concentrations in soil can be higher than in plants, especially for very persistent halogened compounds. Highest concentrations of POPs in soil may be close to 1 μg per kg dry matter for PCDD/Fs and PBDEs, 100 μg/kg dry matter for indicator PCBs, 10 g/kg dry matter for PAHs. The contamination of milk by persistent organic pollutants depends on environmental factors, which are related to the rearing system (feeding system, access to contaminated soil or not, stage of lactation, udder health of the animals) and of the characteristics of the considered contaminants (their chemical properties like molecular weight, halogenations, lipophilicity and metabolic susceptibility). Established transfer rates to milk were lowest for PAHs (generally less than 1%), mainly due to other excretion pathway like urine but also to a certain proportion of metabolization. Transfer rates of halogenized compounds were generally higher: for PCBs the rate of transfer varies widely from 5% to 90%, and for the PCDD/Fs from 1% to 40%. Further studies should clarify the effect of hydroxyl-metabolites of PAHs on Human health and precise the transfer rates of PBDEs where only few data are available.