Temporal trends in dioxin, furan and polychlorinated biphenyl concentrations in bovine milk from farms adjacent to industrial and chemical installations over a 15 year period

The concentrations of polychlorinated dibenzo-p-dioxins, furans and polychlorinated biphenyls (PCBs) were measured in pooled bovine milk samples collected between 1991 and 2005 in County Cork, Ireland. The pooled samples were of bulk-tank milk collected from farms adjacent to industrial, chemical and pharmaceutical installations (target milk) or from rural farms distant from industrial activity (control milk). Comparing data between the first and last 3-year periods of the study revealed a 62% decrease in the mean total dioxin concentration in target milk from 1.58 to 0.60pg toxic equivalents (TEQ)/g fat. On the same basis the dioxin-like PCB concentration in target milk decreased by 80% over the study period (from 0.95pg to 0.19pg TEQ/g fat). The mean 'marker' PCB concentration in target milk from 1991 to 1993 inclusive was 3359pg/g fat. This value decreased by 75% to a mean of 849pg/g fat for the years 2003-2005 inclusive. The results of this study are consistent with low background dietary/environmental PCB contamination in both target and control herds. The total dioxin concentrations in all samples were well below the maximum tolerable limits permitted for marketable milk. The decrease in the total dioxin concentration in target and control milk samples over the study period was chiefly due to decreases in the concentration of dioxin-like PCBs, consistent with significant reductions in the concentration of PCBs in the dairy cow diet over the 15 year study period.