Little monitoring of PCBs in air has been carried out in the UK, and as a result there are few data to show whether the sharp reduction in new uses of the chemicals since the early 1970s has been reflected in air concentrations. However, deposition of PCBs onto the foliar portion of plants is believed to be closely related to their air concentrations, and since PCBs are not taken up by plant roots, indirect evidence of their past air concentrations can be obtained by analyses of historical samples of vegetation.
This was the approach taken by the research team, which measured PCBs in herbage grown on long-term research plots at Rothamsted Experimental Station in Hertfordshire. Samples of each annual harvest have been stored in air-tight containers at the facility.
The study involved "blind" analyses for 25 PCB congeners in the crops grown in each year between 1965-89. Averaged over five-year periods, the results showed that total PCB concentrations peaked in 1965-9 at 1,510ng/g (dry weight). Concentrations declined sharply to 156ng/g by 1975-9, and the reduction then continued more slowly in the 1980s. By 1985-9, the average PCB concentration had fallen to 28ng/g.
This trend was evident for all the PCB congeners analysed. However, while levels of the less chlorinated PCBs had declined to 2-3% of their 1965-9 values by 1985-9, concentrations of the more highly chlorinated and persistent compounds fell less sharply over the period, to 10-16% of their peak values.
The results are markedly at variance with ReChem's claim that PCB contamination increased five-fold "throughout the UK" between 1984-9. The claim was based on analyses of grass samples around its three incinerator sites in Scotland, southern England and south Wales. It was made in response to indications that PCB levels around its south Wales incinerator were rising. The claim appears not to have been repeated following a recent officially-sponsored study which found that PCB levels in grass and soil are significantly elevated around the plant (ENDS Report 201, pp 12-13).