The polybrominated diphenyl ethers (PBDEs), which are used as flame retardants, are widely distributed in the environment, principally because they are blended with the products rather than being chemically bound. So, release is easy, as demonstrated by their occurrence in sediments, soil, air, dust, fish, birds, animals and food. They have also been detected regularly in human breast milk.
The appearance of PBDEs in food is of great concern because they have been associated with endocrine disruption, liver toxicity, thyroid toxicity and neurodevelopmental toxicity, as well as cancer.
In the USA, studies have revealed that 82% of human exposure is from household dust with a minor contribution from inhalation and food, whereas the opposite is the case in the UK. There, two studies reported that the diet contributed 73 and 93% of total human PBDEs.
Dairy products are a recognised, but minor, dietary source, representing less than 10% of the total dietary PBDE intake. However, they should not be discounted. In particular, cows’ milk is a central part of the diet and an important food for infants.
To this end, scientists in the UK have undertaken the first controlled study of PBDE concentrations in food produced from flooded pastures. River sediments are a recognised sink for many pollutants, so flooding might constitute a significant source of local pollution and consequent health hazard.
Iain Lake from the University of East Anglia, Norwich and coresearchers from the Food and Environment Research Agency (FERA), Sand Hutton, Askham Bryan College, York and the Food Standards Agency, London, studied pasture alongside the River Trent in central England. They measured the PBDE contents of soil, grass and milk of cows grazing in those fields.
They selected five flood-prone farms which had neighbouring flood-free farms to use as controls. All ten farms had been free from sewage sludge applications over the past 20 years, eliminating a potential PBDE source.
Soil and grass from the same areas of each field were collected, along with milk from the bulk storage tanks. All samples were analysed by gas chromatography/high-resolution mass spectrometry using a set of stable isotope-labelled analogues as surrogate standards. A total of 16 PBDE congeners, with differing numbers and positions of bromine atoms, were targeted.
In practice, the total levels of seven congeners were taken for comparison because they were present in most samples and constituted the major proportion of PBDEs found.
Fire retardant effects on soil, grass and milk
The levels of PBDEs in soil from the flood-prone pastures ranged from 240-3230 ng/kg dry weight, with a median value of 770 ng/kg. Although theses figures cover a wide range, each individual measurement was greater than those from the corresponding control farm, which were 210-300 ng/kg with a median value of 280 ng/kg.
So, flooding had a notable effect on the PBDE levels in soils. However, cows ingest very little soil as a matter of course, typically less than 3.5% of their total diet by weight. It was of more interest to compare the soil values with those in grass growing in the flooded areas, because grass makes up about 60% of the total dietary intake.
Median grass levels of PBDEs in the flood-prone and control fields were 140 and 150 ng/kg, respectively. This indicates that PDBEs are transferred inefficiently to grass. In addition, the researchers claimed that the differences were not unexpected due to the short-term influences of rain and air temperature on the PBDE levels in grass.
The median PBDE concentrations in milk from flood-prone fields were 300 ng/kg, compared with 250 ng/kg from the control areas. So, there was little evidence to support the transfer of PBDEs from soil to grass and on to milk.
The team cited several explanations for this, including the fact that cattle spend about 40% of their time in fields that are not prone to flooding and that they are also fed on commercial feed.
Comparison of the ratios of the two major congeners, PBDE47 and PBDE99, in the samples indicated that there was a common pollution source between flood-prone and control areas. The ratio was similar to that of the major penta-BDE product on the market. The congener ratios also suggested that the transfer efficiencies of the two congeners were similar for soil and grass but different for milk.
So, the PBDE group of flame retardants present in river sediments do not introduce an increased health risk in cows’ milk following flooding, even though the farmland itself becomes polluted.