Mink more sensitive than rats to fire retardant chemicals.

Zhang, S, SJ Bursian, PA Martin, HM Chan, G Tomy, VP Palace, GJ Mayne and JW Martin. 2009. Reproductive and developmental toxicity of a pentabrominated diphenyl ether mixture, DE-71, to ranch mink (Mustela vison) and hazard assessment for wild mink in the Great Lakes region. Toxicological Sciences doi:10.1093/toxsci/kfp095.

Relying on data from laboratory rodent studies may underestimate the risks that fire retardant chemicals pose to wildlife. 

A team of researchers reports that some low concentrations of a commercial fire suppressant mix – which have no effect in laboratory rats – caused pregnancy failures when fed to female mink. The low levels tested also skewed thyroid hormone levels in juvenile offspring that were exposed during development and weaning. The younger animals were more sensitive to the chemicals than the adult animals.

In this study, scientists determined what levels of a commercial mixture of polybrominated diphenyl ether (PBDE) flame retardants – called DE-71^® – would affect reproduction and thyroid hormones in mink. They compared these concentrations to those known to affect laboratory rats and to levels found in wild populations of mink around the Great Lakes.

The concentrations tested are commonly found in the environment. Interestingly, the individual PBDE chemicals that make up the mixture were present in fairly similar proportions in the experimental and the wild mink livers. However, the wild mink had lower PBDE levels in their livers than those found to cause thyroid problems.

Their findings have at least two implications.

First, DE-71^® caused changes in mink at much lower levels than those reported to cause similar changes seen in laboratory rats. This underscores that not all types of animals react the same to chemicals, such as the class of fire retardants known as polybrominated biphenyl ethers (PBDEs). Using data from other species to directly draw conclusions about the safety of PBDEs in wild animals such as mink could be misleading, unless the uncertainty about species differences is accounted for.

Second, this study suggests that the levels of PBDEs in wild mink are usually below those where overt thyroid or reproductive changes are seen. But, if levels of PBDEs keep rising – as they have for the past couple of decades – wild mink populations may be more likely to suffer the same effects from PBDEs as seen in this study's experimental mink.

PBDEs are a class of fire retardant chemicals widely used in textiles, electronics and other products to slow or suppress burning. Exposure occurs mainly through food, air and water. PBDEs are a common contaminant of fish.

The now banned DE-71^® was used in a wide variety of consumer products for several decades. The chemicals can leak into both indoor and outdoor environments from such products.

Thyroid hormone is a very important developmental hormone, from pregnancy through maturity, in animals and people.

In this study, four strengths (0, 0.1, 0.5 or 2.5 parts per million) of DE-71^® were fed to female mink through pregnancy and nursing. It was also fed to their offspring, which are called kits, until they were 33 weeks old. The amounts tested are similar to those wild mink might eat through the freshwater fish in their diet.

Adult females eating the highest daily dose in their food completely failed in carrying their pregnancies to term. Females in lower dose treatment groups, 0.5 ppm and below, were able to successfully give birth to kits.

The mixture also decreased the active form of thyroid hormone, T3, in the juvenile, 33 week-old mink that were part of the 0.5 ppm treatment group. Yet, the inactive form of thyroid hormone, T4, increased. The increase in T4 may compensate for the decrease in T3 since inactive T4 is converted to active T3 in cells that need thyroid hormone.

PBDE levels in the liver of wild mink trapped by commercial fur trappers in most areas around two of the Great Lakes – Erie and Ontario – were below the levels found to disrupt thyroid hormone. However, at least one wild mink’s PBDE levels exceeded that threshold.

Chemical contaminants, such as dibenzo-p-dioxins, that form when DE-71^® is made make it difficult to attribute the changes seen in the mink solely to PBDEs. However, wildlife are usually exposed to mixtures of chemicals rather than only one pure type of chemical.

Other PBDE mixtures that are considered somewhat safer have replaced DE-71^®. Even though the mixture is no longer used, individual PBDEs in the DE-71^® mixture closely match the PBDEs found in samples taken from the environment, making it a reasonable choice for this study.