Mixing mercury with PCBs reduces PCB effect on hearing in rats.

Powers, BE, E Poon, HJK Sable and SL Schantz. 2009. Developmental exposure to PCBs, MeHg, or both: long-term effects on auditory function. Environmental Health Perspectives doi:10.1289/ehp.0800428.

Testing a pair of environmental contaminants commonly found together in wild Wisconsin fish shows the two affect hearing differently from when either one is tested alone.

Rats exposed to PCBs during development had permanent hearing loss. Developmental exposure to methyl-mercury (MeHg) did not appear to affect hearing.

When mixed, though, MeHg reduced the hearing deficits seen in the PCB exposures.

This study is part of a growing trend to test mixtures that are more likely to reflect “real world” exposures. Previous studies of individual PCB and MeHg exposures found that both pollutants affected hearing.

The MeHg doses were selected to achieve a ratio of PCBs to MeHg similar to that found in walleye caught by fisherman in Wisconsin. The researchers wanted to identify effects at levels similar to human exposures. However while the ratios of the exposures were similar, the doses were higher than those encountered by humans in the environment.

PCBs (polychlorinated biphenyls) and methyl mercury (MeHg) are neurotoxins. Nervous system development is particularly vulnerable to these contaminants. PCBs and MeHg can readily transfer from the mother to the fetus. Normal physiologic changes during pregnancy can redistribute environmental contaminants that have been stored in the mother’s body.

PCBs do not occur naturally, yet they are long lived in the environment and remain in soil and sediment. They were manufactured for their chemical properties and served as good, nonflammable insulators. Before being banned, PCBs were used in dielectric fluids in transformers and capacitors; hydraulic and lubricating fluids; diffusion pump oils; plasticizers; extenders for pesticides; and as ingredients in caulking compounds, paints, adhesives and flame retardants. PCBs were alos used in inks and carbonless carbon paper.

Mercury is a metal found naturally and produced by coal-fired power plants and other industrial sources.  Like PCBs, mercury persists in the environment and can travel long distances from the source of the exposure.

The form of mercury that is a threat to human health is methyl-mercury (MeHg). Bacteria take up mercury that has been deposited from rain and wind and turn it into MeHg. It increases in concentration as it moves up the food chain. Larger, predatory fish have the greatest concentrations.

The most common, non-work related PCB and MeHg exposure for humans is through eating contaminated fish.

PCBs impact the immune, the reproductive, the nervous and the endocrine systems. It may cause cancer. Mercury can affect the nervous system and cognitive development.

In this study, researchers used an otoacoustic emission (OAE) test to assess hearing damage. An OAE test determines the status of the hair cell function of the part of the ear known as the cochlear and is used to determine hearing sensitivity within a limited range, if hearing loss is due to senses or nerves and if the loss is functional. Functional hearing does not involve physical damage to the hearing pathway.

PCBs affected the range of frequencies the rats heard so that they could hear fewer on the low end. The effects may be related to damaged hair cells that catch and carry sound waves to the brain or to damaged brain cells that translate the sound waves.