The predecessor to zinc air batteries for use in hearing aids were mercuric oxide batteries. These batteries contained relatively high amounts of mercury (greater than 50% by cell weight). When zinc air batteries came on the scene in the late 1970s, their superior performance and reliability quickly replaced mercuric oxide as the battery chemistry of choice.

Today, unless the package says “Mercury Free”, standard zinc air batteries still contain added mercury, about 1% by cell weight. Mercury is a good electrical conductor and its properties help suppress internal gassing. Without it, battery cells are prone to leakage or swelling

It is a common misconception that all hearing aid batteries are mercury free. This stems from the 1996 legislation that banned mercury from household alkaline batteries. At this time button cells such as hearing aid and watch batteries were exempt because technology to create a mercury free design was not available.

When mercury containing hearing aid batteries are thrown away, they decay in the landfills. Over time, the battery will start gassing and the mercury inside has a potential of leaking out. This mercury can seep into the ground contaminating drinking water and streams. People are exposed to mercury most commonly when they eat food contaminated with mercury such as fish. Over 66,000 children are born each year with risk of neuro-development issues due to mercury exposure as a fetus.

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