Several states in the U.S. have regulations against using mercury in batteries. Traditional hearing aid batteries have a small amount of mercury and manufacturers are changing their products to comply with the new regulations.
When the mercury is removed from the batteries, the power level drops a little bit. For this reason, it is really important to remove the sticker from the back of the battery and wait for a full two minutes before putting the battery into your hearing aid. This way the chemical reaction in the battery can fully charge and you will maximize your battery life.
The battery manufacturers had recommended this practice previously but we never saw much difference in battery life. However, now it makes a difference with the mercury free batteries and the newer hearing aids.