All batteries contain substances harmful to the environment and must be disposed of conscientiously.
Button batteries are a small single cell battery shaped as a squat cylinder typically 5 to 12mm in diameter and 1 to 6mm high. They are generally used to power small portable electronic devices such as wrist watches, calculators and hearing aids.
See also battery
Before you purchase a product, research the alternatives. Solar-powered calculators and wind-up watches are good options.
The Whanganui Resource Recovery Centre has an household battery disposal wheelie bin.
Please keep from landfill.