Thank you for the question. There are a few reasons why batteries can be harmful when not disposed of properly. Single-use batteries still contain a very small amount of mercury and when not recycled, they can break down and potential hazards will arise from the leaks. Some rechargeable batteries also contain cadmium which can be bad for the environment and people if it were also to leak or be burned. Batteries can also cause soil and water pollution and endanger some wildlife. For example, the cadmium will do damage to the microorganisms in the soil and affect the breakdown of organic matter. It also affects fish which reduces their numbers and make it unsafe for human consumption.

Batteries use chemicals to store electrical energy, these chemicals often being salts that are toxic because they contain heavy metals (e.g. silver, lead, etc.). Not disposing of batteries properly carries a risk that these toxic chemicals will escape into the environment and poison something (or someone).