Alkaline batteries are prone to leaking potassium hydroxide (KOH), a caustic agent that can cause respiratory, eye and skin irritation. The reason for leaks is that as batteries discharge — either through usage or gradual self-discharge — the chemistry of the cells changes and some hydrogen gas is generated. The outer casing of the battery prevents the hydrogen gas from leaking. However, if a battery is left unused in a device for an extended period, the resulting gas buildup can rupture the casing and cause leakage.