Due to inclusion of cadmium, the units aren’t the most environmentally friendly in the market. When they die, you should dispose them properly to avoid harming the environment and your family members.
Nickel-metal hydride: These are cheaper and don’t contain toxic heavy metals. Unlike their cadmium counterparts, they aren’t affected by memory effect thus you don’t have to run them flat before recharging them. You should note that they can get damaged if you don’t recharge them for a long time. To extend their life, recharge them when they get down.