Most radios are comprised of a plastic or metal casing, which houses a printed circuit board, metals such as copper, lead and aluminium, as well as materials such as solder, epoxy and porcelain. Radios come in a large variety of shapes and sizes and can be built small enough to fit in a cell phone or large enough to form part of a stereo system. Analogue radios are slowly being replaced by digital radios, which use larger amounts of energy to operate and contain greater amounts of environmentally hazardous metals. Digital radios form part of what is known as e-waste. 20-50 million tonnes of e-waste is disposed of annually worldwide, putting heavy metals and chemicals such as lead, mercury and arsenic into landfills, which can contaminate soil and groundwater.