Why has e-waste become an issue especially in the 21st century?

"E-Waste" is a term used for discarded computers, mobile phones, office electronic equipment, television sets, entertainment device electronics, and refrigerators. It refers to electronics (electronics waste) which are intended for reuse, resale, recycling, salvage or disposal are also considered as e-waste. These had become an issue, especially in the 21st century because of the lead, mercury, and cadmium that are contained in the electronics, are hazardous. They are hazardous because they can release dangerous toxins into our air and water, or when burned or deposited in landfills improperly.