Ash from an incinerator was spread on the paths between these allotments in Newcastle, and then found to contain hazardous, partly burned cadmium and mercury.

BBC

These are things like arsenic, mercury, cadmium, lead, selenium.

NPR: Coal Ash May Pose Health Hazard

First, electronic waste is currently hazardous waste in that it contains lead, mercury, cadmium, brominated flame retardants etc, which are integrally mixed into components and cannot therefore be easily separated from the non-hazardous components.

FORBES: It's Easy Enough To Solve The Problem Of E-Waste