Typically heavy metals develop in the growth cycle and usually are in the soil or perhaps they can be found in nutrients that have either been added or even nutrients from the soil. Heavy metal is a dense metal that can be toxic at low concentrations. These include but are not limited to lead, mercury and arsenic. This is something that is often tested for in CBD oils. Also, this is why it is so important to have any product that you purchase be accompanied by a 3rd party lab result or Certificate of Analysis.