What kind of EMF do Ground Loops cause?

Hi, I would like to ask what kind of EMF do Ground Loops cause? I understand that it is stated to avoid them as it is unhealthy for one or more devices to be grounded more than once but it hasn't been explained that much.

Could you please explain more about Ground Loops and how they are 'biologically active and potentially harmful' so to speak?

Are they part of the four EMFs such as electric and magnetic fields, radio-frequency and dirty electricity? If so, which one of them do this belong to?

Thanks.