Any electronics that convert AC power to DC and back to AC will cause distortions to the 60Hz fundamental frequency sine wave. These distortions or the creation of additional currents are also called 'noise,' defined as the harmonic content of the particular electric loads. The compounding of noise in the circuit finds its way to the neutral as that is the return path of the circuit. This noise has been part of the electrical distribution system since the 1980s and is an increasing problem globally due to our modern electronics.