Even the simple and small computer mouse can emit an harmful electromagnetic field.
Of course the wireless mouse is a source of radio-frequency emissions which are almost unavoidable, but even the mouse with a wire connected to the computer cause serious problems.
The electromagnetic emissions are lower for PS2 type mice with mechanical ball, while USB mice with optical device give much more EM radiation.
Because the user's hand is just over the mouse circuit, at few centimeters from it, this EM field is a serious problem. The fingers are even nearer to the internal circuit board, and the finger that is used to turn the mouse wheel is most exposed because of the hole in the plastic cover of the mouse where the wheel comes out.
We have noticed strong electromagnetic noise in USB wires also for other devices. Most of the EM noise comes through the USB power line, but the signal line carries a lot of EM noise too. This EMF is originated in part by the computer power supply, which is a switching one, and in part by the USB circuits themselves.
The effect of the exposure of the hand and fingers to these EM emissions is typically a strange pain localized in several points in the hand and fingers, both in muscle and in bones. Usually this pain goes away as soon as the hand is moved away from the USB mouse, but the prolonged exposure may cause more serious damages.
Of course the best solution would be to have an USB circuit with very low electromagnetic noise, after all the PS2 port doesn't cause too much EMF in the mouse.
The USB connection however is very difficult to filter, there are several different versions of USB that work in different frequency bands, even if the power line is replaced with a low-noise one the signal line continues to bring high frequency noise from the computer to the mouse circuit.
Currently the only solution we have found is to apply an internal shield to the mouse cover.