Although exposure reduction can be realized by distance, the common recommendation to sit 28 inches from a VDT display is arbitrary and overly simplistic. Electromagnetic fields drop off rapidly with distance, but the issue is more complex than that. The rate at which field strengths drop off is determined by the form of the radiation source. For instance, if the radiation is emitted by a single wire source, the fields drop off inversely with distance. If the radiation is from a 2-wire source, where currents flow in opposite directions and produce a partial field cancellation, field strength falls off inversely as the square of the distance.
Fields created by toroidal coils fall off inversely as the cube of the distance.
It is widely reported that radiation levels are stronger in the back and sides of a VDT than the front and, therefore, users should be more concerned about their co-workers' displays rather than their own. The principal sources of EMFs are closer to the rear of the VDT, and so when measurements are taken at these locations, the sensor is closer to the radiation source. It isn't the case that VDT electromagnetic fields are directional or that shielding is intrinsic to the front of the display. The VDT face merely has the advantage of distance when measuring field strengths. However, LCDs emit correspondingly high levels of electric fields.