In case of exposure to very high intensities of 50 Hz magnetic and/or electric fields, acute direct effects can occur, e.g. direct stimulation of nerves and muscle tissues and induction of retinal phosphenes (*). These effects are well studied in human volunteers and animals. Recommendations and guidelines are intended to protect us from these short term established effects.
(*) Phosphenes are flashes of light that can be perceived in the eye. These flashes can be directly induced by mechanical, electrical, or magnetic stimulation of the retina. High magnetic fields can induced current at the retina level, that could lead to magnetophosphenes.
Short term effects depend on the local intensity of the electric field in each tissue. This explains why the reference unit is the internal electric field expressed in Volt per meter (V/m).
There is also indirect scientific evidence that brain functions such as visual processing and motor co-ordination can be transiently affected by induced electric fields.
All these effects have thresholds below which they do not occur and can be avoided by meeting appropriate limit values of electric fields induced in the body.