Within the body, currents induced by fields have the same range of effects as currents injected via electrodes, e.g. in an electric shock. But these effects depend entirely on the size of the current. Thus current densities of about 0.1 A m-2 can stimulate excitable tissue and current densities above about 1 A m-2 can cause ventricular fibrillation, as well as producing heating. However these current densities correspond to fields far larger than are ever encountered at 50 Hz.
At lower fields a range of possible effects have been reported. The established effect observed in humans at the lowest magnetic field is the magnetophosphene effect, where a flickering sensation is produced in peripheral vision by 50 Hz magnetic fields above about 10 mT (i.e. 10,000 ยตT). Magnetophosphenes are probably caused by induced current densities in the retina; the threshold at 20 Hz (the most sensitive frequency) is about 20 mA m-2.
Microshocks are a related but separate phenomenon, caused not by a continuous current but by a one-off discharge.
What is a safe level of induced current?
Exposure guidelines are usually designed to prevent all effects of induced currents, on the basis that any effect in the brain or nervous system is potentially harmful. For example, the ICNIRP exposure guidelines currently recommend that people at work should not be exposed to current densities in the head, neck and trunk of greater than 10 mA m-2 (the "basic restriction") with a lower limit of 2 mA m-2 for the general population which may include people who are more sensitive because of medical conditions.
See more on induced currents and how they are calculated