Biological effect or health risk?
Biological effects are measurable modifications in response to a stimulus (e.g. exposure to electromagnetic fields or sunlight). Biological effects are not necessarily harmful to health : reading a document produces a biological effect, but it is not a harmful activity to health.
The human body possesses compensation and regulation mechanisms for many stimuli. There is a health risk whenever the biological effect exceeds the capacity of normal compensation of the organism, thereby resulting in a deterioration of health.
Note: Confusion between biological effect and electromagnetic compatibility
The correct functioning of an electrical appliance can be disturbed by an electromagnetic field emitted by a nearby electrical device. The disturbances caused by this field are called electromagnetic interferences. To avoid such interference, it is necessary to be aware of the electromagnetic compatibility of electrical appliances.
It is important not to confuse biological effects with electromagnetic field interferences from an electronic device. Certain materials are very sensitive to low-frequency magnetic fields. For example a computer screen can be disturbed by a magnetic field of about 1 µT. The interference is due to the refresh rate of the display on the screen. This frequency is indeed close to 50 Hz.