It is shown that the waves electromagnetic spectrum registered at the outside of the Earth spans a great diapason of frequencies. Shields, depending on their properties, attenuate not only the constant ambient geomagnetic field, but also reduce the power of magnetic and electric fields of selected frequencies. Therefore, a biological object that is placed in a shielded surroundings is exposed to magnetic and electric fields frequencies that are different from the ambient magnetic and electric fields spectrum. Changes of biological processes that are noted in the shielded environments may be caused by the non-presence of some frequencies that cannot pass through a protection and/or by a reduce of other frequencies that can pass through a protection.