The international standard on the basic safety of medical nerve and muscle stimulators advises "that stimulation should not be applied across or through the head, directly on the eyes, covering the mouth, on the front of the neck, (especially the carotid sinus), or from electrodes placed on the chest and the upper back or crossing over the heart". The standard also notes that "any electrodes that have current densities exceeding 2 mA/cm² may require the special attention of the operator". It imposes the following limits on the output parameters of stimulators (for therapeutic purposes):