Cell phones produce electromagnetic interference (EMI), which can cause artifacts in physiological recordings and be misinterpreted by the clinician. This study investigated the possible effect of EMI (electrical artifact) on physiological recordings when cell phones are activated/ringing. The procedure consisted of placing the cell phone at varying distances from surface electromyographic sensors. Depending on the orientation of the cell phone's antenna, the EMI produced an artifact in the physiological signal for up to 175 cm (6 ft) that can be misinterpreted by the therapist. To avoid EMI artifacts, clients and therapists should turn off their cell phones when recording physiological signals. This means turning the cell phone off and not just switching it to vibrate. In addition, recent epidemiological studies suggest that long-term intensive cell phone use may increase the risk of gliomas, auditory tumors, and salivary tumors on the side of the head to which the person places the cell phone. Thus, to reduce artifacts and biological harm, the authors recommend keeping the cell phone away from the body and the biofeedback equipment.