Electromagnetic fields, including those used in mobile telephony, have a potential to cause disturbances in sensitive electronic devices. The phenomenon is called electromagnetic interference (EMI).

Mobile phone users should always obey warning signs and restrictions that may exist in hospitals, aircrafts, and other places considered to have sensitive electronic equipment.

Some sensitive medical equipment, including cardiac pacemakers and defibrillators, can be susceptible to electromagnetic fields. However, the immunity of such devices has increased immensely in recent years and work has started to look into the possibility to increase the use of mobile communications in healthcare environments. Private use of mobile phones is allowed in some hospitals, particularly in public areas, such as cafeterias.
The risk that a mobile phone may have an impact on a pacemaker is very small. Pacemaker users are however recommended to keep a distance of 15 cm between their mobile phone and the pacemaker to avoid the risk of electromagnetic interference.
Hearing aids, especially older ones, can sometimes pick up the radio signals from mobile phones. The user can notice an audible sound, which will disappear as soon as the transmitter is moved away from the hearing aid. The problem could often be avoided by using handsfree equipment. Most modern hearing aids have very good immunity to electromagnetic fields, and disturbance from mobile phones is usually not a problem. 

The navigation systems in aircrafts are based on radio communications, and can be sensitive to radio waves from nearby transmitters. In addition, mobile phones used at high altitudes can disturb the mobile networks on the ground. Due to these reasons use of mobile phones is not allowed during flying. The risk of interference caused by mobile phones is, however, very low due to the low output power. Ericsson will in the near future provide solutions that will make the use of mobile phones in aircrafts possible.

Sparks can cause explosions and fires in petrol stations and other potentially explosive areas. The radio frequency energy from a mobile phone is considered too low to cause a spark.