Transportable, mobile, and portable phones are all considered "cellular" phones.

Transportable phones are also known as "bag phones." These operate with equipment stored in a small carrying case; the antenna of the bag phone usually extends from the carrying case. Because they are most commonly stored inside the car with the phone user, or carried by the phone user, bag phones can be a greater source of RF exposure than mobile phones. Transportable telephone use is declining as portable phones become more popular.

Mobile phones, also called "car phones," usually have an antenna mounted on the outside of a car-on the window, fender, roof, or trunk. The antenna of a cellular phone is the phone's primary source of radio frequency. The metal surface of a car provides a shield between the mobile phone user and the energy associated with the antenna. The physical distance between the mobile phone user and the antenna also serves as protection against RF energy. Because of these two obstacles-the metal car surface and the physical separation-users of mobile phones are thought to have little exposure to RF energy.⁶

The antenna of a portable phone is integrated into the body of the phone. Because the antenna of a portable phone is close to the phone user's head, portable phones pose greater RF exposure than the other types of cordless phones.

Cellular phones are an important source of RF exposure for those who use them. The amount of RF to which a person is exposed depends on a number of factors. The number of "cells" in a geographical area depends upon the cellular phone traffic in that area. For example, large cities may have many cells per square mile, whereas a less-populated, rural area may have a single cell stretching over several square miles. The farther away a cell phone antenna is from its base station, the higher the power level needed to maintain the connection. Very small cells are therefore associated with much lower exposures.⁵

Each geographical cell has a different number of available channels. Cellular phones operate ideally with the least amount of interference from neighboring channels. To help achieve optimal operation, cellular phones automatically step down to the lowest power level available that still maintains a connection with the base station. On the other hand, any physical obstacle, such as buildings or trees, interfering with the connection between base station and cell phone forces the base station to increase the power sent to that phone. Therefore the amount of power sent from a base station to a particular cellular phone can vary, even within a single call.⁵

Manufacturers are required to report the specific absorption rate (SAR) of their product to the FCC. The SAR is the amount of RF energy absorbed from the phone into the local tissues. The upper limit of SAR allowed is 1.6 watts per kilogram of body weight.³ Exposure to RF also depends on the duration and frequency of cellular phone use, with more use implying more exposure. Finally, older cellular phones (analog models) involve higher exposure than newer, digital equipment.

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