...The warnings stem from an odd quirk in federal testing procedures designed to ensure the safety of cellular phones. In 2001, the FCC released a set of guidelines for manufacturers that required all cell phones sold in the U.S. to emit a specific absorption rate (SAR) of not more than 1.6 watts of radio-frequency energy per kilogram of body tissue, a standard deemed safe given the state of scientific knowledge about thermal harm from radio-frequency waves. The standard was considered a so-called worst-case scenario, accounting for the energy emitted when the phone was transmitting at full power all of its various signals — such as Bluetooth, wi-fi and cellular. But the FCC testing regulations notably chose not to simulate a situation in which the phone was broadcasting at full power while inside a shirt or pants pocket flush against the body, an odd oversight given the known habits of many cellular-phone users. As a matter of physics, radio-frequency energy generally increases sharply as distance is reduced. "The exposure is definitely related to distance," says Cleveland.
According to the 2001 FCC guidelines, testing of the device in a "body-worn" configuration should be done with the device in a belt clip or holster. If a belt clip or holster was not supplied with the phone, the FCC told testers to assume a separation distance of between 0.59 inches and 0.98 inches (1.5 cm to 2.5 cm) from the body during a test.
"Clearly if it's tested in a holster, it's only guaranteed to be compliant if it's used with a holster," says one current FCC official familiar with these issues, who asked not to be identified by name. "Clearly a lot of people weren't aware of this, and it probably does need to be addressed." Some phones come with a holster included, while others, including Apple's iPhone 4, are not sold with holsters.