Health effects of mobile telephones.

The safety of mobile telephones is a pressing question, now that the brains of nearly half the humans on the planet have become exp sed within a short span of time to a physical agent to which their ancestors' genes could not have adapted. It is no surprise then that epidemiologists have taken an interest in possible health effects of microwave exposure from mobile telephony. Few had experienced this novel exposure until 25 years ago, but nonionizing radiation from mobile telephones now regularly bathes the cerebral cortex of billions of people. This radiation has been demonstrated to affect communication channels across cell membranes by inhibiting or closing gap junctions,1'2 lending some plausibility to the idea that use of mobile telephones might have consequential biologic effects. Conducting epidemiologic research of microwave exposure from mobile telephones, however, has proven to be problematic. The diseases of greatest concern that might be related to microwave exposure are malignancies of the brain and of other tissues that are in close proximity to the mobile telephone antenna when the telephone is placed against the ear. These malignancies include glioma, meningioma, acoustic neuroma, and tumors of the salivary gland. All are rare, thus posing the first challenge to epidemiologists. The second challenge is a low prevalence of relevant exposure. Although mobile telephony is fast becoming ubiqui tous, most theories about the carcinogenicity of microwave exposure posit long induction times, perhaps even decades, between an exposure sufficient to induce a cancer and the appearance of the cancer. If using mobile telephones causes cancer with a long induction time, use of today's mobile telephone may be inducing many future cancers but relatively few of today's cancers, given the comparatively small number of users until recent years. Unfortunately, rare disease and rare relevant exposure barely begin to describe the difficulties of studying the effects of using mobile telephones. By far the greatest obstacle is exposure assessment. Biologic exposure from using a mobile telephone extends just a few centimeters from the telephone, but people use telephones in myriad ways. They are sometimes held to the left ear, sometimes held to the right ear, and sometimes placed away from the head while used in speakerphone mode or with headsets. They are carried by some who seldom make or receive calls, and used by others as a nearly continuous channel of communication. All these factors strongly influence the biologic exposure to micro waves of susceptible tissues. Adding to the difficulty is the problem that the technology itself has been changing rapidly. Most users today employ telephones with "third generation" technology, and "fourth-generation" is on the way, but the study of tumors occurring now might reflect the effects, if any, of firstand second-generation telephones that are no longer used. Finally, the transmission output of telephones is constantly changing, waxing and waning as it adjusts to the distance from cell towers or to shielding materials such as building walls. Thus, without an elaborate metering effort that would be impractical for any but a small subsample of users, a person's actual tissue exposure to microwaves can only be vaguely estimated, even from telephone call records. Even if metering were possible to sample some periods of exposure, it would hardly be possible over the length of time that a cohort is being followed. Furthermore, because the outcomes of primary interest are rare, nearly all of the studies conducted to date have