So what can microwaves do to the brain?

INVESTIGATION: Could your mobile phone give you cancer? Sophie Goodchild hears sufferers' stories. Charles Arthur explains the technology

MOBILE PHONES are remarkable engineering achievements: a hand- held combination of a microwave transmitter and receiver, plus a remarkable amount of computer power, a sensitive microphone and a low-output loudspeaker.

The health concerns are focused on the microwave transmitter - used when you are actually speaking on the mobile phone. (Normal "landline" telephones do not utilise such a transmitter.)

When it is simply turned on, the mobile is "listening" for signals that will tell it to start ringing. That signal is incredibly weak. Thousands of such signals permeate our environment every moment of our lives, although they have no apparent ill-effects.

However, there has been growing concern about the microwaves emitted by the aerial, and to a lesser extent the circuitry, of the phone when it is used in conversation.

Microwaves are part of the electromagnetic spectrum, with a higher frequency than, say, VHF radio waves, but less than infra-red or even visible light. Their ability to heat things - as seen in microwave ovens - comes because their comparatively long wavelength lets them penetrate materials which are opaque to the shorter waves of visible light. Hence you can "microwave" food enclosed in a cardboard box.

That is fine in a microwave oven, but worrying in a hand-held communications device, because the nearest body part to the aerial is the watery tissue of the brain. Water is especially good at absorbing microwave radiation, which is why it's good for cooking - but domestic ovens contain failsafe switches to stop you putting your hand in while the power is on.

The mobile phone industry had always assumed that the tiny power output of a mobile phone - typically about half a Watt - would not do any harm. (Domestic microwave ovens consume at least 650 Watts.)

Now, that opinion is being revised, following careful experiments by Dr Alan Preece at Bristol Royal Infirmary, and other work - sponsored by a telephone company - in Germany. They found measurable effects on memory and blood pressure.

"Passive phoning" poses no risk - the power of the waves falls by a factor of four as you double the distance from the aerial. But for users the problem remains.

One way that companies are trying to cope is by reducing the power output of the phones: using digital rather than analog systems is one way to achieve that. Another is to somehow shield the user from the aerial's emissions.

But new types of mobile phones, intended to "uplink" to satellite systems, will require more power, putting their users at greater risk if the microwaves have a direct effect.