Sir, - In view of recent public debate (The Irish Times, February 4th) concerning the possible dangers posed by microwave radiation from mobile phone masts, it may be helpful to your readers to have some explanation of the physical interaction of microwaves with liquids. To put it at its simplest, many liquids, including water, are composed of bi-polar molecules - that is, molecules which permanently have large positive and large negative centres of electric charge a very small distance apart. Such molecules, when placed in a static electric field, will then orient themselves in the direction of the field on average and so we say that the polar liquid becomes electrically polarised.
Let us now consider the situation when the electric field is no longer static but varies very rapidly with time, as in the microwave region. The polar molecules again try to orient themselves in sympathy with the fast undulations of the field. However, they cannot do this because their surroundings are hot, which has the effect of slowing down the rotation of the molecules. The effect of this is to further heat up the surroundings. For water molecules this heating effect is most pronounced in the microwave region, say wavelengths of about one centimetre, so that an assembly of water molecules acts like a sponge in this frequency region. This is in effect the principle of operation of the microwave oven. In an oven, the microwaves are enclosed in a metal box. In a mobile phone, however, or at a mobile phone mast, the microwave energy must be radiated in order to carry the message and so may be absorbed by water or any polar protein molecules. Since we are composed largely of such substances, such radiations will be absorbed by us and a heating effect as described above will be produced.
In view of the indubitable fact of nature that microwaves produce heating in polar substances, I would have to agree with the statement of Dr Payne that further erection of mobile phone masts should await the results of the World Health Organisation study mentioned in your report. - Yours, etc., W.T. Coffey, F.I.E.E, F. Inst. P., Monkstown,
Co Dublin.