In the previous sections of this article, we have been exclusively concerned with "ionising radiation".  Ionising radiation has sufficient energy to ionise (remove electrons from) atoms of the materials with which it interacts, and it is the subsequent behaviour of the liberated electrons that is responsible for the detrimental effects of ionising radiation.

However, there is another class of radiation, known as "non-ionising radiation", that is also commonplace and which is suspected to have implications for human health.  By far the most important class of non-ionising radiation is low energy electromagnetic radiation.  (In fact, gamma rays and X-rays are types of electromagnetic radiation, but of sufficient energy to induce atomic ionisation).

Electromagnetic radiation, in spite of the rather fancy name, is ubiquitous and indeed we would struggle to live our lives without it.  Listed below are some common types of electromagnetic radiation:

1.    Visible light;

2.    Ultra-violet radiation;

3.    Microwaves;

4.    Radio waves.

Electromagnetic radiation is generated when electrical charges (usually electrons) undergo accelerative motion.  It is in essence a wave that has an electric field component and a magnetic field component.  The speed at which the wave travels depends on the medium in which it is travelling, but in free space it travels at the "speed of light", i.e. about 300,000 kilometres per second.  The following figure (taken from Ohanian's book entitled Physics) shows the various types of electromagnetic radiation in order of the frequency of the wave.

Electromagnetic waves have a structure as set out in the figure below (again taken from Ohanian's book Physics).

In this figure, the black line represents the electric field component (symbol E) of the electromagnetic wave.  The red line is the magnetic field component (symbol B), and is perpendicular to the electric field.  In the 19th century, James Clerk Maxwell developed a set of differential equations that describe how the electric and magnetic components are related to each other, and from these equations was able to deduce the important properties of electromagnetic waves and radiation.

One of the most important classes of electromagnetic radiation in modern life is that emitted by cell phones.  In the UK, a typical frequency of radiation emitted by cell phones is about 900 MHz (i.e. 900,000,000 Hz).  From the figure above, it can be seen that this frequency lies at the upper end of the "radio wave" category, and in fact by some definitions, this frequency lies in the "microwave" category (see below).  To put this cell phone frequency into context, FM radio frequencies are around 100 MHz, and so cell phone frequencies are around an order of magnitude higher.  Because we make such considerable use of cell phones in our everyday lives, the effects of being exposed to cell phone radiation is of interest to many people.

Another important class of electromagnetic radiation is microwave radiation.  There can be very few houses in the developed world that do not have a microwave oven in the kitchen.  Although not shown on the figure above, microwaves lie in the frequency band between radio waves and infra-red radiation.  They are therefore more energetic than radio waves.  Microwave ovens are built to very strict standards in order to protect users from the detrimental effects of microwave radiation.  The radiation frequency is about 2,400 MHz, so around a factor of three higher than is used in cell phones.

It can be seen from the first figure that ultra-violet radiation lies at frequencies just above those of visible light.  Most of us are familiar with the fact that our beautiful sun tan arises from the ultra-violet radiation from the sun, or from the tanning salon that we visit from time to time.

The final category of electromagnetic radiation that is of interest in every day life is that emanating from power transmission lines.  Because the power is transmitted as alternating current, the acceleration of electric charges (electrons) takes place, and so electromagnetic radiation is emitted.  Just as for cell phones, there is considerable interest in the possible health effects of living next to, or close to, electric power lines.

In the next section, the health effects from electromagnetic radiation are examined, and the current state of research on health effects is considered.  Note that the next section is not a detailed treatise of all the research that has been undertaken to date.  Rather, I examine some of the points of view on health effects, and offer some opinions of my own.

On to Health Effects