Science and Risk: Radiation and Radioactivity

How to be a Civil Servant

Radiation, Radioactivity and Cancer

This is the second in a series of information notes which have been prepared to help generalist civil servants get to grips with some of the more technical issues which they might need to understand before providing advice to Ministers. (The first note looks at risks to health and safety.)

Summary

If you get caught up in a radiactivity scare, make sure that your scientific advisers (and those advising lobby groups) tell you what equivalent dose (measured in millisieverts (mSv)) might be received by those exposed to the radiation. We on average receive a dose of 2.6mSv each year so a further dose of less than 3mSv is not worrying.

Detail

Some forms of radiation are dangerous, but most are not:-

Radio waves travel through most things, including our bodies, without doing any harm. Light waves, another form of radiation, are essential to life, as is infra-red radiation which transmits heat from one place to another, and particularly from the sun to the earth.
The air that we breathe is radioactive. This is because cosmic rays entering the atmosphere create carbon dioxide containing carbon 14, a radioactive form of carbon. This slowly turns into normal carbon, but every gram of natural carbon (e.g. in our bodies, or in plants) emits 15 radioactive particles every minute. This gradual reduction in radioactivity provides a way of establishing the age of very old organic material:- "carbon dating".

Radiation and radioactivity are nevertheless very scary. We have a choice about whether to accept the (much higher) risks associated with, for instance, smoking and drinking. But the risks from such things as radioactive spillages are imposed on us without our agreement. Even worse, we cannot detect radioactivity, and it poses threats to unborn generations. The public therefore understandably expect the Government to take every possible measure to protect it from this danger.

Policy makers therefore need to understand that there is a radiation spectrum, ranging from very energetic and dangerous radiation through to very low energy and safe. The following are the three principal types of radiation within the spectrum, in order of decreasing energy.

1. Ionising Radiation The most energetic radiation is very dangerous because it can ionise material in the body. Most ionisations are harmless, but there is a small chance that ionisation of DNA can cause cancer. The main components of this part of the spectrum are:

Bits of atoms, emitted when radioactive material breaks up:- Alpha particles, Beta particles (electrons) and Neutrons.
Next come Gamma rays
and then X rays

2. Ultra-violet radiation comes next. These rays are less dangerous but can still cause skin cancer.

3. Visible light and radio waves make up the low energy end of the radiation spectrum. The main components, in order of decreasing energy, are:

Visible light, ranging from Violet through to the least energetic red light
Infra-red
Microwaves
UHF radio
VHF radio
TV
Short wave radio
Medium wave radio
Long wave radio
Very low frequency magnetic fields

Let's now look at the risk that each of the above three categories of radiation might cause cancer.

1. Radioactivity Luckily, our natural environment contains very little of the most energetic ionising radiation, otherwise known as radioactivity. But great confusion is caused by the fact that our exposure to this radiation can be measured in several ways.

The number of particles emitted by radioactive material depends upon the material's half-life, i.e. the time that it takes for one half of each unit of material to transform into another material.
But if we are handling radioactive material, we are usually more interested in the number of emissions that it creates every second. This is measured in becquerels. The shorter the half-life, and the heavier the material, the greater the number of becquerels.
We are even more interested, of course, in the amount of energy that is absorbed by the body when it is hit by radioactivity. This is measured in grays. For instance, a heavy alpha particle delivers much more energy, or grays, than a lighter beta particle.
And we are more interested still in the damage that is done by the radioactivity, for again a heavier particle, such as an alpha particle, will leave all its energy in one place, causing maximum damage. The damage that is done is called the "equivalent dose" and is measured in sieverts. As most of us are subject to very little radioactivity in our normal lives, our personal doses are usually measured in one thousandths of a sievert or millisievert (mSv).
- The average person in the UK receives a dose of 2.6mSv each year. 1.3 of this comes from the naturally occuring gas radon. Another 0.9 comes from other natural sources, and 0.4 comes from medical X rays etc. Only 0.013 comes from radioactive discharges from power stations etc, or fallout from e.g. Chernobyl.
- But some of us are subject to higher doses. Radon in some homes can give annual doses of up to 100mSv, and some medical procedures can give doses of up to 10mSv. And every 20 hours in a jet plane adds 0.1mSv to our total.
Finally, what about the risk of getting cancer? This is generally thought to be about .005% per sievert over a lifetime.

What does this mean in practice? An average person in the UK receives 2.6mSv each year, which is about 200mSv, over a lifetime. The chances of this causing cancer are therefore 0.005% of 200 which is 1%. In other words, 1% of us will die from cancers caused by radioactivity. In contrast, a huge proportion of us die from cancers caused by other factors, including smoking.

Therefore, if you are asked to give advice about the effect of e.g. a radioactive spillage, you should do your best to ensure that scientists tell you what equivalent dose might be received by those exposed to the radioactivity. They might well need to make a number of assumptions in calculating the dose(s), and these should be made explicit. But once you have a possible figure, you can compare it with the average annual dose and decide how to react. Clearly, a one-off dose of less than 3mSv per person need not cause huge concern. And it is hard to understand why the HSE once prosecuted the Natural History Museum for accidentally displaying some rocks which exposed visitors to 0.044mSv per hour.

But do bear in mind that almost all predictions about the effect of radiation on the body are based on a model (the ICRP model) which was developed by using the observed cancer rates in the survivors of atomic bombs exploded over Japan during the Second World War. These were high dose, high dose-rate exposures to external radiation. These risks are extrapolated, in the ICRP model, to lower doses, lower dose-rates and internal radiation. Some scientists believe that these extrapolations are, and therefore the model itself is, seriously flawed. There is therefore real concern about the damage that might be done by substances such as depleted uranium dust which might have found its way into the lungs of soldiers and others as a result of military operations. (See a a separate note on this subject).

2. Ultraviolet light Although it is non-ionising, ultraviolet light is still energetic enough to cause cancer. None of us can totally avoid UV radiation, of course, but we should try to avoid prolonged exposure to sunlight, especially if we have fair skin. A failure to avoid these precautions, together with the depletion of the ozone layer, which lets through more of the high energy UV radiation, has led to a doubling in the incidence of skin cancer in the UK, to about 5000 cases a year.

3. Visible radiation, microwaves, radio waves and electro-magnetic radiation It can be seen from the position of this radiation, in the lower part of the above spectrum, that it is very unlikely that it could be doing us any damage. In particular, there is no evidence that these waves, and in particular microwaves, could cause cancer (e.g. when using a mobile phone).

However, microwaves, like other radiation, do warm the material through which they pass. Intense microwaves are therefore used to warm food in a microwave cooker. Could such warming damage us, e.g. when using a mobile phone? The temperature effect is so slight - less than sitting near an open fire - that, again, it hardly seems likely that damage could be caused. Nevertheless, some scientists have found some evidence that microwaves do have a biological effect, e.g. by making some tiny worms grow a little faster. Unfortunately, they have not yet thought of a reason why this could happen, so it is possible that the experiments are faulty in some way. And, anyway, humans are not much like worms! Further work is therefore being carried out to resolve the uncertainty. In the meantime, mobile phones still seem relatively safe, although it might be best if they were not used too much by smaller children, until the issue has been resolved one way or another.

There is also some concern about the effect of magnetic fields found in some houses and near electicity pylons. A March 2001 report by the NRPB (see below) found a weak but statistically valid link between such exposure and childhood leukaemia. At most, the fields might be causing 2 cases of leukaemia each year, out of a UK total of 500. But it is not clear how these very low energy waves could be causing cancer. It might well be that the apparent link was caused by the design of the research, or the lifestyles or other characteristics of the families that are exposed to this radiation, rather than being caused by the magnetic fields themselves. The report certainly did not merit the Evening Standard's headline:- 55,000 children are 'at risk from power radiation'.

For Further Information, start with the clear and authoratitive web site of the National Radiological Protection Board.

And see my separate note on depleted uranium for a review of the handling of one particular radiation scare in early 2001.

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