Guest Contribution: A Discussion on Radiation and Radioactive Material

Editor’s Note: Alan Stinchcombe is a retired physics teacher who resides in Suffolk, UK. He is currently collaboratively writing a school textbook on computer science.

Radioactive fission or other decay processes occurring in radioactive materials can produce nuclear radiations such as gamma rays and neutrons that have substantial ranges in air.  Industrial or therapeutic exposure to such radiation can occur without any release of radioactive material from its container.  However, the currently raised levels of radiation dose rate in some parts of Japan are the result of environmental pollution by radioactive material following recent damage to several nuclear reactors and spent fuel storage facilities.

The New York Times has been careful to distinguish between radioactive material and radiation (see, for example, its article on reactor status).  However, the BBC has been less scientifically literate in its reporting, using less scientifically-accurate terminology in an effort to simplify complex notions such as using the term ‘radiation’ to refer to both radiation and radioactive material.  For example, one BBC report states: ‘Tepco will have to compensate farmers for losses caused by the nuclear radiation leaking from its power plants’.

Obviously, the two concepts are intimately related and radiation is the simplest method of detecting the presence of radioactive material. Yet we need to distinguish between radiation as a tell-tale marker of radioactive materials and the damaging dose of radiation that radioactive material can deliver once it contaminates our environment and ultimately finds its way into our bodies.

Outside the evacuation zone around a nuclear accident, at a distance of tens or hundreds of kilometres, the intensity of even the most penetrating radiation from the accident is very low.  However, radioactive material ejected into the atmosphere or washed into waterways can travel long distances, although it becomes more and more diluted as it spreads.  A relatively low level of radiation from a relatively small amount of a radioactive substance is not very hazardous provided that we can remove ourselves from the vicinity of the material and keep it out of our drinking water and food chain. The radiation travels rapidly, in some cases at the speed of light, so it does not itself persist.  Depending on the type of radiation, it may be harmlessly absorbed by our outer, dead layers of skin, be absorbed within our bodies, or even travel through our bodies with relatively little absorption.  Once it’s gone, it’s gone, although living cells can accumulate damage from radiation that can cause cancer later in life.

Some radioactive substances are relatively slow to undergo radioactive decay, so that they are long-lived.  These can persist in the soil so that they continue to contaminate water, food crops and animal feed.  If even small amounts of long-lived radioactive materials become incorporated into our bodies, they may remain there irradiating our tissues for decades and present a significant health hazard.

RDTN.ORG is working on ways of visualising the geographical distribution of crowd-sourced radiation dose rates in its Google Maps mashup.  It is worth bearing in mind that, at the time of writing, the highest dose rate currently reported on the site is little more than three times the average individual background radiation dose rate for Americans, who themselves experience slightly above the global average for geological reasons.

The infographic by XKCD helps to put these dose rates into perspective.  However, very little background radiation normally comes from airborne radioactive material, except on rock that releases radon gas.  The current Japanese estimated radiation dose rates are based on measurements of external radiation.  While pollution by radioactive materials presents a relatively low hazard outside the body, it presents a greatly-increased, radiotoxic hazard if these materials enter the body by inhalation of radioactive dust or gases or by ingestion of radioactive substances in food or drink.  The resulting internal radiation is more hazardous since, when radioactive materials are in intimate contact with living tissue, even short-range radiation such as alpha and beta particles can damage the body.

Since both soil and atmosphere naturally contain tiny concentrations of radioactive substances, our whole food chain contains low levels of radioactivity, so that our own bodies are mildly radioactive and are therefore sources of low-level internal radiation, as well as irradiating our nearest and dearest!  For example, it has been estimated that eating a banana can expose a person to a radiation dose that could be equivalent to several hundred hours of exposure to the average background dose rate.

Ingestion of radioactively-contaminated food or drink or inhalation of radioactive smoke or dust has the potential to bring considerably larger quantities of radioactive substances into contact with body tissues.  It is important to avoid excessive permanent accumulation of radioactive substances in the body, as this leads to ongoing exposure to internal radiation, even when the environmental dose rate falls.  For example, when exposed to radioactive iodine, it is important to take iodide tablets to guard against absorption of the radioactive iodine by the thyroid gland. Unfortunately, there is no simple test for the presence of radioactive iodine, so people are dependent on timely information from official sources.

The opinions expressed in this commentary are solely those of Alan Stinchcombe.