The secure knowledge we have of the natural world comes from science. Nevertheless, results produced by scientific investigations often come in shades of grey or subtle hues of pastel.
On the other hand, the general public lives in a world of black and white opinions and vividly coloured argument. It is difficult to effectively communicate the subtle shades of science into the stronger hued general world.
Consider ionising radiation (IR), the type of radiation emitted by radioactive substances. How dangerous is exposure to low-level IR? More is known about ionising radiation as a toxic agent than about any other toxic agent.
The health effects of exposure to higher levels of ionising radiation are well-established but it is difficult to gauge the precise effects of exposure to low-level IR. Nevertheless, I believe we do have reliable estimates of the risks associated with low-level IR. It can cause cancer and genetic defects. Our bodies are composed of trillions of cells organised into tissues - muscle, nerve, liver, etc.
DNA is the chemical that controls the day-to-day activities of the cell and is also the hereditary material passed on to future generations. IR can damage the cell's DNA.
If DNA damage occurs in an ordinary tissue cell, it may initiate changes which eventually produce a cancer. If the damage occurs in germ cells (egg in female, sperm in male), it may produce a genetic defect that is passed on to offspring of the irradiated individual.
We are exposed to ionising radiation from two different sources - natural and man-made IR. The earth is showered by IR from space (cosmic rays) and the rocks in the earth are naturally radioactive. Natural radioactivity gets into our food and we cannot avoid ingesting it. The rocks irradiate us externally and also emit a radioactive gas called radon which we cannot avoid inhaling.
Radon can build up to dangerously high levels in some modern houses, but simple remediation steps can reduce these levels. You can have radon levels in your home measured for a nominal fee by contacting the Radiological Protection Institute of Ireland 01269 7766. Man-made IR includes medical ionising radiation (mainly X-rays), IR from the nuclear industry, IR from nuclear weapons fall-out, and IR from miscellaneous appliances, e.g. TV. Under normal circumstances, almost all the IR we receive is natural IR (about 87 per cent of total) and almost all the rest is medical IR. Ionising radiation deposits energy in our cells when it interacts with them.
The amount of energy absorbed per unit mass of tissue is called the radiation dose and is measured in units called Sieverts (Sv). Onethousandth of a Sv is a millisievert (mSv).
In Ireland, the average annual individual dose from natural ionising radiation is about 2.6mSv.
We accurately know the effects of acute (over hours to days) exposure to large doses of radiation (from a large fraction of a Sv upwards). But what are the implications of chronic exposure to low-level IR, additional doses comparable to the dose we normally receive from natural IR?
If we are to receive doses of artificial ionising radiation these are easily the most likely sized doses we will receive. The ill-health effects of low-level exposure to IR are difficult to determine for the following reasons - the effects are small; there is a long latent period (five-40 years) between initiation of radiation damage and appearance of clinical symptoms; huge numbers of people must be studied and compared to appropriate controls to get meaningful results; and you must accurately know the radiation doses received by the population under a long-term study period.
If the ill-health effects of exposure to lowlevel radiation were big, they would be unmissable and easily studied.
If the DNA in one of my tissue cells suffers damage from ionising radiation today which initiates a sequence of events culminating in cancer, I will have no knowledge of my misfortune for many years. Blood cancer (leukaemia) takes several years to announce itself and solid tumours can take up to 40 years.
The inheritance of genetic effects, if my damaged cell is a sperm, by definition takes at least one generation to show up. Therefore, estimation of the effects of low-level IR requires meticulous medical surveillance over many years. When effects are small you must study huge numbers of people (preferably hundreds of thousands) to get meaningful results.
It is crucially important to have a valid control group against which to compare the irradiated group. The members of the control group must resemble the test group but not receive the extra ionising radiation the test group received.
And finally, you must accurately know the radiation doses received by the test group. In most cases, it is extremely difficult to know this information precisely.
The best study to-date of the effects of exposure to low-level IR is the long-term study of the survivors of the atomic bombings of Hiroshima and Nagasaki in 1945.
This study has, so far, uncovered no evidence of inherited genetic damage. Our estimates of the genetic effects of IR come from animal studies. As regards induction of cancer it is estimated that if one million people each received 1mSv dose, 50 will develop a fatal cancer.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC. He will deliver the opening lecture at this year's Merriman Summer School at Lisdoonvarna, Co Clare, at 8.30 p.m. on Saturday, August 18th, The title of the lecture is "Creativity, Intuition, Inspiration and Hard Work in Science".