RADIATION is a general term and covers many different forms, e.g. visible light, ultra violet light, microwaves, X rays etc.
Radiation can be divided into two broad categories: ionising and non ionising. Ionising radiation is the particular type emitted by radioactive substances. It differs from non ionising radiation in that it is very energetic and has the capacity to ionise matter when it interacts with it.
Matter is composed of atoms. The atoms in matter are normally electrically neutral, i.e. positive and negative charges are balanced. Ionising radiation can strip negatively charged electrons off atoms to produce positively charged atoms, and the process is called ionisation.
It is disruptive of the orderly structure of matter. If the matter is biological tissue there is a risk that the damage will initiate the development of a cancer or, if the damaged tissue is in the germ line, lead to a genetic defect in a future generation.
Broadly speaking, the general public is exposed to ionising radiation from two sources: natural and artificial. The world is naturally radioactive and always has been.
The main form of artificial radiation is medical ionising radiation (principally diagnostic X rays). Other forms of artificial radiation include emissions from nuclear industry, fallout from nuclear weapons tests and radiation from miscellaneous appliances (e.g. televisions).
Under normal circumstances most of the radiation we receive comes from natural sources (mainly from the radioactive gas radon). On the artificial radiation side, most comes from medicine. Under normal circumstances we receive very little from the nuclear industry, nuclear fallout and miscellaneous appliances.
When ionising radiation interacts with biological tissue it deposits energy there. The amount of energy deposited per unit mass of tissue is called the absorbed dose, and the size of this biological dose determines the probability that harm will ensue.
The unit of dose is called the Sievert (Sv). One thousandth of a Sv is a millisievert (mSv). When calculating overall doses of radiation to the population (collective dose), you multiply the average individual dose by the total number of exposed people. Collective dose is expressed as man sieverts, manmilisieverts etc.
The average individual annual dose in Ireland from ionising radiation is about 4 mSv. Of the annual collective dose received by the Irish population approximately 92.6 per cent comes from natural radiation, 6.8 per cent from medical radiation, 0.02 per cent from nuclear discharges, 0.23 per cent from nuclear fallout, and 0.3 per cent from miscellaneous sources.
It is very important to understand the risk from ionising radiation in order to decide how to take sensible protective precautions. Ill effects resulting from exposure to radiation are divided into early and late effects.
Early effects are associated only with accumulation of large doses of radiation (greater than 1 Sv) over a short period (hours days). These effects include radiation sickness (diarrhoea, vomiting) and possible death within days or weeks.
The radiation dose that is lethal within 30 days for 50 per cent of those exposed to it is about three Grays (at these high acute doses, the term Gray is used instead of Sievert). An acute dose of eight Grays or more will almost certainly result in early death: an acute dose of 1.5 Grays or less will very probably not.
These high doses could only be received as a result of a nuclear war, or being close to a nuclear accident. A more practical and important matter is to know the risks associated with low doses of radiation (in the mSv range) accumulated over long periods, i.e. the sort of doses that we received in our everyday lives.
These doses carry a risk of long term effects, cancer and hereditary defects. With cancer there is a long latent period between the initial biological damage from the radiation and the subsequent clinical manifestation of the tumour. This can range from several years to 40 years, and more. Hereditary effects, of course, cannot be expressed until the next generation at the earliest.
For late effects, the relationship between risk and dose is known reasonably accurately for larger doses, i.e. significant fractions of a Sievert and upwards. It is known with much less precision for lower doses. The main reason for this is that at low doses the effects are small and therefore difficult to detect with statistical validity.
If the effects were large there would be no difficulty in detecting and quantifying them. Consider the difficulties. Because the effects are small you must study a large population exposed to a low level of radiation. You must have a similarly large population of controls against whom you will compare your test group.
The two groups must be as similar to each other in every way as possible, except that the test group receives a defined amount of dose extra to the controls. You must know accurately how much extra radiation the test group received. You must carefully monitor the groups for clinical symptoms over a period of 50 and more years, because of the latency phenomenon in cancer. It is easy to see that such a study is very difficult.
The current general scientific consensus on the form of the relationship between risk (fatal cancer) and dose (for lower doses over protracted time) is the linear no threshold model, i.e. risk is directly proportional to dose.
Risk is reduced to zero only at zero dose, and there is no low threshold of dose below which there is no risk. This is the basis for the statement often heard, "There is no safe level of radiation".
This statement is frequently misunderstood as meaning that radiation is equally dangerous regardless of the dose. The truth is that a tiny dose carries a tiny risk, a medium dose carries a medium risk, a large dose carries a large risk. The popular saying would be more accurately phrased, "There is no absolutely safe level of radiation".
Another consequence of the linear model is that a given collective dose has the same health consequences no matter how it is distributed, e.g. 1,000 people each receiving 1 mSv will have the same consequences as 100 people each receiving 10 mSv.
THE single most useful body of data used to determine risk of ionising radiation is the Japanese study of the survivors of the atomic bombings at Hiroshima and Nagasaki in 1945. Other useful data have come from long term follow up studies of patients who received defined doses of radiation in therapeutic medicine, e.g. in the treatment of ringworm.
Two major international committees continually evaluate the radiation risk data - the International Committee on Radiological Protection (ICRP) and the United Nations SubCommittee on Atomic Radiation (UNSCEAR) - and issue their findings and recommendations.
These bodies have come up with a nominal mortality risk of 5 per cent per Sv for cancer from irradiation at low doses for a population of all ages. In other words, if a million people, of all ages, each receive 1 mSv of radiation this will result in 50 fatal cancers. 1 mSv to each of one million people will also result in the expression of six serious hereditary defects in their descendants over all future generations.