Ionizing Radiation and Health

Although such tragedies as the atomic bombings in Hiroshima and Nagasaki in 1945 should never be repeated, these unfortunate experiences have greatly enhanced our knowledge of the health effects due to ionizing radiation. Studies on the late health effects of ionizing radiation among the atomic bomb survivors have been conducted since 1947 by the Atomic Bomb Casualty Commission (ABCC) and its successor, the Radiation Effects Research Foundation (RERF) which is equally funded by the two governments of Japan and the United States. The results thus far obtainedup to the present can be classified into the following three categories: (1) The effects for which a clear increase has been found include malignant neoplasms, cataracts, chromosomal aberrations, small head size and mental retardation among the in utero exposed. (2) A suggestive increase has been found in the several sites of cancers and immunological abnormalities. (3) No difference has been observed between the exposed and the non-exposed in some types of leukemia, osteosarcoma, accelerated aging, sterility and hereditary effects.


What Are The Properties Of Ionizing Radiation?
Alpha rays and beta rays are sub-atomic particles that travel at close to the speed of light (300,000,000 metres per second). Alpha rays can be stopped (energy absorbed) by a piece of paper, while beta rays can be stopped by one or two centimetres of human tissue. Gamma rays and X-rays are waves of energy similar to visible light, the stuff that comes out of the domestic lamp; except they have more energy and are invisible. They travel at the speed of light and penetrate matter more easily than the particulate radiations. Metals such as lead are normally required to absorb their energy.

What Units Are Used To Measure Radioactivity?
Radiation is measured in decays (disintegrations) per second which corresponds to the number of nuclei losing energy each second. One becquerel (abbreviation Bq) is equal to one decay per second: one megabequerel is equal to one million disintegrations per second. The human body is naturally radioactive due to the presence of radioactive potassium: A 70 kilogram person would contain about 3500 Bq.

How Does Radiation Interact With Matter?
When the energy from radiation is absorbed by matter, chemical changes occur at the atomic level. If the exposure is large enough these changes can be readily observed. For example, if glass is heavily irradiated it changes colour. Some precious stones are coloured for commercial purposes using this method. When the body is subjected to a medical X-ray the bones absorb most of the energy and a photographic fi lm can then give an image of the skeleton. The amount of radiation absorbed per gram of matter is called the absorbed dose.

What Units Are Used To Measure Absorbed Dose?
Absorbed dose is measured in grays (abbreviation Gy). One gray corresponds to one joule of radiation energy deposited in one kilogram of matter. (Note: It would require 320,000 joules of energy to boil one kilogram [one litre] of water). A uniform dose of 3 to 5 Gy to the whole body will kill fi fty percent of people exposed in one to two months. This is a large unit and the milligray (mGy), which is one thousandth of a gray, is more commonly used.
When radiation interacts with living tissue the effect it has varies with the type of radiation. Alpha rays are 20 times more effective than beta and gamma rays at causing tissue damage. To allow for this, the dose in grays is multiplied by an effectiveness factor and the new units are called sieverts (abbreviation Sv) and

Fact Sheet 17
the dose is called the equivalent dose. A one milligray dose of alpha rays is equal to 20 mSv (millisieverts) of equivalent dose. A one milligray dose of beta rays is equal to 1 mSv equivalent dose because the effectiveness factor is 1 for beta rays. In most cases the effectiveness factor is unity and the dose in grays is equal to the dose in sieverts.

How Does Radiation Interact With The Human Body?
When radiation is absorbed in the body it causes chemical reactions to occur which can alter the normal functions of the body. At high doses (above 1 sievert) this can result in massive cell death, organ damage and possibly death to the individual. At low doses (less than 50 mSv) the situation is more complex.
The body is made up of different cells. For example we have brain cells, muscle cells, blood cells etc. It is the genes within a cell that determine how a cell functions. If damage occurs to the genes then it is possible for a cancer to occur. This means the cell has lost the ability to control the rate at which it reproduces. Radiation can cause this effect and at low doses it is the only known deleterious health effect. This type of event is very unlikely to occur, and an estimate of its frequency can only be obtained by measuring the effect at higher doses and calculating the probability at low doses. A dose of one millisievert corresponds to a chance of 6 in 100,000 of contracting a cancer. This fi gure can be compared with the normal incidence of cancer which is 25,000 cases per 100,000 over a lifetime2.
If the damage occurs in the testes or ovaries then hereditary effects in descendents may become apparent. No fi rst generation hereditary effects were observed amongst Hiroshima survivors. Based on other studies the ICRP recommended a risk factor of 2 per thousand per sievert effective dose. A dose of one millisievert to a large population will produce two cases of severe hereditary effects per million births 3. This fi gure can be compared with the normal incidence of severe congenital abnormalities which is 23,000 per million births 4. (See UNSCEAR 2000 report for more detail)

The Natural Background
The effect of radiation on health must be discussed within the context of the natural background. Background radiation consists of cosmic rays from space and radiation present in the earth from when it was formed. Cosmic radiation increases with altitude and so airline pilots receive a high exposure from this source; the dose rate at 12,000 metres being about 150 times the sea level dose. The terrestrial radiation comes from naturally occuring radiosiotopes of potassium and rubidium and from decay products of uranium and thorium. On average two thirds of the dose people receive comes from terrestrial sources. Most of this dose comes from the gas, radon, which is a decay product of uranium and thorium. Radon emanates from the soil and tends to concentrate in buildings. Overseas radon contributes a high proportion of background dose. However in Australia studies have shown that radon contributes only a small proportion of background dose.

Exposure Limits
The International Commission on Radiological Protection (ICRP) has set the following limits on exposure to ionizing radiation: • The general public shall not be exposed to more than 1 mSv per annum (over and above natural background).
• Occupational exposure shall not exceed 20 mSv per annum These limits exclude exposure due to background and medical radiation.

Monitoring Of Radiation Exposure
People who are occupationally exposed to ionizing radiation can be monitored with a dosemeter which is worn as a badge attached to clothing. At monthly intervals the dosemeter is sent to a laboratory where the radiation exposure can be read. In Australia the average radiation worker receives a dose of 0.12 mSv per annum. * Age standardized lifetime probability for whole population. **Age standardized incidence rate for whole population (not necessarily fatal).

Man's Exposure To Ionizing Radiation
The risk of obtaining cancer from 1 mSv of radiation exposure is equivalent to the risk of getting cancer from smoking approximately 100 cigarettes.

Risk Normal Incidence
Risk of cancer from 1 mSv of radiation 1 in 17,000* 57 in 17,000** Risk of severe hereditary effect from 1 mSv of radiation 1 in 77,000 1,770 in 77,000