Radiation and Life

N.B. Though the intrinsic radioactivity is the same, the radiation dose received by someone handling a kilogram of high-grade uranium ore will be much greater than for the same exposure to a kilogram of separated uranium, since the ore contains a number of short-lived decay products (see section on Radioactive Decay), while the uranium haas a very long half-life. 

Half-life

 

Atoms in a radioactive substance decay in a random fashion but at a characteristic rate. The length of time this takes, the number of steps required and the kinds of radiation released at each step are well known.

The half-life is the time taken for half of the atoms of a radioactive substance to decay. Half-lives can range from less than a millionth of a second to millions of years depending on the element concerned. After one half-life the level of radioactivity of a substance is halved, after two half-lives it is reduced to one quarter, after three half-lives to one-eighth, and so on.

All uranium atoms are mildly radioactive and decay through a number of steps on the way to becoming stable lead. Each step has a different half life, and a characteristic type of radiation. The shorter-lived each kind of radioisotope in the decay series, the more radiation it emits per unit mass. Much of the natural radioactivity in rocks and soil comes from the uranium-238 (U-238) decay chain (but not from the uranium itself).
 

 

 

 

 

 

 

 

 

 

Types of Ionising Radiation

Here we are concerned mainly with ionising radiation from the atomic nucleus. It occurs in two forms, rays and particles, at the high frequency end of the energy spectrum.

Ionising radiation produces electrically-charged particles called ions in the materials it strikes. This process is called ionisation. In the large chemical molecules of which all living things are made, the changes caused may be biologically important.

X-rays and gamma rays, like light, represent energy transmitted in a wave without the movement of material, just as heat and light from a fire or the sun travels through space. X-rays and gamma rays are virtually identical except that X-rays are generally produced artificially rather than coming from the atomic nucleus. But unlike light, X-rays and gamma rays have great penetrating power and can pass through the human body. Mass in the form of concrete, lead or water are used to shield us from them.

Alpha particles consist of two protons and two neutrons, in the form of atomic nuclei. They thus have a positive electrical charge and are emitted from naturally-occurring heavy elements such as uranium and radium, as well as from some man-made elements. Because of their relatively large size, alpha particles collide readily with matter and lose their energy quickly. They therefore have little penetrating power and can be stopped by the first layer of skin or a sheet of paper. 

However, if alpha sources are taken into the body, for example by breathing or swallowing radioactive dust, alpha particles can affect the body's cells. Inside the body, because they give up their energy over a relatively short distance, alpha particles can inflict more severe biological damage than other types of radiation.

Beta particles are fast-moving electrons ejected from the nuclei of many kinds of radioactive atoms. These particles are much smaller than alpha particles and can penetrate up to 1 to 2 centimetres of water or human flesh.  They can be stopped by a sheet of aluminium a few millimetres thick.

Cosmic radiation consists of very energetic particles, mostly protons, which bombard the Earth from outer space. It is more intense at higher altitudes than at sea level where the Earth's atmosphere is most dense and gives the greatest protection.

Neutrons are particles which are also very penetrating. On Earth they mostly come from the splitting, or fissioning, of certain atoms inside a nuclear reactor. Water and concrete are the most commonly used shields against neutron radiation from the core of the nuclear reactor.

It is important to understand that alpha, beta, gamma and X-radiation does not cause the body to become radioactive. However, most materials in their natural state (including body tissue) contain measurable amounts of radioactivity.

Measuring Ionising Radiation

Grays and Sieverts 

The human senses cannot detect radiation or discern whether a material is radioactive. However, a variety of instruments can detect and measure radiation reliably and accurately.

The amount of ionising radiation, or 'dose', received by a person is measured in terms of the energy absorbed in the body tissue, and is expressed in gray. One gray (Gy) is one joule deposited per kilogram of mass.

Equal exposure to different types of radiation expressed as gray do not however necessarily produce equal biological effects. One gray of alpha radiation, for example, will have a greater effect than one gray of beta radiation. When we talk about radiation effects, we therefore express the radiation as effective dose, in a unit called the sievert (Sv).

Regardless of the type of radiation, one sievert (Sv) of radiation produces the same biological effect.

Smaller quantities are expressed in 'millisievert' (one thousandth) or 'microsievert' (one millionth) of a sievert. We will use the most common unit, millisievert (mSv), here.

What are the health risks from ionising radiation?

It has been known for many years that large doses of ionising radiation, very much larger than background levels, can cause a measurable increase in cancers and leukemias ('cancer of the blood') after some years delay. It must also be assumed, because of experiments on plants and animals, that ionising radiation can also cause genetic mutations that affect future generations, although there has been no evidence of radiation-induced mutation in humans. At very high levels, radiation can cause sickness and death within weeks of exposure - see Table.

The degree of damage caused by radiation depends on many factors - dose, dose rate, type of radiation, the part of the body exposed, age and health, for example. Embryos including the human fetus are particularly sensitive to radiation damage.

But what are the chances of developing cancer from low doses of radiation? The prevailing assumption is that any dose of radiation, no matter how small, involves a possibility of risk to human health. However there is no scientific evidence of risk at doses below about 50 millisievert in a short time or about 100 millisievert per year (40 times average annual dose from natural background). At lower doses and dose rates, up to at least 10 millisieverts per year (4 times average background), the evidence suggests that beneficial effects are as likely as adverse ones.  

Higher accumulated doses of radiation might produce a cancer which would only be observed several - up to twenty - years after the radiation exposure. This delay makes it impossible to say with any certainty which of many possible agents were the cause of a particular cancer. In western countries, about a quarter of people die from cancers, with smoking, dietary factors, genetic factors and strong sunlight being among the main causes. Radiation is a weak carcinogen, but undue exposure could certainly increase health risks.

The body has defence mechanisms against damage induced by radiation as well as by chemical and other carcinogens. These can be stimulated by low levels of exposure, or overwhelmed by very high levels.

On the other hand, large doses of radiation directed specifically at a tumour are used in radiation therapy to kill cancerous cells, and thereby often save lives (usually in conjunction with chemotherapy or surgery). Much larger doses are used to kill harmful bacteria in food, and to sterilise bandages and other medical equipment. Radiation has become a valuable tool in our modern world. See also The Peaceful Atom in this series.

Tens of thousands of people in each technically advanced country work in medical and industrial environments where they may be exposed to radiation above background levels. Accordingly they wear monitoring 'badges' while at work, and their exposure is carefully monitored. The health records of these occupationally exposed groups often show that they have lower rates of mortality from cancer and other causes than the general public and, in some cases, significantly lower rates than other workers who do similar work without being exposed to radiation.

Radiation levels and their effects 

The following table gives an indication of the likely effects of a range of whole-body radiation doses and dose rates to individuals:

Background Radiation

Naturally-occurring background radiation is the main source of exposure for most people. Levels typically range from about 1.5 to 3.5 millisievert per year but can be more than 50 mSv/yr. The highest known level of background radiation affecting a substantial population is in Kerala and Madras States in India where some 140,000 people receive doses which average over 15 millisievert per year from gamma radiation, in addition to a similar dose from radon. Comparable levels occur in Brazil and Sudan, with average exposures up to about 40 mSv/yr to many people.

 

Several places are known in Iran, India and Europe where natural background radiation gives an annual dose of more than 50 mSv and up to 260 mSv (at Ramsar in Iran). Lifetime doses from natural radiation range up to several thousand millisievert. However, there is no evidence of increased cancers or other health problems arising from these high natural levels.

Ionising Radiation 

The Earth is radioactive, due to the decay of natural long-lived radioisotopes. Radioactive decay results in the release of ionizing radiation. As well as the Earth's radioactivity we are naturally subject to cosmic radiation from space. In addition to both these, we collect some radiation doses from artificial sources such as X-rays. We may also collect an increased cosmic radiation dose due to participating in high altitude activities such as flying or skiing. The average adult contains about 13 mg of radioactive potassium-40 in body tissue – we therefore even irradiate one another at close quarters! The relative importance of these various sources is indicated:

 

Sources: Australian Radiation Protection & Nuclear Safety Agency, Health & Safety Executive (UK), Australian Nuclear Science & Technology Organization, various  

Man-made Radiation

Ionising radiation is also generated in a range of medical, commercial and industrial activities. The most familiar and, in national terms, the largest of these sources of exposure is medical X-rays. A typical breakdown between natural background and artificial sources of radiation is shown in the pie chart.

Natural radiation contributes about 88% of the annual dose to the population, and medical procedures most of the remaining 12%. Natural and artificial radiations are not different in kind or effect.

Protection from Radiation

Because exposure to high levels of ionising radiation carries a risk, should we attempt to avoid it entirely? Even if we wanted to, this would be impossible. Radiation has always been present in the environment and in our bodies. However, we can and should minimise unnecessary exposure to significant levels of man-made radiation.

Radiation is very easily detected. There is a range of simple, sensitive instruments capable of detecting minute amounts of radiation from natural and anthropogenic sources.

There are four ways in which people are protected from identified radiation sources:

Limiting Time: For people who are exposed to radiation in addition to natural background radiation through their work, the dose is reduced and the risk of illness essentially eliminated by limiting exposure time.

Distance: In the same way that heat from a fire is less the further away you are, the intensity of radiation decreases with distance from its source.

Shielding: Barriers of lead, concrete or water give good protection from penetrating radiation such as gamma rays. Radioactive materials are therefore often stored or handled under water, or by remote control in rooms constructed of thick concrete or even lined with lead.

Containment: Radioactive materials are confined and kept out of the environment. Radioactive isotopes for medical use, for example, are dispensed in closed handling facilities, while nuclear reactors operate within closed systems with multiple barriers which keep the radioactive materials contained. Rooms have a reduced air pressure so that any leaks occur into the room and not out from the room.

Standards and Regulations

Radiation protection standards are based on the conservative assumption that the risk is directly proportional to the dose, even at the lowest levels, though there is no evidence of risk at low levels. This assumption, called the 'linear no-threshold (LNT) hypothesis', is recommended for radiation protection purposes only such as setting allowable levels of radiation exposure of individuals. It cannot properly be used for predicting the consequences of an actual exposure to low levels of radiation. For example, it suggests that, if the dose is halved from a high level where effects have been observed, there will be half the effect, and so on. This could be very misleading if applied to a large group of people exposed to trivial levels of radiation and could lead to inappropriate actions to avert the doses.

Much of the evidence which has led to today's standards derives from the atomic bomb survivors in 1945, who were exposed to high doses incurred in a very short time. In setting occupational risk estimates, some allowance has been made for the body's ability to repair damage from small exposures, but for low-level radiation exposure the degree of protection may be unduly conservative.

In any country, radiation protection standards are set by government authorities, generally in line with recommendations by the International Commission on Radiological Protection (ICRP), and coupled with the requirement to keep exposure as low as reasonably achievable (ALARA) - taking into account social and economic factors. The authority of the ICRP comes from the scientific standing of its members and the merit of its recommendations.

The three key points of the ICRP's recommendations are: