There would be no life on Earth without lots of sunlight, but we
have increasingly recognised that too much of it on our persons is
not a good thing. In fact it may be dangerous, so we control our
exposure to it.
Sunshine consists of radiation in a range of wavelengths from
long-wave infra-red to short-wavelength ultraviolet, which creates
Beyond ultraviolet are higher energy kinds of radiation which
are used in medicine and which we all get in low doses from space,
from the air, and from the earth and rocks. Collectively we can
refer to these kinds of radiation as ionising
radiation. It can cause damage to matter, particularly
living tissue. At high levels it is therefore dangerous, so it is
necessary to control our exposure.
While we cannot feel this radiation, it is readily detected and
measured, and exposure can easily be monitored.
Living things have evolved in an environment which has
significant levels of ionising radiation. Furthermore, many of us
owe our lives and health to such radiation produced artificially.
Medical and dental X-rays discern hidden problems. Other kinds of
ionising radiation are used to diagnose ailments, and some people
are treated with radiation to cure disease. We all benefit from a
multitude of products and services made possible by the careful use
of such radiation.
Background radiation is that which is naturally and inevitably
present in our environment. Levels of this can vary greatly. People
living in granite areas or on mineralised sands receive more
terrestrial radiation than others, while people living or working
at high altitudes receive more cosmic radiation. A lot of our
natural exposure is due to radon, a gas which seeps from the
Earth's crust and is present in the air we breathe.
Ionising radiation comes from the nuclei of atoms, the basic
building blocks of matter.
Each element exists in the form of atoms with several different
sized nuclei, called isotopes.
Most atoms are stable; a carbon-12 atom for example remains a
carbon-12 atom forever, and an oxygen-16 atom remains an oxygen-16
atom forever. But certain atoms change or disintegrate into
totally new atoms. These kinds of atoms are said to be 'unstable'
or 'radioactive'. An unstable atom has excess internal energy, with
the result that the nucleus can undergo a spontaneous change
towards a more stable form. This is called 'radioactive decay'.
Unstable isotopes (which are thus radioactive) are called
radioisotopes. Some elements, eg uranium, have no stable
When an atom of a radioisotope decays, it gives off some of its
excess energy as radiation in the form of gamma rays or fast-moving
sub-atomic particles. If it decays with emission of an alpha or
beta particle, it becomes a new element. One can describe the
emissions as gamma, beta and alpha radiation. All the time, the
atom is progressing in one or more steps towards a stable state
where it is no longer radioactive.
Another source of nuclear radioactivity is when one form of a
radioisotope changes into another form, or isomer, releasing a
gamma ray in the process. The excited form is signified with an "m"
(meta) beside its atomic number, eg technetium-99m (Tc-99m) decays
to Tc-99. Gamma rays are often emitted with alpha or beta radiation
also, as the nucleus decays to a less excited state.
Apart from the normal measures of mass and volume, the amount of
radioactive material is given
in becquerel (Bq), a measure which
enables us to compare the typical radioactivity of some natural and
other materials. A becquerel is one atomic decay per second*, and
each disintegration produces some ionising radiation.
|1 adult human (100 Bq/kg)
|1 kg of coffee
|1 kg superphosphate fertiliser
|The air in a 100 sq metre Australian home (radon)
|The air in many 100 sq metre European homes (radon)
||up to 30 000 Bq
|1 household smoke detector (with americium)
||30 000 Bq
|Radioisotope for medical diagnosis
||70 million Bq
|Radioisotope source for medical therapy
||100 000 000 million Bq (100 TBq)
|1 kg 50-year old vitrified high-level nuclear waste
||10 000 000 million Bq (10 TBq)
|1 luminous Exit sign (1970s)
||1 000 000 million Bq (1 TBq)
|1 kg uranium
||25 million Bq
|1 kg uranium ore (Canadian, 15%)
||26 million Bq
|1 kg uranium ore (Australian, 0.3%
||500 000 Bq
|1 kg low level radioactive waste
||1 million Bq
|1 kg of coal ash
|1 kg of granite
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
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
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
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.
There are several types of ionising radiation:
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.
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.
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.
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
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
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
Measuring Ionising Radiation
The human senses cannot detect
radiation or discern whether a material is radioactive. However, a
variety of instruments can detect and measure radiation reliably
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
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
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 -
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
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 levels and their
The following table gives an indication
of the likely effects of a range of whole-body radiation doses and
dose rates to individuals:
|10,000 mSv (10
sieverts) as a short-term and whole-body dose would cause immediate
illness, such as nausea and decreased white blood cell count, and
subsequent death within a few weeks.
Between 2 and 10 sieverts in a short-term dose would cause severe
radiation sickness with increasing likelihood that this would be
|1,000 mSv (1 sievert) in a short-term
dose is about the threshold for causing immediate radiation
sickness in a person of average physical attributes, but would be
unlikely to cause death. Above 1000 mSv, severity of illness
increases with dose.
If doses greater than 1000 mSv occur over a long period they are
unlikely to have health effects, but they may create some risk that
cancer will develop many years later.
|250 mSv as short-term dose was maximum
allowable for workers controlling the Fukushima
|Above about 100 mSv
, the probability of cancer (rather than the severity of
illness) increases with dose.
The estimated risk of fatal cancer is 5 of every 100 persons
exposed to a dose of 1000 mSv (ie. if the normal incidence of fatal
cancer were 25%, a 1000 mSv dose would increase it to 30%).
|50 mSv is, conservatively, the lowest
dose at which there is any evidence of cancer being caused in
adults. It is also the highest dose which is allowed by regulation
in any one year of occupational exposure. Dose rates greater than
50 mSv/yr arise from natural background levels in several parts of
the world but do not cause any discernible harm to local
|20 mSv/yr averaged over
5 years is the limit for radiological personnel such as employees
in the nuclear industry, uranium or mineral sands miners and
hospital workers (who are all closely monitored).
|10 mSv/yr is the maximum actual dose rate
received by any Australian uranium miner.
|3-5 mSv/yr is the
typical dose rate (above background) received by uranium miners in
Australia and Canada.
|3 mSv/yr (approx) is the typical
background radiation from natural sources in North America,
including an average of almost 2 mSv/yr from radon in air.
|2.5 mSv/yr (approx) is
the typical background radiation from natural sources, including an
average of 0.7 mSv/yr from radon in air. The minimum dose received
by all humans anywhere on Earth is about 1.5 mSv/yr.
|0.3-0.6 mSv/yr is a typical range of dose
rates from artificial sources of radiation, mostly medical.
|0.05 mSv/yr, a very small
fraction of natural background radiation, is the design target for
maximum radiation at the perimeter fence of a nuclear electricity
generating station. In practice the actual dose is less.
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.
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:
|Terrestrial + house: radon
|Terrestrial + house: gamma
|Cosmic (at sea level)
+20 for every 100m elevation
|Food, drink & body tissue
|1500 (plus altitude adjustment)
|From nuclear weapons tests
|Medical (X-ray, CT etc. average)
||up to 75,000
|From nuclear energy
|From coal burning
|From household appliances
||8 per week
|Air travel in jet airliner
||1.5-5 per hour
||up to 5000/yr
The International Commission for Radiological Protection
recommends, in addition to background, the following exposure
for general public, 1,000 (i.e. 1 mSv/yr)
for nuclear worker 20,000 (i.e. 20 mSv/yr) averaged over 5
Sources: Australian Radiation Protection & Nuclear Safety
Agency, Health & Safety Executive (UK), Australian Nuclear
Science & Technology Organization, various
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
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
There are four ways in which people are
protected from identified radiation sources:
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
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
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
In any country, radiation protection
standards are set by government authorities, generally in line with
recommendations by the International Commission on Radiological
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
The three key points of the ICRP's
- Justification. No practice should be adopted unless its
introduction produces a positive net benefit.
- Optimisation. All exposures should be kept as low as reasonably
achievable, economic and social factors being taken into
- Limitation. The exposure of individuals should not exceed the
limits recommended for the appropriate circumstances.
National radiation protection standards are based on ICRP
recommendations for both Occupational and Public exposure
The ICRP recommends that the maximum
for occupational exposure should be 20
millisievert per year averaged over five years (ie 100 millisievert
in 5 years, about 8 time average dose from natural background) with
a maximum of 50 millisievert in any one year.
For public exposure, 1 millisievert per
year averaged over five years is the limit. In both categories, the
figures are over and above background levels, and exclude medical
Ionising radiation has been
studied very intensively for more than a century. Compared with
many things which influence human health, it is well understood
scientifically. The main internationally-recognised authority on
ionising radiation is the UN Scientific Committee on the Effects of
Atomic Radiation (UNSCEAR), set up in 1955. Its
mandate is to assess and report levels and effects of exposure to
Public, and even medical practitioners', understanding of ionising
radiation is generally low, which gives scope for generating
misinformation on the subject resulting in fear. This is compounded
by the invisible nature of radiation, and a frequent confusion of
units used to describe both radioactivity and radiation exposure.
However, radiation is easily detectable and precisely measurable.
Furthermore, the effects are well-known, though contested from
non-scientific sources and popular folklore.
Lack of understanding and properly measuring radiation has major
public health effects. In 1987 in Brazil an old radiotherapy source
the size of a small cup stolen from an abandoned hospital caused
four deaths, 20 cases of radiation sickness and significant
contamination of many more. In 1986 the Chernobyl nuclear accident
caused a few (preventable) deaths from thyroid cancer and massive
psycho-social impact due to relocation of over 100,000 people,
mostly unnecessarily. (It also caused 28 deaths among clean-up
workers who received high radiation exposure.) For members of the
public, fear of radiation was much more devastating than radiation
The body has defence mechanisms against
damage induced by radiation as well as by chemical carcinogens.
These can be stimulated by low levels of exposure, or overwhelmed
by very high levels.*
* 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.
The occurrence of cancer is not uniform
across the world population, and because of local differences it is
not easy to see whether or not there is any association between low
occupational radiation doses and excess cancers. This question has
been studied closely in a number of areas and work continues, but
so far no conclusive evidence has emerged to indicate that cancers
are more frequent in radiation workers (or those living near
nuclear facilities) than in other people of similar ages in western
Further information on the subject can be found in the
information paper on Nuclear
Radiation and Health Effects.
The ARPANSA web site section on
Radiation and Health is also valuable.