Foreword
This booklet aims at providing you with basic
information about atomic radiation and its applications, health
effects and the programme of monitoring and control in Hong
Kong.
Introduction
Radiation, and its effects, are a topic which
arouses a great deal of concern and controversy - often without
a proper understanding of all the facts. How much do we actually
know about it? What is it? Where does it come from and how
does it affect us?
I. What is radiation?
Matter is made up of tiny units called atoms.
Every atom has a nucleus and a surrounding cloud of electrons.
The nuclei of some atoms, like those of 40Potassium,
are more unstable than others. They may change their structure
and consequently their physical and chemical properties spontaneously.
When an unstable nucleus undergoes changes, invisible particles
or waves are released. The particles and waves are called
radiation, the unstable nucle us is said to
be radioactive. Radioactive nuclei are called
radionuclides.
The Universe is filled with radiation. Since
the inception of time, lives on Earth have been exposed to
radiation in the natural environment. Radiation cannot be
felt, smelt, seen, heard or tasted. However, with the use
of instrument, it can be detected and measured.
The unit to measure radiation dose to tissues
is sievert (Sv). One millisievert (mSv) is one-thousandth
of a sievert.
II. Ionizing Radiation
Radiation with energy high enough to remove
electrons from an atom to create an electrically charged ion
is called ionizing radiation.
This ionization process often results in
chemical changes in living tissue, which can lead to injury
in the organism.
Types of Ionizing Radiation
The various types of ionizing radiation include :
- alpha particles, which are swiftly moving nuclei
of helium atoms and carry positive charges. They have little
power of penetration and can be easily stopped by a sheet
of paper or the outer layer of the skin. However, alpha
emitting materials are harmful to health if they enter the
body by inhalation or along with food or water.
- beta particles, which are high speed electrons
and are more penetrating than alpha particles. A sheet of
aluminium a few millimetre thick can stop beta particles.
- X-rays and gamma rays, which are both very penetrating
and can pass right through the body. Dense materials such
as lead or concrete are more effective in absorbing these
rays.
- neutrons, which do not carry any electric charge
and are constituents of atomic nuclei. Hydrogen-rich materials,
such as water or paraffin can shield against these highly
penetrating particles.
III. Radioactive
"Half-Life"
The radioactivity of a material decreases
with time. The time it takes for a material to lose half of
its original radioactivity is its half-life. For example,
131 iodine has a half-life of 8 days. It loses
half of its initial radioactivity in 8 days, and half of the
remaining radioactivity in another 8 days, and so on. Each
radionuclide has a characteristic half-life. The half-lives
of various radionuclides may vary from millionths of a second
to millions of years.
|
Radionuclide
|
Half-life
|
|
219 Radon
|
4.0
|
second
|
|
38 Potassium
|
7.6
|
minute
|
|
73 Selenium
|
7.2
|
hour
|
|
131 Iodine
|
8.0
|
Day
|
|
60 Cobalt
|
5.3
|
Year
|
|
137 Caesium
|
30.2
|
Year
|
|
14 Carbon
|
5 730
|
Year
|
|
129 Iodine
|
15 700 000
|
Year
|
|
235 Uranium
|
703 800 000
|
Year
|
|
40 Potassium
|
1 277 000 000
|
Year
|
IV. Sources of radiation
There are two generic sources of radiation
- natural and artificial.
(i) Natural Radiation
We are continuously exposed to cosmic ray
from space and the radiation emitted by radioactive substances
that exist in everything on Earth, including our food and
habitat. Natural radiation accounts for about 80% of the
radiation doses to which we are subjected. It may vary from
place to place.
(ii) Artificial radiation
Radiation has many useful applications
in medicine. These include x-ray radiology, nuclear medicine
imaging and radiation therapy. Medical use of radiation
is the most significant source of human-made radiation.
Other major artificial sources of radiation
exposure are radioactive fallout from nuclear testings,
x-ray emission from vacuum tubes, such as television and
video display units, and use of radioactive materials in
consumer products, such as radioluminous articles and smoke
detectors. Air travelling enhances our exposure to cosmic
ray and living indoor enhances our exposure to radon, which
is a radioactive gas from the decay of radium in terrestrial
materials.
V. Application of radiation
Put to proper use, radioactive substances
and radiation can be used to the benefits of the society.
(i) Electricity Generation
The demand for energy increases with the
world's booming population and expanding economy. We are
consuming energy at a pace much faster than the Earth can
replenish it. Nuclear energy is a source to meet this ever
increasing demand of energy. To date, there are over 430
commercial nuclear power reactors in operation around the
world. These reactors generate 17% of the electricity demand
world-wide. They are designed and operated to meet the very
high safety standards.
In these nuclear power reactors, energy
comes from splitting, or fissioning nuclei of uranium or
plutonium.
Since 1994, a pressurized water reactor
(PWR) type nuclear power station has been in operation to
the northeast of Hong Kong at Daya Bay, about 50km away
from the city centre. The power station delivers about 70%
(percent) of its electricity output to Hong Kong.
(ii) Medical Applications
Radioactive materials when administered
internally to the body provide images of the function of
body organs and tissues. And x-ray, when applied externally,
can provide images for the identification of abnormal changes
in body organs and tissues.
Radiation is also a major tool in the treatment
of certain kinds of cancer. Irradiating tissue affected
by a tumour has proved to be effective in inhibiting the
tumour's growth or in destroying it.
(iii) Industrial Applications
Because of its penetrating characteristics,
gamma radiation is used in the imaging of defects in welds
and metal castings.
Radiation is widely used to provide automatic
quality adjustments in production lines, such as the gauging
of level in beverage cans or density of tobacco in cigarettes.
It is also used to measure the thickness of electroplating
and to eliminate static charges in industries, for example,
in the manufacture of electronic components.
(iv) Applications in Consumer Products
Radioactive materials are used in some
consumer products where the benefits and safety significantly
outweigh the radiation risks. These products include smoke
detectors, luminous signs and lightning conductors.
(v) Archaeological Application
The presence of natural radioactivity on
Earth can be used in the assessment of the age of objects.
Popular techniques include 14carbon dating and
thermoluminescence dating.
VI. The health
effects of radiation
Lives on Earth have always been exposed to
a certain level of natural radiation. Although radiation may
cause damages to body cells and tissues, health effects are
insignificant unless the dose of radiation is large. The effect
depends on the intensity of the radiation, the length of the
exposure, and the type of body cell exposed.
Sudden large doses of magnitude above 1 000
mSv to the whole body can cause acute radiation injuries,
with short-term symptoms like nausea, vomiting, extreme tiredness
and hair loss. A dose of 10 000 mSv or more can be fatal unless
good medical attention is available.
Besides, exposure to radiation can increase
the risks of cancers to the exposed individuals and genetic
defects to their offspring.
Radiation dose received by the general public
in daily life are very low. Even for workers who are exposed
to radiation in their work, the expected radiation induced
mortality rate is small when compared with some common causes
of death as shown in the table below :
|
Average annual risk of death in Hong
Kong from some common causes
|
|
Cause
|
Annual risk of death
|
|
Smoking 10 cigarettes a day
|
1 in
|
2001
|
|
Malignant diseases
|
1 in
|
6302
|
|
Occupational accidents
|
1 in
|
55 0003
|
|
Traffic accidents
|
1 in
|
22 2002
|
|
Radiation exposure in work for local
workers
(average 0.15 mSv/year)5
|
1 in
|
170 3334
|
1. From Living with Radiation, UK, National
Radiological Protection Board, 1998
2. Based on Annual Report, Department of
Health, 1997/98
3. Based on Annual Report, Labour Department,
1997
4. Based on a risk factor of 4 x 10-5
per mSv adopted by ICRP, publication 60, 1990
5. Occupational monitoring data, Department
of Health, 1998
VII.
Environmental radiation monitoring programme in Hong Kong
From 1987 to 1991, the Hong Kong Observatory
conducted a comprehensive radiation monitoring programme to
establish the baseline radiation levels in various environmental
media in Hong Kong. These levels are compared with those obtained
after the Guangdong Daya Bay Nuclear Power Station was put
in commercial operation in 1994. In the programme, samples
of air, water, foodstuff, soil and the like are collected
and analyzed for their radioactivity content. Environmental
gamma radiation levels were also measured. The measurement
results show that there is no observable increase in the radiation
level in the environment of Hong Kong. Details of measurements
are published in the monthly and annual reports of the Hong
Kong Observatory.
VIII. Legal controls
The import, export, possession and use of
radioactive substance and irradiating apparatus in Hong Kong
is governed by the Radiation Ordinance (Cap 303), Laws of
Hong Kong, which provides for the establishment of the Radiation
Board.
A licence from the Board is required for
any person to carry out any activity involving radioactive
substance or irradiating apparatus. The Radiation Health Unit
of the Department of Health serves as the licensing office
of the Radiation Board. The unit also serves as Government's
adviser on radiation health matters, provides occupational
and environmental radiation monitoring services, and maintains
the radiation metrology standards of Hong Kong.
IX. Further Information
You are welcome to contact the Radiation
Health Unit of the Department of Health at Tel. no. 2886 1551,
Fax no. 2834 1224, or visit the websites of the Department
of Health, http://www.info.gov.hk/dh-rhu
and the Hong Kong Observatory , http://www.info.gov.hk/hko
for more information.
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