Radioactivity and Food
An
annual report is published jointly by MAFF and the Scottish Environment
Protection Agency (SEPA) showed that consumers' exposure to radioactivity
via the food-chain in 1999 remained well below nationally and internationally
recommended limits of 1 millisievert
per year.
An American study
found that radioactivity of food is due mainly to accumulations of Ra226,
Th232, K40, C14 and H3. These
contribute about 0.2 mSv to the total annual dose from natural radiation
sources.
Public water supplies
are unlikely to contain significant amounts of radon.
However, private supplies (such as from wells or boreholes) can contain
high levels of dissolved radon. This can happen if ground waters have
filtered though uranium or radium rich rocks. This is rare but can result
in high levels of dissolved radon. Radon released from water usage in
these cases then contributes to room air concentrations of the gas and
may also increase intestinal cancer risks though direct ingestion of the
water.
Radioactive Isotopes
in food due to human activity
The accident at the
Chernobyl nuclear plant made us very aware of how quickly radioisotopes
are incorporated into the food chain. Almost immediately increased radiation
levels were found in leafy green vegetables growing in contaminated fields
and in cows' milk (due to the 'radioactive grass'). Radioactive particles
that went up into the atmosphere fell down to Earth dissolved in raindrops,
steadily increasing the radioactivity of the soil for several weeks. Early
estimates of how quickly radiation levels would drop were found to be
wrong. Instead of decreasing levels of radioactivity
crops were found to have successively increased levels.
Animals grazing on
contaminated fields concentrated radioisotopes in their tissues, and passed
them on to the carnivore that eats them.
Humans eating the
animals or vegetation that are contaminated then get a larger than normal
dose of radioisotopes that produce ionizing
radiation within their bodies. This can lead to DNA
damage and mutations and cancers.
Discharges from nuclear
power plants are carefully monitored and restricted by the Government
departments that have responsibility for the environment and food. The
highest estimated level of exposure to consumers from artificially produced
radionuclides in food from around the Sellafield area is only 10% of the
recommended limit compared to 14% in 1996. This is mainly due to the decreased
consumption of some shellfish. However the report also demonstrates that
natural radionuclides such as
lead-210, polonium-210 and carbon-14 are by far the most important source
of exposure in the average diet of consumers.
|
Carbon-14
C14
|
half
life of 5700 years |
Found
in all organic matter (plant and animal) |
|
Cesium-137
Cs137
|
half-life
of 33 years |
- Cesium-137
is produced in a fission reactor
- it
is distributed throughout the body, taking the place of potassium.
|
|
Calcium
45
Ca45
|
half-life
of 164 days |
- Calcium-45
is produced in fission reactors
- Calcium
supplements obtained from animal bones or oyster shells will
contain environmental contaminants, including radioisotopes,
but calcium obtained from limestone (dolomite) will not be contaminated
by recent radioactive fallout.
|
|
Iodine
131
I131
|
half-life
of 8 days |
- Iodine
131 is also produced in fission reactors
- it
competes with dietary supplies of normal iodine, and may be
concentrated and stored in the thyroid gland.
- concentration
and storage of the isotope in the thyroid increases the risk
of cell mutation, and the subsequent emergence of cancer. Immediate
impairment of thyroid function may also occur.
- iodine
supplements reduce the opportunity for the radioactive isotope
to be taken up. Doses as high as 100 mg of potassium iodide
for 7-10 days have been recommended for those directly exposed
to radioactive fallout. This is a toxic dose of iodide under
normal circumstances and should not be taken unless there is
real danger.
|
|
Lead-210
Pb210
|
half-life
of 19 years |
- A
decay product in the Uranium (4n+2) series
|
|
Polonium-210
Po210
|
half-life
of 138 days |
- A
decay product in the Uranium (4n+2) series
|
|
Potassium-40
K40
|
half-life
of 1.3 x 109 years |
- Constitutes
0.011% of naturally occurring potassium
- vital
for transmission of bio-action potentials
- emits
beta rays
- The
"average" banana contains 400 mg of Potassium
|
|
Plutonium
(all
isotopes are radioactive)
|
half-life
of 24,000 years |
- Plutonium
is the most dangerous isotope produced in nuclear reactors.
It is toxic but if inhaled or ingested the biggest danger is
from radiotoxicity
|
|
Radium
(all isotopes are radioactive)
|
half-life
of 1600 years - for Ra-226 |
- Brazil
nuts contain Radium
|
|
Strontium
90
Sr90
|
half-life
of 20 years |
- Strontium-90
has got into the environment from weapon testing fall out.
- It
gets into food supplies and then is concentrated in animal and
fish bones.
- The
slow accumulation of this isotope in human bone can be expected
to increase the mutation rate in bone and bone marrow, increasing
the incidence of bone cancer and leukaemia.
- Calcium
supplements are useful as they may compete with strontium-90
and reduce its storage in bones.
|
: Useful
URLs :
http://www.foodstandards.gov.uk/surv/rifeprevious.htm
http://www.maff.gov.uk/inf/newsrel/1998/980918b.htm
http://www.maff.gov.uk/inf/newsrel/fsa/fsa2099.htm
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