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Dosimetry

Dosimetry is the science of measuring the dose of ionizing radiation a person receives.  While the word radiation includes all forms of energy, only ionizing radiation can penetrate cells and create ions in the cell contents.  Ionizing radiation can damage cells because when an ion is created a molecular bond may have been broken. (The electron may have been holding two atoms together in a covalent or ionic bond.)

Ionizing radiation includes:

bulletUV light rays
bulletX rays
bulletgamma rays
bulletneutrons
bulletelectrons ("beta" particles)
bulletalpha particles (helium nuclei)

The effect of this radiation on a living body is the Dose.

Units of Dose Measurement

Absorbed Dose (Units: grays  Gy)
 The gray (Gy) represents the amount of energy (joules J) absorbed by 1 kg of tissue. It is a unit of concentration.
                                1 Gy = 1 J/kg

If we could uniformly expose the entire body to radiation, the absorbed dose received would be the same whether we were speaking of a single cell, an organ (e.g., an ovary) or the entire body (just as the concentration of salt in sea water is the same whether we consider a cupful or an entire ocean).

Effective Dose (Units: seiverts Sv)
Some forms of radiation are more efficient than others transferring their energy to the cell. To have a level playing field, it is convenient to multiply the dose in Gy by a quality factor (Q) for each type of radiation. The resulting unit is the Sv (seivert). Thus, Sv = Gy x Q.    The quality factor compares the damage done by each type of radiation to the damage done by the same dose of gamma rays:

Type Q
X or Gamma rays 1
alpha 10~20
beta ~10
neutrons ~5

X rays and gamma rays have a Q about 1, so the absorbed dose in Gy is the same number in Sv.   Neutrons have a Q of about 5 and alpha particles have a Q of about 20.   An absorbed dose of, say, 1 Gy of these is equivalent to 5 Sv and 20 Sv respectively.

Older units: rad and rem

All the earlier work on radiation was done in units based on the energy measured in ergs absorbed per gram of tissue.  The absorbed dose in these units is 100 times greater than in modern units.  The units of absorbed dose were rad ("Roentgen Absorbed Dose) and rem (Roentgen Equivalent Man).  We no longer use these units, but in the US there is still a lot of information available in rads and rems.  Just divide by 100!

1 Gy = 100 rad
1 Sv = 100 rem

Common Units:
Because the seivert is a large unit, we more commonly express doses in milliseiverts (mSv):

 

Estimated "Worst Case" Risks From Low Doses of Ionizing Radiation:
 - based on a linear extrapolation of health effects of Hiroshima and Nagasaki bombs
 - does not take into account spreading the exposure over a long time period which appears to dramatically reduce the health effects (see: Health Physics Society position paper)

Effect Risk Normal Incidence
Risk of cancer from 1 mSv of radiation 1 in 17,000*
or 0.006% per lifetime		 57 in 17,000**
or 0.3% per year
Risk of severe hereditary effect from 1 mSv of radiation 1 in 77,000
or 0.001% per lifetime		 1,770 in 77,000
or 2.3% per lifetime

 





* Age standardized lifetime probability for whole population.
**Age standardized incidence rate for whole population (not necessarily fatal).

 

Canadian Cancer Rates
About 27% of Canadians die of cancer.
The risk of obtaining cancer from 1 mSv of radiation exposure is equivalent to the risk of getting cancer from smoking approximately 100 cigarettes.

 

The Health Effects of Radiation:

Source of Nuclear Radiation Effective Dose (Sv) Effective Dose (mSv) Probable Health Effect
Massive exposure due to nuclear accident or bomb 6.0 6 000 ~100% Death due to radiation damage
Nuclear accident or bomb - Approximate lethal dose ("LD50") if no treatment and given to the entire body in a short period 4.5 4 500 50% death due to radiation sickness
~57% chance of contracting cancer.    
(Assumes normal rate of cancer to be ~27% + a 30% chance due to the radiation)
Causes radiation sickness (when absorbed in a short period) 1 1000 ~27% + 1% = ~28% cancer rate
Annual dose in some houses in Ramsar, Iran 0.79 790 No known health effects - dose is per year
Annual dose in Guarapari, Brazil 0.175 175 No known health effects - dose is per year
Apartment building in Taiwan accidentally built using steel contaminated with radioactive Co-60 0.015 15 Possible health benefits?
Cancer mortality was 97% less than normal!
Nuclear accident - workers exposed in one-time exposure 0.010 10 Possible increased chance of cancer, but at limit of detectibility (~0.6% increase in cancer possible).
Annual exposure for people living in control zones near Chernobyl 0.010 10 Unknown since this exposure is spread over a period of time.
Received by the bone marrow during a barium enema 0.008 7 8.7 Possible health effects but below level of detectability
Maximum possible dose received by breast during mammogram 0.007 7 No known health effects
Average annual Canadian exposure from all sources 0.003 3 3.3 No known health effects
Average annual dose (excluding natural background) for medical x-ray technicians 0.003 2 3.2 No known health effects
Flight crew and cabin attendants annually due to cosmic radiation 0.002 2 No known health effects
Hourly dose to skin holding piece of the original "Fiesta Ware" (a brand of pottery once available in the US which used a lead glaze high in radioactive isotopes of lead.) 0.002 2 No known health effects
Maximum permissible annual dose (excluding natural background and medical exposure) to general public due to the operation of nuclear power plants. 0.001 7 1.7 No known health effects
Natural Radiation (per year, excluding radon) 0.001 5 1.5 No known health effects
Most heavily exposed person (a fisherman) near Three Mile Island 0.001 1.0 No known health effects
Natural Radiation (Cosmic radiation at sea level) - total annual exposure 0.000 3 0.3 No known health effects
Received by the bone marrow during a chest x ray 0.000 1 0.1 No known health effects 
Average airline passenger (10 flights/year) 0.000 03 0.03 No known health effects
Average dose to person living within 10 miles downwind of the Three-Mile Island accident of 28 March 1979 0.800 0.08 No known health effects
Additional annual dose if you live in a brick rather than a wood house 0.000 07 0.07 No known health effects
Approximate dose received by a person spending 1 year at the fence surrounding a nuclear power station  0.000 002 0.002 No known health effects
Annual dose to the gonads from TV sets 0.000 002 0.002 No known health effects 
Annual dose due to nuclear fallout
(former weapons testing plus Chernobyl)		 0.000 000 6 0.000 6 No known health effects
Average annual dose to each person due to nuclear power plants 0.000 000 02 0.000 02 No known health effects 

Calculate your annual dose!
Note: to convert rem to Sv divide by 100

An interesting article on the possible health benefits of low doses of radiation!


Background Radiation

About 27% of our annual exposure to radiation is from background radiation:

bulletcosmic radiation (0.027 Sv). The value increases with altitude, so the dose for people in a high location such as Denver, Colorado is about 0.50 Sv.
bulletrocks and soil (0.028 Sv). This value varies with the geology of a region: people in Louisiana (a flood plain) get as little as 0.015 Sv/yr; people in Northern Ontario (Canadian Shield) get 0.14 Sv/yr.  Ten times as much!
bulletfrom within the body (0.4 Sv). Most of this comes from potassium-40. About 0.02% of the potassium in nature is in the form of the radioactive isotope 40K. Living tissue cannot discriminate between radioactive and non-radioactive versions, so the same 0.02% (about 2.7 g in a 70-kg person) is radioactive

 

Information on this page adapted from:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/R/Radiation.html
 http://www.arpansa.gov.au/is_rad.htm