Radiation and the Human Body
A. Sources -
Data from "Health effects of low levels of ionizing radition",
Radon (gas) is from the decay of 238U in rocks. It can accumulate in house basements with little air circulation as it seeps through cracks in walls and floors. It is heavier than air.
Rn-222 undergoes radioactive decay to form Po-218 with the emission of an alpha particle.
B. Why does radiation do damage?
Radiation such as gamma rays, x-rays, alpha particles, and beta particles have sufficient energy to break apart chemical bonds. Such radiation is collectively termed ionizing radiation because it causes molecules to fragment and ionize.
Biological systems > 60% water.
The hydroxyl radical is an example of a free radical, a species with one unpaired electron:
Radicals are unstable and highly reactive. The hydroxyl radical can react with a variety of molecules essential to the operation of the body thereby resulting in damage. Furthermore, this process generally regenerates another radical thereby setting up a long sequence of damage.
C. Most Harm is to fast dividing cells within the body: Particularly harmful to cells in
1013 Cells in human body, cell cycle between 8 hours and 100 days or more
1. bone marrow - produces blood cells: red blood cells (erythrocytes) - carry oxygen; white blood cells (leucocytes) - fight infection; lympohcytes - immune respone. Bone marrow produces red blood cells at 2 million / sec)
2. digestive tract (cell cycle - 11 hours)
3. hair folicles
4. reproductive organs
D. Measuring radioactivity
The damage radiation does is proportional to the number of particles and their energy
Measures of the output of a radiation source:
1. Curie (Ci) - Based on rate of radioactive decay - 3.7 x 1010 (37 billion) disintegrations / sec = # of disintegrations / sec for 1 g of pure radium. Essentially measures numbers of particles
normal background: 2-3 disintegrations / sec
Shortcoming - says nothing about the energy of the radiation
Ci very large unit - more typical for medical uses is the mCi or mCi
2. Roentgen (R) - capacity to cause ionization, not the rate at which the source disintegrates, essentially measures intensity of x-rays or gamma rays - energy
600 R in a week generally fatal
Measures used to characterize biological effects:
3. rad (radiation absorbed does) -
energy absorbed by an object exposed to a radiation source. 1 rad deposits 10-2 J of energy / kg of tissue
For soft tissue - the delivery of 1R results in the absorption of 1 rad.
4. rem (roentgen equivalent in man, roentgen equivalent medical) - different types of radiation cause differing levels of damage even if delivering the same amount of energy:
dose equivalent in rem = Q x absorbed dose in rad
Q = relative biological effectiveness
Gamma rays, beta particles, Q = 1
1 rad of alpha radiation causes 15 times more radiation damage than 1 rad of gamma rays; a 1 rem dose of alpha radiation is therefore 15 times less radiation than a 1-rem dose of gamma-rays.
Remember, ionizing radiation is dangerous and does serious damage. It does not, however, make things radioactive. Very high doses (100,000 rad) are used in food irradiation to kill bacteria, insects, and mold.
D. Medical Uses
1. Radioactive iodine 131I --> b + g
iodine in thyroid regulates growth
radioactive I goes to thyroid to provide an image as b particles and g rays emitted (small doses)
Large doses: stops excessive release of thyroid hormones
, a "special" form of this undergoes radioactive decay with the emission of a gamma ray. Used as a radiotracer to help image the brain, kidneys, liver, thyroid, heart, and bone.
t1/2 = 6 hours
3. positron emission tomography
positron given off from unstable nuclei
positrons are similar to an electron but with a positive charge
used to image tissue
o1p , when a positron combines with tissue, burst of light (gamma rays) given off and detected.
Radioisotopes such as carbon-11, nitrogen-13, oxygen-15, or fluorine-18 emit positrons
One use involves labeling glucose with fluorine-18. Glucose is the only food used by the brain. By using labeled glucose it is possible to follow the pathway of glucose through the brain.
E. Scientific uses
After plant dies, ratio of 14C / 12C decreases.
For example: if 14C / 12C = 2x10-7 for a living plant
14C / 12C = 1x10-17 for a dead plant, the plant died 5730 years ago (one half-life).