Home Up E1 E2 E3 E4 Topics - E1 Topics E2 Topics E3 Topics E4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 27 28 29 30 31

 

Radiation and the Human Body

A. Sources - 

Data from "Health effects of low levels of ionizing radition", 
committee on biological effects of ionizing radiation, BEIR V, 
National Academy Press, Washington D.C., 1990, p. 19)

Radon

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.

H2O

-----------> 

H2O+

+ e-

gamma
radiation

 

H2O+ + H2O

-----------> 

H3O+  +

 .OH

gamma
radiation

hyrodxyl radical

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
Alpha particles Q = 15, alpha particles more damaging.

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.

0.3 rad

dental x-ray

0-25 rad

reduces white blood cell count

25-100 rad

fatigue, blood changes, nausea

100-200 rad

nausea, vomiting, probable death

200-400

bone marrow damage - 1/2 exposed will die without treatment

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

Inside body:

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

2. Technetium

  ,  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

Carbon dating

 

----> ----> Death
In atmosphere,

147N + 10n-->146C+11H

14C + O2 --> 14CO2

In living systems, 14C continually decaying and being replenished.

Assumption:

14C/ 12C = constant

146C-->147N+b

t1/2 = 5730 yrs

After plant dies, ratio of 14C / 12C decreases.

For example:  if 14C / 12C = 2x10-7 for a living plant

and

14C / 12C = 1x10-17 for a dead plant,  the plant died 5730 years ago (one half-life).