Safety

Radiation Effect on Life

Manhattan project workers in Los Alamos, Oak Ridge, Hanford, and atomic workers in the former U.S.S.R. suffered various degrees of anorexia (loss of appetite), fatigue, headache, nausea, vomiting, and diarrhea. Their physical states provide information on the delayed effects of ionizing radiation.

Radiation effects are divided into somatic effects and genetic effects. The former affects the function of cells and organs, whereas the latter affects the future generations.

Somatic Effects

Somatic effects are cell damages that pass on to succeeding cell generations.

Radiation affects rates of cell division. Hastening and slowing down cell division affect embryonic tissues. Damages to cell membranes, mitochondria and cell nuclei result in abnormal cell functions, affecting their division, growth and death.

Exposure to radiation strongly affects the rapidly dividing tissues and cells. Effects on some of these tissues are listed below:

skin - skin disease and cancer
lining of the gastrointestinal tract - hinders digestion and absorption
bone marrow - anemia
embryonic developments - mature to have disproportionate parts

Genetic Effects

Genetic effects are damages to genes and chromosomes that affect future generations.

Genetic information for the production and function of a new organ is contained in the chromosomes of germ cells - sperm and ovum. In reality, genetic effects are due to the damages to DNA molecules.

Normal human somatic cells contain 46 chromosomes. They determine not only the physiological characteristic but also personality of a individual.

Each chromosome contains a deoxyribonucleic acid (DNA) molecule, which is a complex macromolecule (gigantic polymer). Each DNA consists of two complementary strands, each of which consists of a backbone made up of phosphoric acid and sugar. Attached to each sugar molecule are two types of bases: purine bases (adenine and guanine), and pyrimidine bases (cytosine and thymine).

Adenine (A) always pair up by thymine (T), and guanine (G) by cytosine (C) via hydrogen bonds. The A-T and G-C pairs stack on top of each other, but the two strands spiral around each other. Two successive turns have a distance of 3.4 nm, and 10 pairs of bases pack in this distance. The diameter of the double helix is about 2 nm, but its length reaches several centimeters to meters in some mammalian species.

A gene Each DNA molecule is responsible for many genes.

The hydrogen-bonded base pairs are shown below:

The base pairs are made up of four nucleotides.

DNA molecules carry out two functions: replication and transcription. Replication gives a copy of the DNA molecules to the daughter cells so that all the genetic information is preserved. Transcription synthesizes messenger ribonucleic acid (mRNA) using DNA molecules as templates. The mRNA molecules specify the amino acid sequence in the formation of polypeptide and proteins. Thus, the sequences of base pairs in DNA molecules are accountable for the biochemistry of the subject as well as for the genetic information of future generations. Thus, damages to DNA molecules by radiation result in delayed somatic and genetic effects.

When two or more DNA molecules in the same nucleus are broken by radiation, the broken ends recombine, perhaps not in the previous order. This phenomenon is called translocation. The consequence is acute deformation and mutation.

The Quality Factor and rem

Radiations Q or rbe
X-, gamma- & beta-rays 1
Thermal neutrons (n) 3
Fast n, a, and protons 10
Recoil nuclei 20
The factor is called the quality factor (Q) or relative biological effectiveness (rbe) in older literature. This Q factor, reflects an estimate of the relative harmfulness of various radiation with respect to X-, gama- and beta rays. The Q factors are listed in the Table here.

Multiplication of the absorbed dose (rad or Gy) by the Q factor converted it to a dosage equivalent to that of X-ray or gama-ray. The cgs unit used for the dose equivalent is roentgen equivalent man (rem) whereas the SI unit for it is the sievert

1 Sv = 100 rem Thus, the rem is the dose from any radiation that produces biological effects in man equivalent to one rad or R of X-ray, gamma- or beta rays.

All dosages related to health physics or safety are usually given in rem, but newer literature uses the SI units Sv.

A unit for total energy absorbed by a particular organ called integral dose is gram-rad or g-rad.

Similar to g-rad, the unit g-rem is used for the integral dose of an organ (used in medicine) when other radiation than X-rays, gamma rays, or beta particles are in question.

©cchieh@uwaterloo.ca