On Nuclear Radiation Part 3: Effects of Different Radiation

This is a third part to a series of FAQs which I received from my friends and family after the Nuclear Accident in Japan Fukushima.  Note that most of the notes came from a set of lecture notes that I compiled 2 years ago.  The lecture notes were integrated from various sources which was used for lecturing Nuclear Physics for A-levels. 

(Picture from Reuters:  The No.3 nuclear reactor of the Fukushima Daiichi nuclear plant is seen burning, March 14, 2011. REUTERS/Digital Globe.)

Question:  Of the Types of Ionising Radiation, which is the most harmful?  How do they affect the human body?

This is a complex question, as each type of ionising radiation impacts the body differently. In general, the effect of ionizing radiation on the human body or any other living things depend on three things:

1. The type of ionizing radiation which was absorbed,
2. The number of cells affected, the number and
3. The amount and the rate at which ionizing radiation was absorbed.

  Let us look at the effect of each type of ionizing radiation

• Alpha-particles:

Even though alpha-particles have a short range in air and can be stopped by the skin or a sheet of paper, it will still pose a danger if it gets into the body through ingestion and inhalation.

Alpha particles are the largest of the radiation and carry a charge of (+2e) - hence possessing the greatest ionization power, the particles will deposit their energy over a smaller volume (possibly only a few cells) if they enter the body and cause more damage to these few cells.

Radon gas (one of the key sources of lung cancer in US) is an example of an isotope that radiates alpha particles during of its course of decay and hence, there has been suggestions for buildings to be tested for Radon gas.

•   Beta Particles:

Beta-particles have a longer range than alpha-particles, but ionise much less strongly. Therefore, they do around 1/20 th of the damage done by the same dose of alpha particles.  But with greater penetrating power, affect a greater number of cells.

•   Gamma Rays, X-rays :

Gamma and x-ray are pure energy (photons) and have no mass.  They are however very penetrating and can easily pass through the whole body, and hence will still interact with many atoms in the body as they pass through.  Both X-rays and gamma rays spread their energy over a large volume, which causes less damage per collision. Of course at very high levels of exposure, they can still cause a great deal of tissue damage. Furthermore, because of their penetrating ability, they can easily reach internal organs and bones which is why large doses can be used to damage cancer tissue.

Interaction of Radiation and Cells
So how do the radiation affect the cells and human body?
The effects of the radiation can be characterised as:

• Direct effects
• Indirect effects

Direct Effects
For direct effects, the radiation interacts with the atoms of the DNA molecule, or some other cellular component critical to the survival of the cell.

Such an interaction may affect the ability of the cell to reproduce and, thus, survive. If enough atoms are affected such that the chromosomes do not replicate properly, or if there is significant alteration in the information carried by the DNA molecule, then the cell may be destroyed by “direct” interference with its life-sustaining system.

Indirect Effects
However, the probability of the radiation interacting with the DNA molecule is very small since these critical components make up such a small part of the cell. The radiation has a higher probability interacting with the water that makes up most of the cell’s volume.

When radiation interacts with water, it may break the bonds that hold the water molecule together, producing fragments such as hydrogen (H) and hydroxyls (OH). These fragments may recombine or may interact with other fragments to form toxic substances, such as hydrogen peroxide (H2O2), which can contribute to the destruction of the cell.


In general, the following are possible effects of radiation on the cells:
1. Cells are undamaged.
Ionisation may form chemically active substances which in some cases alter the structure of the cells. These alterations may be the same as those changes that occur naturally in the cell and may have no negative effect.


2. Cells are damaged, but the damage is repaired and they continue to operate normally.
Some ionizing events produce substances not normally found in the cell. These can lead to a breakdown of the cell structure and its components. Cells can repair themselves if the damage is limited. Even damage to chromosomes is usually repaired.


3. Cells are damaged, repaired but is mutated.
When cells divide to reproduce, an exact copy of the cell's chromosomes are created for the new cell. If the DNA of the chromosome is damaged, the instructions that control the function of the cells and reproduction are also damaged.

If the cells reproduce instead of die, a new mutated cell may be produced. In many cancers, the instruction that turns off cell growth are somehow damaged causing cells to reproduce out of control, creating tumours.

Ionizing radiation, along with many other substances such as some chemicals, heavy metals and intense e.m. waves can damage cells in this manner.

Radio-Sensitivity of Cells:
Not all cells in the human body respond in the same way to radiation.  The most radiosensitive cells are those which :

*have a high division rate

*have a high metabolic rate

*are of a non-specialized type, and

*are well-nourished.

Examples of radio-sensitive cells :

*reproductive cells

*Blood forming tissues

* Epithidium of skin

*Epithidium of gastrointestinal tract.

This is why when people go on radiotherapy, their alocepia (hair loss) tends to occur as hair is a fast growing cell.  This also why young children/babies and foetuses carried by pregnant mothers are especially susceptible to radiation. This is because children/babies are growing rapidly, with more cells dividing and thus a greater opportunity for radiation to disrupt the process.

Related Links  and Previous Related Posts:

1)  On Nuclear Physics Part I :  What is Radiation?  Does a person continue to radiate radiation after he has been irradiated?

2) Can we therefore make the radioactive isotopes non-radioactive by chemical reactions etc.?

3)  Japan's Nuclear Concerns Explained by CNN

4)  Why do the Spent Fuel Rods Need to Be Cooled Down?  What happens If they are Not?  - External Blog :  Fukushima's Spent Fuel Rods Pose Great Dangers.

5) Scientific American Article : Radiation's Complications: Pinning Health Problems on a Nuclear Disaster Isn't So Easy