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By Charles Rhodes, P.Eng., Ph.D.

An overview of radiation safety matters is contained in the slide presentation titled:
Radiation and Health
by Dr. Alex Cannara.

A video that puts nuclear radiation safety issues into perspective

Significance of failed historical foundation of LNT model

LNT and Cancer Risk Assessment Part 1 Radiation and Leukemia

LNT and Cancer Risk Assessment Part 2 How Unsound Science Came to be Accepted

Photon radiation can be divided into two categories, ionizing radiation and non-ionizing radiation. Ionizing radiation is very short wavelength radiation such as x-rays and gamma rays. Each photon of ionizing radiation carries enough energy that, if the photon is absorbed by a multi-atom molecule, the photon energy is sufficient to break the atomic chemical bonds. Non-ionizing radiation is longer wavelengh radiation that only weakly interacts with matter and generates heat. Ionizing radiation can damage biological tissue by destroying the chemical structure of the tissue or by creating chemically agressive ions in the tissue which then overcome the immune system and then cause secondary damage. Non-ionizing radiation only causes heating of the tissue.

Examples of non-ionizing radiation are radiation associated with AC power lines, radio transmissions and microwave transmissions. Some care is required close to high power radio and microwave sources because high local heat generation can cause tissue damage.

A sievert (Sv) is a unit indicating the cumulative effect of absorbed ionizing radiation on biological tissue. Typical ionizing radiation exposures are expressed in mSv / year and hourly exposures are expressed in uSv / hour.

Through the process of evolution existing animal species have become tolerant to the level of background ionizing radiation typically found on Earth. This level is typically about 3 mSv / year in North America but can be as high as 50 mSv / year in countries such as Iran that have high levels of background radiation due to local mineral deposits that slowly decay emitting the radioactive inert gas radon.

At an ionizing radiation exposure of 100 mSv / year the effect of radiation exposure on the incidence of cancer is statistically noticeable.

In an accident situation a short term radiation exposure of 1.0 Sv will cause about 5% mortality. A short term radiation exposure of 5.0 Sv is lethal without immediate medical intervention.

Hazardous nuclear emissions fall into six categories:
1. Alpha particles (energetic Helium-4 nuclei);
2. Beta particles (energetic electrons and positrons);
3. Gamma and x-ray photons;
4. Neutrons;
5. Protons (energetic hydrogen-1 nuclei);
6. Deuterons (energetic hydrogen-2 nuclei);
7. Ions (energetic nuclei of all types).

Strictly speaking out of this list of hazards only gamma and x-ray photons are radiation. The other hazards are all energetic particles that have rest mass. However, for regulatory purposes, because energetic particles cause biological tissue damage analogous to gamma and x-ray radiation the energetic particle hazards are loosely referred to as forms of "nuclear radiation".

We do not live in a radiation and energetic particle free environment. We are surrounded by rocks containing small amounts of radio isotopes with half lives greater than 300 million years. We ignest food containing shall amounts of radio isotopes such as potassium-40. We breath air containing small amounts of the radio isotope C-14. We are constantly being bombarded by high energy particles from outer space known as cosmic rays. We refer to this naturally occurring nuclear radiation as "background radiation".

All life forms on Earth have evolved to tolerate background radiation. As long as the man made nuclear radiation level is small compared to the background nuclear radiation level man made nuclear radition poses no health risk. In fact medical studies have shown that small amounts of whole body nuclear radiation above the background level actually improves average human health because this limited whole body radiation stimulates the human immune system. However, we are cautious about increasing the nuclear radiation level to which human reproductive organs are exposed for fear of possibly causing birth defects due to potential damage to reproductive DNA molecules.

DNA molecules have elaborate mechanisms which generally ensure that during reproduction and tissue growth these molecules make correct copies of themselves. However, a large amount of nuclear radiation can overwhelm this copy protect mechanism causing sterility or birth defects. Hence the issue of nuclear radiation safety is taken very seriously by engineers in both the scientific and medical communities.

Alpha particles are He-4 nulcei. Alpha particles are particularly dangerous when absorbed in the human body because their relatively high mass leads to high local tissue damage. On a per unit of energy basis alpha particles are considered to be 20X as toxic as beta particles and gamma rays. Alpha particles are emitted during natural decay of some high atomic weight radio isotopes.

Beta particles are usually high energy electrons. However, in some cases beta decay refers to positron emission which can cause more tissue damage than electron emission.

Gamma rays are high energy photons. High energy gamma photons can penetrate a large thickness of shielding mass and hence are much more difficult to contain than alpha or beta particles.

Neutrons, high energy protons, high energy deuterons and high energy ions are all hazardous. However, significant fluxes of these particles are generally only found in outer space and in the immediate proximity of special purpose man made equipment such as nuclear reactors and particle accelerators. Large fluxes of these particles do not normally occur as a result of natural radioactive decay on Earth. Neutrons of any energy are hazardous because on absorption they cause other substances to become radio active. Quantification of these specialized particle hazards is beyond the scope of this web page.

A unit of absorbed ionizing radiation energy is a gray (Gy). One gray is defined as the amount of ionizing radiation that dissipates 1 joule / kg in a material. A short term whole body gamma radiation dose of 1 Gy is a dangerous radiation exposure with about a 5% mortality. At a whole body dose of 5 Gy the mortality rate without sophisticated medical intervention is almost 100%. The hazard decreases if the dose is spread out over a period of years. For example, life saving medical procedures may cumulatively give a patient a x-ray / gamma ray dose of up to 500 mGy per year.

Another frequently used unit of absorbed ionizing radiation energy is a rad. One rad equals 0.01 Gy.

A unit of biological radiation exposure is a sievert (Sv). This is a large amount of exposure. One Sievert is defined as the amount of ionizing radiation that causes a biological effect equal to dissipating 1 joule / kg of gamma radiation in a body. Note that 0.05 Gy of absorbed energy from alpha particles causes 1.0 Sv of biological radiation exposure.

The regulated maximum safe limit for general public exposure to non-medical ionizing radiation is 1.0 millisievert per year (1.0 mSv / year). For individually monitored nuclear workers the maximum regulated limit is 50 mSv / year subject to a maximum dose over a 5 year period of 100 mSv.

For nuclear workers the nuclear radiation dose rate is usually expressed in microsieverts per hour (uSv / hr). An ongoing workplace biological dose rate in excess of 10 uSv / hr is a matter for immediate administrative concern as it may lead to worker activity restrictions.

This web page last updated February 6, 2021

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