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

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.

Since prior to 1965 the operating assumption has been that the only acceptable level of radiation exposure to the public is a level that is small compared to the background radiation. However, there is now a major body of evidence which indicates that some whole body radiation exposure is actually beneficial because it stimulates the immune system to make DNA repairs that otherwise would not spontaneously occur. This issue may be important in cancer prevention and in treatment of Alzheimer disease and Parkinson disease. Presently available experimental evidence seems to suggest that the maximum "safe whole body photon radiation dose" is somewhere in the region of 500 mSv per year at about 40 mSv / month.

However, caution should be used because certain combinations of critical life saving medical procedures can expose a patient to that much x-ray and gamma radiation in a single year. In order to safely implement these medical procedures the patient should not have had other significant radiation exposure within the year prior to the procedure. Hence presently medical doctors need to be able to assume that their patients' exposure to non-medical radiation is small compared to 500 mSv per year. Thus the nuclear worker limit of 50 mSv per year is well chosen. The choice of a limit of 1 mSv per year for the general public perhaps errors on the side of safety but provides an allowance for unplanned, extraordinary natural and unregulated radiation exposures.

A significant problem in the medical profession is poor or non-existent inter-institutional communication. For example, cardiac doctors at hospital A may be unaware of X-ray or gamma ray exposure that a patient has received as a result of unrelated diagnostic procedures at hospital B or as a result of working in nuclear facility C.

Hence before radiation exposure can be safely used for large scale medical treatment there should be reliable integration of health records at all hospitals, radiation diagnostic/treatment facilities and work places and there must be certainty as to the state of the patient's heart. Some cardiac diagnostic and therapeutic procedures involve substantial radiation doses.

Recently Dr. Jerry M. Cuttler and his associates have found that limited doses of ionizing radiation (40 mGy of x-ray) at approximately monthly intervals are beneficial because they stimulate a patient's immune system in a manner that substantially reduces the incidence of cancer and relieves the symptoms of Alzheimer's disease and Parkinson's disease. It has also been show that radiation treatments extend the average lifetime of dogs. Details of this work are available from the following documents and published papers:

1989 Article by Nobel Prize winner Dr. Rosalyn Yalow (nee Sussman)

Cuttler-2014_DR_Remedy Rad Fear--Discard politicized science


Cuttler-Feinendegen-2015 Feb5_Inhaled PuO2 in dogs-lung cancer Dose Response

Cuttler-2016Apr_Dose-Resp_AD treatment

Cuttler-et al-2017_Update AD patient treated CT scans

Cuttler-2017_PEO-York Chapter_Treat AD with CT-R

Cuttler-et al-2017_Evidence LD rates increase dog lifespans

Cuttler-2017_PEO-York Chapter_Low dose rates increase lifespan

3.5 hour lecture by Jim Welsh and Bill Sacks on Cancer and the Immune System

Alzheimers Disease

Safe Radon Level

Henriksen-2016_Radon, lung cancer and LNT model


Treatment of Cancer and Inflammation With Low Dose Ionizing Radiation

Treating AD with CT Scans


This web page last updated April 4, 2017

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