Radiation Hormesis

T. D. Luckey

• Éditeur: CRC Press
• Année: 2020
• ISBN:
  9781000141856
• Édition: 1
• Sujet: Law
• Sous-sujet: Forensic Science

Radiation Hormesis presents the only critical review of the effects of whole-body exposure to low doses of ionizing radiation in animals. This is a "must read" book for radiobiologists, health-conscious individuals, and serious environmentalists. Topics discussed include our radiation environment, radiation hormesis in cancer mortality, growth and development, reproduction and mutation, immunity, and cancer. Data is presented that indicates that low doses of ionizing radiation may actually be beneficial to human health. This information could invalidate the "zero thesis" and linear models used by most regulatory agencies. The implications regarding eliminating linear models and accepting radiation hormesis are also discussed.

Chapter 1

OUR RADIATION ENVIRONMENT

“We have met radiation and it is us!”

Pogo paraphrased

I. INTRODUCTION

Ionizing radiation is ubiquitous. The earth and everything in it, including you, are radioactive. Background radiation has changed little since the emergence of humans four million years ago. However, life began when earth’s radiation was about ten times more than it is now. Early life forms were molded by ionizing radiation, as well as other environmental agents, and probably used high energy photons from radionuclide decay and cosmic rays to promote radiogenic metabolism.

Evolution is conservative. Animals store so much DNA information for old systems that most of our DNA is metabolically inactive. If radiation were once useful to an organism, the DNA necessary for its use is probably in the storehouse for all subsequent progeny. Vestigial systems remain available in genetic stores. New metabolic pathways routinely branch from old, well used routes. Since ionizing radiation undoubtedly provided free radicals useful in anaerobic fermentations by primitive microorganisms, biopositive effects of ionizing radiation in mammals are reasonable from an evolutionary viewpoint.

The purpose of this book is to review all available evidence and present a basis for the conclusion that hormetic exposures to ionizing radiation are beneficial for humans. Most studies which meet rational criteria for whole-body exposure have data that indicate small doses evoke biopositive effects. In this review doses to specific tissues, such as lungs, bones, or gonads, are generally ignored to provide emphasis on whole-body exposures. The thesis to be examined is: high and low doses of ionizing radiation produce qualitatively opposite effects.

This chapter summarizes exposures to ionizing radiation from the many sources which constitute our radiation environment. These include cosmic, cosmogenic, air, terrestrial, buildings, food, endogenous, medicine, industry, and fallout. Cumulatively, they provide a basis for estimating how many ionizing rays and particles impact us each hour. Knowledge of our radiation environment and its biopositive effects should replace the ignorance, fear, and perception of harm from low levels of ionizing radiation.^{88,184,358,500,634,723,849,966}

A. CHARACTERISTICS AND UNITS OF IONIZING RADIATION

The common denominator of ionizing radiation is the ability to disrupt molecular structures with the production of ions and free radicals. A concise explanation of different types of radiation and their chemical “indirect” reactions are presented in the first chapter of BEIR V.⁵⁸ However, it should be noted that one event registered in a scintillation counter may produce two to ten thousand free radicals within the body.

Ionizing radiation includes several types of rays: alpha rays are helium nuclei; beta and delta rays are electrons; X- and gamma rays are high energy photons; and the subatomic particles include neutrons, protons, and other atomic nuclei.^811,977 Alpha particles and other nuclei literally bowl through molecules of cell membranes, cytoplasm, and nuclei; their paths are broad and short. Beta rays flit like a butterfly, kissing first this and then that molecule. Gamma and X- rays zap through a thousand cells leaving a shaft of free radicals as long as their energy allows. Delta rays feebly reach out to nearby molecules with barely enough energy to cause disruption.

The energy of ionizing radiations (Figure 1.1) begins at about 10 eV (electron volts) and extends into little explored cosmic rays, some of which pass through the earth with little energy loss. Low energy ultraviolet rays have too little energy to penetrate skin; only those photons with higher energies are ionizing. Alpha rays have plenty of energy; however, most alpha rays do not penetrate more than 1 mm of tissue. Alpha rays with >6 MeV of energy can penetrate skin. Most beta rays have energy enough to penetrate through 10 to 100 cells. Alpha and beta rays become dangerous when the radionuclides emitting them are breathed, ingested, or enter wounds where they can affect living cells. After causing many ionizing events, about half the gamma and X- rays with energies >1 MeV will emerge from the body.

FIGURE 1.1. Energy of various electromagnetic radiations.

The amounts and effects of Auger electrons, called “delta rays” when they are strong enough to be ionizing, are not generally stated; they should be. Delta rays are emitted by an atom in transition following electron capture or during the rearrangement of electrons in their orbitals. One example is

${}^{40}\text{K}+\text{e}{\to }^{40}\text{Ar}+\text{delta rays and other Auger electrons}$

Delta rays have very low energies and penetrating power. The most numerically effective have energies of about 200 eV; these can penetrate only 0.01 μm of tissue. Their abundance and proximity to sensitive cell structures make delta rays important in radiobiology.^{433,570,607,608,633,837,889,890} For example, 16% of ⁴⁰K disintegrations release delta rays with 200 eV. Delta rays can be extremely harmful when they are emitted close to the DNA of proliferating cells....