Effects of radiation on living organisms1. BackgroundThe term radiation is elastic. Unfortunately it hascome to mean “something terrible1” in ordinaryconversation. Literally radiation means “anythingemitted from a localized source”, but in the contextof biology we mean something quite specific. Hu-man beings live amidst a sea of radiation of all sorts.It is useful to classify these radiations according totheir biological effects, especially on human be-ings.Since the early part of this century we have beenaware that all radiation, including electromagneticfields, consists of particles. The more energy theparticles of radiation transmit to living cells, themore they can affect them. We classify radiationaccording to increasing energy per particle becausethis is the same as listing them according to in-creasing harmfulness. We generally classify radia-tion as “penetrating” or “non-penetrating”:penetrating means radiation such as x-rays, thatpass through our skin, whereas non-penetratingradiation stops at our skin.Types of radiationTypes of radiationTypes of radiationTypes of radiationElectromagnetic spectrumElectromagnetic spectrumElectromagnetic spectrumElectromagnetic spectrumnonpenetrat-ing, non-bioactivelowfrequencymagnetic fields,power linesradio AM, FM radio; infrared Heat lamps,radiant heaters,wood stovesnonpene-trating,bioactiveultraviolet UV tanning lights,sunlightPene tr ating,bioactivex-rays medical, dentalx-ray machines;TV & computercathode ray tubesγ-raysradioactive decays,cosmic raysNuclear and particle spectrumNuclear and particle spectrumNuclear and particle spectrumNuclear and particle spectrumPene tr at-ing, verybioactiveα-particlesnuclei of heliumatoms, emittedmainly inradioactive decaysof heavy elements(uranium, radium,radon)β-particleselectrons, emittedin radioactivedecays of variousisotopes (such as40K); also producedin electronacceleratorsneutrons produced mainly innuclear explosionsand nuclearreactorsVerypene-trating,bioactiveCosmicrayprotonsandshowerparticlesultra-high energyprotons from outerspace collide withatoms in the upperatmosphere toshower us withleptons, γ-rays andmesons; there is noway to shieldagainst themexcept in thedeepest mines.Physics of the Human Body 107Chapter 12: Effects of radiation on living organisms1. …in the sense of terrifying, as in “Ivan, the Terrible”.In medicine, energy at radio frequencies can beused to produce local heating in tissues—at lowintensities this is called diathermy. At high inten-sities and higher frequencies, this effect is appliedin surgery to excise tissue while cauterizing it. TheRF curette is the instrument of choice for opera-tions in heavily vascularized areas of the body, suchas the scalp and prostate glands. Bowel polyps canbe easily excised with minimal blood loss using asimilar instrument that can be passed through acolonoscope.Microwave ovens emit radio waves at wavelengthsespecially absorbed by water molecules. The mole-cules absorb energy and become hot, thereby cook-ing the food. As far as hazard to life and limb isconcerned, a leaky microwave oven can give youordinary burns from too much heat.Powerful radar systems can kill at short ranges—basically, the organism absorbs too much power.They are also known to stimulate cataracts in thelenses of mammalian eyes. That is, it is definitely abad idea to work near radar antennas when theyare in operation.Certain authors find it both profitable and enjoy-able to frighten the American public with techno-logical bogey-men. One such has claimed boththat microwave ovens pose an unacceptablehealth risk to their users and that there exists a vastconspiracy (involving among others the US Gov-ernment) to cover up these facts2.Similar claims of hazard have been advanced (bythe same author—surprise!) regarding the delete-rious effects of the electric and magnetic fields ofpower lines3. 2. Effects of penetrating radiationsIn this chapter we discuss the effects of penetratingradiation on living cells and complex organisms.We begin with the electromagnetic spectrum. Oureyes detect electromagnetic waves in the wave-length region 4000 Å (violet) to 7000 Å (red), i.e.the range is 4−7×10−5 cm. The energy range of thecorresponding photons is 3.1-1.8 electron volts4,according to the Planck-Einstein relationE = hν .That is, the energy of visible-light photons is suffi-ciently great to excite atoms and molecules, or tobreak the weaker types of chemical bond (for ex-ample the hydrogen bond whose typical strengthis 0.2 eV). However, most matter is opaque toradiation in the visible range, so these photonsneither penetrate past our skins nor influence ourinternal body chemistry5 (except through the eyes,of course).Physics of the Human Body108 Effects of penetrating radiations2. Paul Brodeur, The Zapping of America: microwaves, their deadly risk, and the coverup (Norton, New York,1977).3. Paul Brodeur, Currents of death: power lines, computer terminals, and the attempt to cover up their threat to yourhealth (Simon and Schuster, New York, 1989); The great power-line cover-up: how the utilities and the govern-ment are trying to hide the cancer hazard posed by electromagnetic fields (Little, Brown and Co., Boston, 1993).See the Web site http://www.phys.virginia.edu/classes/304/electrophobia/em_fld.htm for a more complete dis-cussion of this.4. An electron volt is a unit of energy equal to the energy gained by an object with the charge of an electron,falling through an electrostatic potential of one volt. Since the electron’s charge is (in magnitude) 1.6×10-19 Coulombs, 1eV=1.6×10-19 Joule.5. Visible light photons are the primary energy source in plant photosynthesis. The chlorophyll molecule isspecifically evolved to capture visible light, converting it into chemical energy that is then used to promotethe synthesis of energy-rich carbohydrate molecules. See, e.g., Xiche Hu and Klaus Schulten, “How NatureHarvests Sunlight”, Physics Today (Aug 1977) p. 28ff.Next in order of particle energy are ultraviolet(UV) photons, with wavelengths6 in the range 100to 4000 Å, and energies ranging from 4 to 124 eV.These penetrate into the skin where they cancause damaging chemical reactions whose effectscan include sunburn or skin cancer. The skin pig-ment melanin is manufactured by skin cells pre-cisely to prevent UV rays from penetrating theepidermal layer to the basal skin cells.However some UV must penetrate to the basalcells in order to convert