Lesson 3 Reading

Biological Effects of Ionizing Radiation

All waves on the electromagnetic spectrum travel at the speed of light. The human body can sense only a narrow range of the spectrum. We can feel (as heat) some of the infrared rays closer to the visible range, and we can see a narrow range of the waves in the form of visible light (the rainbow colors on the chart). Although most of the ultraviolet range is generally not considered ionizing, at the extreme high end of its range, ultraviolet light edges into the X-ray range; therefore, the higher portion of the ultraviolet range can be ionizing. However, the ultraviolet rays that cause skin cancer do so by altering DNA through a photochemical reaction, not by an ionization.

We are constantly exposed to ionizing radiation. It is in the air we breathe and in the world around us. It is in rocks and soil. It reaches us from outer space. It is in all the food we eat. Quite simply, it’s not possible to avoid radiation and radioactivity. So we need to be aware of its potential effects on us.

Ionizing Radiation

Some important forms of ionizing radiation are the alpha and beta particles, and gamma rays emitted from radioactive materials. The cosmic radiation that reaches the Earth from outer space is also ionizing. So are X-rays.

Gamma rays and X-rays are waves of pure energy, without mass or electrical charge. They are ionizing. They appear at the highfrequency (high-energy) end of what we call the electromagnetic spectrum shown in the figure. The various non-ionizing radiations (radio and television waves, microwaves, visible light, etc.) occupy the lower frequency (lower energy) part of the electromagnetic spectrum.

All types of electromagnetic radiation, ionizing and non-ionizing, travel at the speed of light. Their energies are not determined by their speed (the speed of light) but by their frequencies, the number of waves or cycles per second. Alpha and beta particles, on the other hand, are not part of the electromagnetic spectrum. They travel very fast but slower than the speed of light.

Potential for Radiation Injury to Human Cells

Because ionizing radiation can knock electrons out of the atoms and molecules in its path, it can cause chemical and/or physical changes in human cells and, later, tissues and organs. The various types of ionizing radiation, however, differ widely in their abilities to penetrate tissue and deposit energy.

Alpha particles, for instance, are relatively large and carry a double positive charge. (Remember, alpha particles have two protons and two neutrons, the equivalent of a helium nucleus.) Because of their relative size and weight, they are not very penetrating and can be stopped by a piece of paper. They travel extremely short distances in human tissue but deposit all their energy along their short paths, doing a relatively large amount of damage to nearby atoms and molecules, and thus to cells and tissues.

Beta particles (electrons) are thousands of times smaller than alpha particles. As electrons, they carry a single negative charge. They are more penetrating than alpha particles, but they can still be stopped by something as thin as a sheet of aluminum foil. Beta particles travel much longer distances in human tissue, but they deposit much less energy along their pathways.

Alpha particles can be stopped by a sheet of paper, and beta particles by a sheet of aluminum foil. Gamma rays (and X-rays) are so penetrating that it takes something very, very dense — such as lead, or enough water or concrete — to absorb and stop them. If you are reading this online, scroll your mouse over the names of the different radiation types to see how they are stopped.

Gamma rays and X-rays deposit less energy per unit-path than do alpha or beta particles. (Unit-path can be any accepted unit of measure by which the pathway of exposure is apportioned, e.g., centimeters or millimeters.) Even though gamma rays and X-rays deposit less energy per unit-path, their overall deposition of energy in a body is not necessarily less than that of an alpha or beta particle. The angle of the rays relative to the body, the amount and types of tissue they pass through, and so on, all can influence the amount of dose absorbed by the body’s cells, tissues, and organs.

Acute Exposures

The effect of radiation on the body depends mostly on how long the exposure lasted, how much energy was absorbed, and the type and number of cells that were affected. Having accumulated a great deal of data about high exposures received in a short time (acute exposures), scientists generally agree on the effects of high doses.

Some scientists believe there is a radiation exposure level so low that there is no risk associated with it. Yet it’s impossible to say how low is low enough.

The risk of injury clearly does decrease with decreasing exposure, but most experts believe it is reasonable to err on the side of caution. While there may be very little likelihood of injury from exposures to low levels of radiation, some degree of risk, however small, is generally assumed when people are exposed to even small amounts of ionizing radiation.

Because ionizing radiation is high-energy radiation capable of knocking electrons out of atoms and molecules, it can damage human cells and tissues. The effects of ionizing radiation on the body depend on many things. First, how much radiation energy was absorbed by the body? Second, what type of radiation was it? Third, what kind of cells — and how many of them — were exposed?

The damage done by radiation results from the way it affects molecules essential to the normal function of body cells. Four things may happen when radiation strikes a cell: 1) It may pass right through the cell without doing any damage. 2) It may damage the cell, but the cell repairs the damage. 3) It may damage the cell so that the cell not only fails to repair itself but reproduces itself in damaged form over a period of years. 4) It may kill the cell. The death of a single cell may not be harmful, but serious problems occur if so many cells are killed in a particular organ that it can no longer function properly. Over time, incompletely or incorrectly repaired cells may produce delayed health effects, such as cancer or genetic mutations or, in babies exposed prior to birth, birth defects.*

It is important to note that genetic mutation caused by radiation to a parent does not cause a future child to receive damaged genes — unless the radiation damage is directly to the parent’s gonads (the mother’s ovaries or the father’s testes). If one of the parent’s gonads is damaged by radiation, there is a chance that it will produce damaged ova or sperm, which could result in genetic damage to the child later conceived. However, if the parent’s kidney, for example, is damaged by radiation, there is no chance that the damage will be passed on to the next generation.

If, on the other hand, a baby herself or himself is directly exposed to a high enough dose of ionizing radiation before birth, the exposure potentially could cause birth defects, but they would not be genetically passed on. As noted before, genetic effects in children whose parents were exposed to elevated levels of radiation have not been observed.

In all cases, the greater the exposure to radiation, the greater the chance that damage will occur. However, if enough time passes between exposures, a higher total exposure may be tolerated better than if the total exposure is received all at once. This effect is similar to the body’s response to solar radiation — too much sun can cause a severe sunburn, but short exposures and sun block can limit the sun’s burning effect on the skin.

If the body is given enough time to repair cellular damage, it will usually do so. For any given total exposure, the longer the time period during which it is received, the more time the body has to repair some of the damaged cells, helping to reduce the overall effect. A damaged area may also be healed by healthy cells from an area not exposed to radiation. Radiation damage to large numbers of cells may be partially repaired, but in such cases some damage is usually permanent.

Likewise, the more tissue exposed to radiation, the greater the chance of injury. Exposure of the whole body, for instance, presents more risk than exposure of an arm or a leg or a single organ to the same dose of radiation.

Sensitivity of Cells, Tissues, and Organs

Some cells, tissues, and organs are significantly more sensitive to radiation than others. Generally, organs with rapidly dividing cell systems — such as the bone marrow, gonads, and intestines — are more sensitive than systems like the kidneys, liver, and brain. Cells that perform specialized functions are less sensitive than those that do not.

In adults, the differences in sensitivity to radiation exposure depends largely on the type of organ exposed: the more rapidly the cells of the organ reproduce, the more sensitive the organ is to radiation. In children, the differences between organs are less important for cellular sensitivity. This is because the younger the child, the more cellular reproduction there is going on throughout the child’s body.

Because rapidly dividing cells are generally more sensitive, babies and young children are more sensitive in general to ionizing radiation. No matter the organ, the cells of babies and children are all rapidly dividing and reproducing as part of the normal growth process. So they are naturally more susceptible to radiation damage.

Different Effects of Different Types of Radiation

This drawing shows what the pathways of different types of radiation might look like as they pass into or through human tissue. Ions can form when a particle, a gamma ray, or an X-ray penetrates tissue. Because alpha particles deposit all their energy along a short path, they are more likely to cause damage than the other forms of radiation -- but only along that path. Notice the number of red dots, indicating ions that could cause damage, for each type of radiation. Source: Adapted from Radiation Activities for Youth Series, The Pensylvania State University, Nuclear Engineering Department, 1988. Permission for use granted.

The type of radiation is also important. Alpha, beta, and gamma radiations differ in both their abilities to penetrate tissue and in the likelihood that they will cause biological damage.

Alpha particles are relatively large and carry a double positive charge. They travel only a short distance in living tissue, but they deposit all their energy in that short path. This increases the likelihood that they will cause damage — but only in the region they manage to penetrate.

Beta particles carry a single negative charge and are thousands of times smaller than alpha particles. They travel longer distances in living tissue than alpha particles, but they deposit their energy over a longer path. For this reason, they are less likely to cause damage.

Because neither alpha nor beta particles can penetrate very far, if present outside the body, they mainly damage skin and surface organs. However, if substances giving off alpha and beta radiation are swallowed or inhaled, they can lodge inside the body and affect internal organs on an ongoing basis. The longer they remain lodged in the body, the more chance that they will damage nearby molecules and cells. There is also a chance, however, that they will be flushed out by the body’s normal elimination processes.

Gamma rays and X-rays share similar properties, but gamma rays are even more energetic. Their biological effects are similar. Their paths in living tissue are long. But because they can penetrate the body more deeply, they can affect internal organs as they pass through.

Gamma rays and X-rays can be partially stopped by dense bodily material, such as bone mass (hence the ability to create internal images with X-ray-sensitive film).

Low Exposures: How Risky?

Less information is available about the effects of human exposure to low doses of radiation. As a result, different theories predict different effects for low exposures.

Basically, it’s a question of “How low is low enough?” Or, considering that it’s literally impossible to avoid ionizing radiation, how much concern should be placed on limiting one’s normal, unavoidable exposure?

Some scientists believe there are no radiation effects below a certain threshold level of radiation since they are impossible to measure. Other scientists believe that the effects of radiation exposure are proportional to the radiation level, regardless of the actual level. This implies that any exposure has some effect, even if we can’t measure it.

Moreover, there are some scientists who believe that low levels of radiation — slightly to moderately above normal background levels — may actually be good for us. Several studies have suggested that there may be an adaptive response, commonly called radiation hormesis. Some people theorize that that a little extra radiation stimulates the immune system. Very high radiation doses, of course, can significantly weaken and even destroy the immune system.

Measuring Potential Health Effects

The accumulated total dose of ionizing radiation absorbed by human cells over a given time frame is what counts the most in estimating potential health risks. The longer the time period required to accumulate a particular total absorbed dose, the less the chance that it will have a negative health effect. That is, a dose absorbed over the course of months or years may have little, if any, long-term effect, but the same dose absorbed in a few minutes could have serious health effects.

To understand potential health effects of ionizing radiation, we need a standard system for measuring radiation absorbed by the different cells, tissues, and organs of our bodies. In the U.S., the basic unit for measuring radiation received is the rad (radiation absorbed dose). One rad equals the absorption of 100 ergs in every gram of tissue exposed to radiation. (See the definition of erg in Lesson 2 of this unit.)

To show biological risk, the absorbed dose is adjusted to correspond to the risk caused by exposure from different types of radiation to different organs or tissues. Such a quantity is expressed in units of rem (roentgen equivalent man).

The rem is adjusted to take into account the type of radiation absorbed, i.e., the differences in likelihood of damage from the different types of radiation and for the types of organs or tissues exposed. Because almost all human exposures are mere fractions of a rem, to arrive at a precise unit for measuring human dose, the rem is further subdivided by 1,000 to get the millirem or one one-thousandth of a rem. (Additional details can be found in Lesson 2 of this unit and in the background notes to this lesson.)

For understanding risks, several key points are important to remember. Ionizing radiation is everywhere on Earth. It is, quite literally, unavoidable. Every year the average American absorbs a dose of about 360 millirem of ionizing radiation from both natural and manmade sources. Moreover, in terms of health effects, it makes no difference whether the ionizing radiation is natural or manmade. It’s the dose that counts, not the source.

Important in determining the dose from a given source, however, is whether the source is inhaled, ingested, or simply encountered for a prolonged period.

Biological Effects

There are two categories of effects from ionizing radiation: somatic effects and genetic effects. Somatic effects appear in the exposed person. (“Soma” refers to the entirety of an organism, except for the germ cells, which in human beings are the ova and the sperm cells; the germ cells could be damaged if the ovaries or testes that produce them are damaged.)

Somatic effects result from radiation damage to body cells that are not reproductive cells. They can occur soon after the exposure or they can take a number of years to become obvious. Somatic effects can not be inherited.

Although such effects have never been observed in humans, genetic effects may theoretically appear in children conceived after a parent has been exposed to radiation only if that parent’s egg or sperm cells were directly affected, or if the ovaries or testes were exposed to enough radiation that they cannot repair themselves and thus they produce damaged ova or sperm. Genetic defects can be inherited.

Although such effects have never been observed in humans, genetic effects may theoretically appear in children conceived after a parent has been exposed to radiation only if that parent’s egg or sperm cells were directly affected, or if the ovaries or testes were exposed to enough radiation that they cannot repair themselves and thus they produce damaged ova or sperm. Genetic defects can be inherited.

Large enough exposures in short periods of time (acute exposures) can produce injuries to a person within weeks or even hours. The severity depends on the amount of radiation received and the type of cells exposed. For example,100,000 to 400,000 millirem causes radiation sickness if received over a short period of time. (This is an extremely high dose. For comparison, in the United States workers in nuclear facilities are not allowed to receive more than 5,000 workrelated millirem, or 5 rem, in any given year, and should not receive more than 10 rem over a lifetime.)

Acute exposure to low amounts of radiation (1,000 millirem or less) produces no observable effect. Larger exposures, if received over weeks or months, may not produce visible symptoms, either. There is a slight risk, however, that a delayed effect (such as cancer) could develop 10 to 40 years later. It’s also possible (but even less likely) that damage to a reproductive cell could have occurred. At such low levels, however, it’s impossible to tell whether such effects are actually due to radiation exposure. They could be caused by many other factors.

Ionizing Radiation and Cancer

Many things can cause cancer, including exposure to ultraviolet light, smoking tobacco; drinking alcohol; breathing polluted air; consuming a poor diet; and exposure to asbestos, certain pesticides, and many other chemicals. Statistics show that 17 percent of the people (170,000 out of 1 million) in the United States die from cancer, from all causes.

The average annual exposure to ionizing radiation for people living in the United States is about 300 millirem from natural sources, including radon gas. About another 60 millirem are from manmade sources. It is estimated that from one to three percent of all cancer deaths could be the result of this yearly natural exposure, but there’s no way to prove this is the case.

After studying the cancer death rates of groups of people exposed to high levels of ionizing radiation, we know that it can cause cancer. The most significant groups are 1) survivors of the atomic bombs dropped on Japan; 2) U.S. radiologists who used ionizing radiation from the 1920s through the 1940s to diagnose and treat medical problems; and 3) people given high X-ray exposures to treat a disease of the spine in the early days of radiation treatments.

Scientists agree that high levels of radiation can cause cancer. No direct data exist, however, to estimate the risk of death from cancer caused by low levels of ionizing radiation, such as we receive from our natural environment year after year (chronic exposures).

Therefore, the table attributing cancer deaths to various sources draws on many lines of evidence. Among these lines, tobacco, our natural environment, and some aspects of modern medicine all have a radiation component that may contribute to cancer deaths.

Tobacco is especially important to consider. In addition to the nonradioactive, but still carcinogenic, chemicals in tobacco, habitual smokers receive extra radiation doses. Routine smokers receive many rems to their bronchial tubes every year from the radionuclide polonium- 210 and its parent, lead-210, both present in cigarette smoke. In fact, someone who smokes 30 cigararettes per day receives an extra 16 rem (16,000 mrem) per year. This is almost 45 times the average American’s annual average dose from all other sources of ionizing radiation combined. Clearly, smoking increases the likelihood of radiation-induced cancers.

Effects on the Unborn

Many genetic and environmental factors affect developing babies before they are born. Because there are so many factors, it is difficult to say with certainty how radiation exposure affects an unborn child. Therefore, information gathered from animal studies is generally used to estimate these effects.

Animal studies have shown that radiation exposure before birth can result in a very wide range of results — from no observable damage, to malformations of major organs of the body, to slowed growth, to damage to the central nervous system, and even to death. Effects vary depending on the stage of development at the time of exposure and the level of exposure.

Since it would be unethical to experiment on unborn babies, the major sources of information about effects on human beings are survivors of the atomic bombs in Japan and unborn patients exposed during medical diagnosis or treatment. The most commonly reported abnormalities to babies exposed to high doses while in the womb are defects to the central nervous system and slowed growth.* Rapidly dividing cells are especially sensitive to radiation. Thus, before birth and during infancy when children grow rapidly, medical exposures to radiation carry a risk.

Scientists generally agree, however, that exposure to ordinary diagnostic X-rays is not likely to be harmful. Nevertheless, it is important to balance the benefits of radiation exposure against the possible risks. It is wise for a woman of child-bearing age to determine whether or not she is pregnant before being exposed to radiation above background levels. This precaution can help avoid unnecessary risk.

Approximately 10 percent of all children born (100 thousand out of a million) have some identifiable genetic defect or abnormality. These defects range from those so mild they are never even noticed to those that are severe and even fatal. Spontaneous abnormalities occur when unknown factors cause cells not to work properly. Abnormalities may also occur because cells are affected by something in the environment, including ionizing radiation.

Genetic effects are effects that are inherited by the offspring of an individual whose egg or sperm cells have been damaged. No hereditary defects caused by ionizing radiation have been observed in humans, not even among Japanese survivors of the atomic bombs. (Only babies in their mothers’ wombs at the time of the bombs had birth defects; babies conceived later by survivors of the bombs showed no genetic defects from their parents’ exposure.) However, there are enough data from laboratory studies (most often using mice) to predict such effects.

DNA: Mighty Molecules

A molecule of DNA, with some sections damaged. The top “step” of the double helix spiral that makes up the molecule is intact and undamaged. The second section has a break in the spiral; the third section has a break in one of the cross-links; and the fourth section shows a cross-link that has been incorrectly repaired.

Our bodies are made up entirely of cells. You were formed — and continue to grow and live — because cells continue to divide and reproduce themselves. Very large molecules in our cells determine what types of cells will form, making us what and who we are. These molecules are called DNA, which stands for deoxyribonucleic acid.

Take a close look at the drawing of a DNA molecule. Notice that the top section shows a normal, healthy arrangement. The second and third sections, on the other hand, show two different types of damage to DNA. The second section shows the most common type of injury, a break in the spiral. The third section contains a break in one of the crosslinks. The fourth section illustrates how the body might incorrectly repair a crosslink. An abnormality like this can be passed on to many future copies of the cell when it reproduces itself.

Our bodies are composed of billions of cells. It’s not surprising, then, to learn that damage to DNA occurs all the time. For the most part, our bodies simply go about their business of repairing any damage.

DNA repair does take a certain amount of time, however, and too much injury within a given time can overwhelm the ability of the body to repair the damage. If the damage is not repaired — or if it is repaired incorrectly — the results are passed along to a great many cells.

One of the many causes of damage to DNA is ionizing radiation. Low exposures usually do not affect the body’s ability to repair damage. But high levels of exposure can damage such a large number of DNA molecules that repair (or proper repair) is less likely. This increases the possibility of harmful health effects.

There is some indication that radiation causes cancer by a DNA defect or incorrect repair. However, exactly how radiation or any other cancercausing agents cause cancer is still not completely understood. It is also uncertain whether low exposures to ionizing radiation cause cancer.

Genetic disorders are also related to defects in DNA itself. Many things may cause genetic disorders, and many disorders caused by radiation can’t be distinguished from those with other causes. Nevertheless, as with cancer, some small risk of a genetic disorder is assumed for any level of radiation exposure.

*Source: Primer on Radiation, FDA Consumer (HEW Publication No.) (FDA) 79- 8099.

*UN Scientific Committee on the Effects of Atomic Radiation, Sources, Effects and Risks of Ionizing Radiation, 1988