Microwave ovens, cellular phones, and cancer risks

by Wm. Robert Johnston
last updated 15 February 2004

Do cellular phones cause brain tumors? Do microwave ovens cause sterility? The answer to both is NO, but the fact that many people think the answer is yes illustrates some major shortcomings in what Americans know about science...

Why cellular phones and microwaves can't cause cancer

Both cellular phones and microwave ovens make use of electromagnetic radiation. The first reason for the persistence of the myths about these causing cancer is the term "radiation." Radiation includes more than just high-energy nuclear radiation; it also includes the visible light which is now allowing you to read this sentence.

Electromagnetic radiation is any type of energy transmitted by electric and magnetic field waves. They travel at the speed of light and can cause charged particles (like electrons in atoms) to oscillate and move. Depending on the amount of energy in individual waves, electromagnetic radiation is classified as follows:

<-------------lower energy                                                 higher energy------------>
    radio waves     microwaves     infrared     visible      ultraviolet     x-rays     gamma rays

All these together are called the electromagnetic spectrum. The very small portion of this electromagnetic spectrum that our eyes are sensitive to is called visible light. Different colors of light represent different energies. From low to high energy, visible light includes red, orange, yellow, green, blue, and violet. Radio waves are the lowest energy. They are used to transmit radio and television signals. Microwaves are used by cellular phones and microwave ovens. Infrared is radiated by objects with temperatures like we encounter on Earth. Infrared goggles take this invisible infrared glow and convert it to visible light for the viewer to see. (Low light goggles, on the other hand, enhance small amounts of visible light.) Ultraviolet light from the Sun can cause sunburns. X-rays tend to penetrate human tissue, allowing its use for medical imaging. Gamma rays are the highest energy category, produced by nuclear reactions.

These categories are based on different ways that these waves interact with matter (which also gives different applications). White paper reflects all colors of visible light. Blue paper reflects blue and absorbs the rest. Visible light does not go through wood and brick walls, but radio waves and gamma rays do, to a degree. Visible light goes through glass windows, but some ultraviolet does not.

Gamma rays, x-rays, and the highest energy ultraviolet waves have enough energy in the individual wave packets (photons) to knock electrons from atoms, ionizing the atoms. These types of radiation are thus called ionizing radiation. The individual wave packets for any other type of electromagnetic radiation cannot ionize matter. (More on this later.)

We do not currently understand cancer well enough to cure it. However, we do know many things about cancers, including a good general understanding of the biological roots of cancers. Each cell in a human (or any other organism) contains genes--the DNA molecules containing all the chemical instructions for cellular reproduction and function. In the human body cells are constantly reproducing to replace others as they die. The genes in each cell are the source of the chemical controls on the rate of reproduction, normally keeping this rate to an equilibrium level so that newly produced cells balance the loss of those that die. Cancer occurs when these genetic controls are corrupted in a cell, allowing the cell to reproduce unnecessarily. Since the cell passes on this genetic error to all cells descending from it, this can create a growing population of incorrectly reproduction cells. The body has natural mechanisms to correct such errors or eliminate such cells, but if a defective cell is not caught cancer can develop. Cancer actually includes a variety of diseases, all involving this basic mechanism.

Causes of cancer fall in four categories. The natural chemical processes that take place in the cell frequently disrupt or damage DNA molecules in the cell. These are mostly but not always corrected by natural mechanisms. Inherited genetic errors are present in an organism from conception and thus would be present in all the organism's cells. These two causes produce the majority of cancers in humans.

The other two causes are environmental. Carcinogens are chemicals which interfere with cellular reproduction, and can be natural or man-made. Natural carcinogens cause more cases of cancer than do man-made chemicals. (Note also that a few viruses are known to cause or facilitate cancer by chemically interfering with cellular reproduction.) Finally, if ionizing radiation ionizes an atom in a DNA molecule, it can disrupt the chemical information and interfere with cellular reproduction. Most people receive more ionizing radiation from natural sources than from man-made sources.

Individual electromagnetic wave packets must have a certain minimum energy to ionize atoms. Only by ionizing atoms in DNA molecules can they possibly cause cancer. Individual photons can't combine to ionize an atom. Consider this analogy:

Imagine you want to bridge a 3-meter-wide bottomless canyon using pieces of wood. However, you may not combine individual pieces--they can't be stacked, glued, or nailed together. Thus, you can bridge the canyon with a 4-meter-long board, or one 10 meters long, or one a kilometer long. But with a 1-meter long board, you can't bridge it--even if you have many of them, since they can't be combined. With a toothpick, you can't bridge the canyon--not with a truckload of them, or even a forest's worth of toothpicks.

The canyon is like the energy threshold needed to ionize atoms. Remember that radiation can only cause cancer by ionizing atoms. High energy ultraviolet, x-rays, and gamma rays attain the threshold. Visible light is lower energy--like the 1-meter long board--and can't cause cancer. No matter how long a flashlight shines on you, it cannot cause you to get cancer. Microwaves are much lower energy still--like the toothpicks--and can't cause cancer.

Now you can combine 1-meter long boards or toothpicks to make a fire as warm as the fire from a 10-meter long board. Large amounts of microwaves can be used to heat an object. A microwave oven uses microwaves of a specific energy level chosen in part because it penetrates deeply into food. Water molecules and other molecules in food absorb the microwave energy and vibrate more--this is heat. The microwave oven is opaque to microwaves, but the glass window is transparent to visible light, so you can see your food but no microwaves escape.

Why do you see signs saying "Caution--microwave oven in use"? Microwaves may emit small amounts of radio energy that could interfere with artificial pacemakers. Those signs are a warning to people with pacemakers. Similarly, hospitals ask you not to use cellular phones in the hospital to avoid the possibility of the microwaves interfering with medical equipment.

The reasons that microwave energy can't cause cancer also apply to sterility and birth defects. Ionizing radiation that damages the DNA in reproductive cells can cause genetic defects in future generations or can cause sterility. Cellular phones and microwave ovens can't do this any more than flashlights can.

Since the large amounts of microwaves in a microwave oven cook food, can sufficiently large amounts of microwaves affect people? Yes: a microwave oven is designed to cook, as is a conventional oven; the amount of heat produced inside either one, while operating, is enough to cook food--and is enough to cause burns! People are occasionally burned from hot liquids or foods just heated by a microwave oven; at the same time, people are sometimes burned using conventional ovens. But realize we are talking about heat, a hazard we are familiar with. There are slight differences, because of the manner in which microwaves heat objects--for example, there are reported cases of overheating water in microwave ovens to a point where the water boils explosively, spraying hot water. Very high amounts of microwaves can cause cataracts, simply because of the eye's sensitivity to heat. But note that this involves cumulative amounts of energy far higher than those from operating microwave ovens or cellular phones. The total energy output of a cellular phone, for example, is only a few percent of the heat output of the human brain, even without considering how little of the energy could be absorbed by the body.

Common mistakes in science

Recently some news sources have reported that some studies do show that cellular phones are linked to cancer. If they conclude that cellular phones cause cancer, then they are making several mistakes.

To understand the first mistake, look at how these statistical studies are conducted. Remember that the occurrence of cancer is random. Certain conditions will increase the probability of cancer occurring, but in a group of individuals with the same risk of developing cancer there is no way to predict which individuals will develop cancer and which ones won't.

For the sake of illustration, assume that the average chance of developing cancer is 10%. Then in a group of 100 people, an average of 10 people would develop cancer and 90 would not. In the diagram below each "*" represents a person who develops cancer and each "o" a person who does not:

      |---|     |---|         |---|
   o o|o o|* o o|o o|o o o o o|o o|o o o o
      |   |     |   |         |   |
   o o|o o|o o o|o o|o o * o o|o *|o o o *
      |   |     |   |         |   |
   * o|o *|o o o|o o|o o o o o|o o|o o o o
      |   |     |   |         |   |
   o o|o o|o o *|o o|o o o o o|o o|o o o o
      |   |     |   |         |   |
   o o|o o|o o o|o o|o * o * o|o *|o o o o
      | A |     | B |         | C |
      |---|     |---|         |---|

Many "cancer studies" take groups of people like this, count how many develop cancer, and compare that proportion to the normal proportion. (More on what is "normal" later.) Now suppose we take a smaller group from the above 100 people for a study. Group A has 1 person out of 10 developing cancer, which is the normal 10%. But suppose instead we had randomly selected group B: in that group 0% developed cancer. Or alternately, if we had picked group C, we would show 20% developing cancer--twice the normal rate!

The smaller a study group we take, the more likely that purely random variation will show significantly higher or lower cancer rates than the average. In the above example, if we combine the smaller studies (or if we take the large group of 100) we find the cancer rate to be the expected 10%. For this reason, sample size is an extremely important factor in the reliability of studies like these. Unfortunately, large studies are more expensive and take more time, so some researchers use smaller sample groups.

Suppose three different researchers decided to do cheap, quick cancer studies each using 10 people--and that they respectively took samples A, B, and C. Two studies will show cancer rates the same as average or below. One study will show higher than normal cancer rates. Which study do you think will be reported in the news media?

Thus, among any collection of studies some will show higher-than-normal cancer rates and some will show lower-than-normal cancer rates, all due to random variation alone. If all the smaller studies are lumped together, however, this variation cancels out. This has happened with studies concerning cellular phones and cancer: a few (smaller) studies show higher rates of cancer, but larger studies or merged studies show no difference.

Another potential mistake involves making the comparison to what is "normal." In the above example, each group was compared to the normal rate of 10%. In practice, researchers compare the test group to a control group--which in theory represents "normal." A study on cellular phones and cancer would compare a test group of cellular phone users to a control group of non-users. This control group must match the test group in all factors that affect cancer rates. In practice this is extremely difficult to do: cellular phones users, for example, may have different lifestyles than the general population, which might be evidenced in say, dietary differences, which affects cancer risk...

This issue brings us to a final point: correlation does not prove causality. This means that if two things are observed together, that does not prove that one caused the other. For example, ice cream sales are correlated with numbers of cars stolen: those times of the year when ice cream sales are high, auto theft is also high. This does not prove that ice cream causes people to steal cars. It turns out that both result from warm weather: more people consume ice cream and more people leave windows rolled down on their cars. During winter weather, fewer people consume ice cream and fewer thieves are willing to go outside.

This means that even if appropriately large studies with scientific controls did show an increased risk of cancer associated with a certain activity or chemical, that would not prove that the activity or chemical caused the cancers. It would be cause for researchers to investigate further and see if there is a cause-and-effect relationship, or if the true cause (like the warm weather) was disguised, or if it was simply coincidence.


Think about this: if I just convinced you not to worry about cellular phones, look at how much work it took me! If I had stopped with the first paragraph, would that have changed your mind? Probably not--and it shouldn't have! On the other hand, did news reports or rumors take this long to scare you in the first place? Probably not. Most people have a tendency to accept bad news far more readily than good news--especially when it comes to a subject they don't understand. This is why a lack of science understanding is capable of adversely affecting a democratic society.

© 2001-2002, 2004 by Wm. Robert Johnston.
Last modified 15 February 2004.
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