By Greg Lapin, N9GL, Chairman
ARRL RF Safety Committee
March 17, 2000
Understanding the scientific basis of RF Safety can be a lifetime's work. Here's the second in a series of column by Greg Lapin, N9GL, the chairman of the ARRL RF Safety Committee, that's aimed at clearing away the confusion and misconception.
This month we will examine specifically how RF energy can affect biological tissue. To recap, cancer can occur when something causes the structure of a cell's DNA to change in such a way that the cell has instructions to continually replicate itself. To prove that a given effect causes cancer, it must be shown to be capable of changing a cell's DNA.
First, it's back to high school for a little review of basic physics and chemistry. The electromagnetic spectrum includes all energy that has certain characteristics: it has an electric and a magnetic field, its unmodulated amplitude varies sinusoidally, and it can propagate through space. The propagation property is often referred to as radiation. All electromagnetic waves are classified as radiation for this reason and, as we will soon see, the word does not connote something dangerous even though it has come to mean this in many people's minds.
Electromagnetic waves actually consist of energy particles, called photons, that can do three things when they come in contact with matter: they can pass through (transmission), bounce off (reflection) and be absorbed. Electromagnetic photons contain energy; the amount of energy is directly related to the frequency of the sinusoidal oscillations (you can think of the photons as vibrating as they move through space and if you attach a pen to one, you will get a tracing of a sinusoidal wave).
If you've ever heard someone describe a new whizz-bang receiver as being able to hear from "DC-to-daylight," that's only part of the electromagnetic spectrum that I am referring to. In fact, it is the stuff higher in frequency than visible light that has some of the most dramatic biological effects. It is also instructive to make the distinction between energy and power at this point. Energy is contained by each photon and is related to its frequency. Power indicates the number of photons that are present at one time. A related term, Power-Density, is the number of photons that pass through a square unit of area at one time.
Everything in the universe is made up of chemical compounds, which are combinations of atoms that are held together by chemical bonds. Each atom is made of a nucleus, containing a number of protons, and an equal number of electrons orbiting it. A chemical bond exists between two atoms where one or more electrons are shared; a shared electron is attracted by both atoms and orbits both of the nuclei. Unlike the electrons that are closer to a single nucleus in inner orbits, the shared electron is held weakly by both nuclei and can be pulled out of orbit with a little push; this is how chemical reactions occur.
Now, let's get back to our topic. Since DNA is a chemical compound with trillions of chemical bonds, all it takes to disrupt the structure of DNA is to knock one shared electron out of orbit. This does not seem like such a difficult thing to do, and, indeed, it is not. Some of the things that can cause a chemical bond in DNA to break are the presence of certain other chemicals, the addition of heat, and exposure to ionizing radiation. Last month's column mentioned a number of chemical compounds that were determined to be carcinogenic. One potential mechanism for their action is that the chemical manages to come in proximity to DNA and has a stronger attraction for the shared electrons in DNA, stealing them and breaking the bonds. This month, we are concentrating on RF energy.
Ionizing radiation consists of electromagnetic photons with enough energy to free a shared electron, thus breaking a chemical bond and creating ions (chemicals with too few or too many electrons). How much energy is needed for this? The simple answer is that enough energy must be added to the electron so that its total energy level is higher than the energy that the nucleus has to hold it in orbit.
The actual answer is not so simple, since the electron must absorb the energy, which must not pass through, be reflected, or be absorbed by other structures (we'll get to the latter form of absorption soon). In practice, it has been found that the lower frequency limit for electromagnetic energy to be considered as ionizing radiation is about 1015 Hz (1,000,000,000,000,000 Hz), which is the frequency of ultraviolet light. Electromagnetic energy above this level includes the names that we have learned to be afraid of being exposed to: x-rays, gamma rays, and so on.
Electromagnetic photons at frequencies below the ionizing threshold simply do not contain enough energy to break shared electrons free from their chemical bonds. Typically, most of RF communications occur below 10 GHz (10,000,000,000 Hz), which is 100,000 times lower than the energy threshold for ionizing radiation. This is why we refer to RF energy (which includes microwaves) as nonionizing radiation.
Just because RF is nonionizing radiation does not mean that it is totally safe. It just means that RF energy cannot harm you by ionizing chemical compounds in your body. Also, it is important to remember that no matter how much RF power biological tissue is exposed to, the individual photons do not contain enough energy to break chemical bonds. When a photon of RF energy collides with tissue, it still can do three things: it can pass right through, it can bounce off, and it can be absorbed. We are concerned with the last case. When a photon is absorbed by tissue, whether by the electrons or the nuclei of atoms, its energy is transferred to the atom. The increased energy causes the atom to vibrate more quickly, which results in the generation of heat. This, of course, is how the microwave oven works.
What happens when a cell heats up? The body strives hard to maintain its internal temperature to within about one degree of normal core temperature, 98.6° F (38° C). It can selectively increase blood flow to carry excess heat to the surface of the body, where is it radiated into the air, often with the aid of sweat. If the heating is very localized, the heat sensors in the brain may not know that a temperature increase is occurring.
Cell function is very dependent on the ambient temperature. A complex set of chemical reactions keeps a cell alive. Chemical reactions depend on heat and a change in the expected temperature can disrupt the cell's operation to the point of killing it. Enzymes, proteins, and even DNA have specific shapes, and increased heat can cause them to lose their shapes (a process called denaturing) and their ability to function properly often depends on their normal shape. A rise of cell temperature to about 113° F (45° C) will cause this to happen. While this is a very high temperature for the body to reach, local heating due to the focussing of RF waves in tissue can happen, causing a small area to rise in temperature while the core body temperature changes very little. Preventing this type of RF heating is what the FCC permitted levels of exposure have been developed for.
Back to our core question: Can exposure to RF energy cause cancer? While we can't say for sure either way, we can hypothesize mechanisms that might cause this effect. The most obvious would be heat related. Let's say that a person is exposed to high enough levels of RF energy to cause local heating in the body. The heating is highly localized, so this person's body does not realize that it is occurring. A few cells die each time the heating occurs; not enough cells are involved to disrupt the functioning of any organs and the body soon replaces them by causing the other cells around them to replicate.
Further complicating the process, the cell replication in an overheated environment might be more prone to DNA errors, though this has not been verified experimentally. Over a lifetime of this happening, there may have been many inadvertent mutations that either die on their own or are killed by the body's immune system. One kind of mutation, however, might be cancerous and not recognized as such by the immune system. Over time, that cell could replicate at a higher rate than normal and eventually form a cancerous tumor.
Just because a possible mechanism may exist for a cell to become cancerous, it is not necessarily something that actually happens. Our bodies are bombarded with cosmic rays from the moment we are conceived until the end of our lives. Cosmic rays are ionizing radiation, which we would expect to cause DNA damage in cells. Since the beginning of life on Earth, our biology had adapted to the presence of this energy by developing DNA repair mechanisms. A recently analyzed form of bacteria, Deinococcus radiodurans, has gone a lot farther toward perfecting this than we have, and is capable of surviving, and actually being able to digest, radioactive matter without incurring any permanent harm.
A mechanism by which RF might cause cancer has been presented above. Actually, it is not specific to RF but rather it is the excessive heat that is theorized to lead to the disease. Just as my pre-braces teeth could have irritated my cheek enough times to cause a similar effect, the irritation of the tissue by the high levels of heat are the problem.
Since I mentioned saccharin last month, let me continue with its story. After much investigation, it was determined that the test rats that were fed massive quantities of saccharin had a chemical reaction between their urine and the saccharin, causing small crystals to form. The crystals irritated the lining of the bladder, eventually leading to cancer when the right mutations occurred.
The hypothesis presented above is a very similar situation: massive quantities of RF cause stress in tissue that might increase the probability of a cancerous mutation occurring. In both cases, the irritation can be avoided by decreasing the massive quantities to more reasonable amounts. With saccharine this means eating normal amounts of the sweetener. With RF this means limiting human exposure so excessive heating is avoided. This is what the FCC guidelines were designed to achieve.
Next month I plan to start a discussion of scientific investigation. As we have seen here, it is possible that thermal exposure to RF energy can cause cancer. But, does it really happen, even at excessive levels of RF exposure? Introducing a plausible mechanism for something does not mean that it ever occurs. Epidemiology and laboratory study help to clarify the picture of what is happening out there in the real world. Of course, the ever-popular term, "bad science" will be discussed, as well. Until then, keep the RF heating in the microwave oven and enjoy the propagation of the energy that goes where it is meant to go.