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By Greg Lapin, N9GL
Chairman, ARRL RF Safety Committee
February 28, 2001
The topic of athermal effects is far too large topic to cover in a single column. This month, I plan to define what most people mean when they refer to athermal effects and give an example of their existence.
Something that has received a lot of attention lately is the concept of Athermal Effects of RF energy. Some people are convinced that some kind of athermal effect is slowly injuring the public health. Others do not believe that such a thing causes any harm, while still others doubt the existence of athermal effects.
As I've written in the past, it is fairly easy to describe the damage that can occur to biological tissue due to excessive heating. Few, if any, deny the potential for RF to cause harm by overheating tissue. A common misconception is that the safety standards that we follow are based solely on heating. This, as I have mentioned in past articles, is not true. The safety standards are designed to prevent any recognized effect in humans and laboratory animals. Even though this method of setting safety standards attempts to turn a blind eye on the cause of an RF bioeffect, it does not mean that the people who study them are not interested in the mechanisms of biological interaction. Far from it. There have been hundreds of scientific studies that have attempted to understand mechanisms that lead to various observed bioeffects.
Let's define an athermal bioeffect. On the surface this would seem to be a very easy task. However, it is not as straightforward as it would seem. Any amount of RF energy that is absorbed in tissue is turned into heat. Just because heat is generated does not necessarily mean that a significant temperature increase will occur. Once heat is generated some of it is taken away by normal thermal conduction (Have you ever tried to solder a huge piece of metal with a small soldering iron? You just can't seem to get it hot enough for the solder to melt). The human body has more effective means of removing excess heat from tissue than simple thermal conduction. In many organs, the body can selectively redistribute the blood flow it needs to remove heat (Again, back to our soldering analogy. Try soldering water pipes with a small propane torch while there is water in the pipes). Blood flow is a very effective means of carrying heat away from tissue. Thus "athermal" becomes a relative term with respect to a level of RF exposure.
It is sometimes difficult to distinguish whether or not an effect is thermal. One laboratory investigator placed cancer cells in a cell culture dish and exposed them to an SAR dose of 20 W/kg. He placed the culture dish in a water bath to cool the cells during the exposure and claimed that he was producing an athermal effect when he saw the cancer cells grow faster when exposed to RF. This research has always been questionable since the extremely high dose of RF (for comparison, the safety standard limits SAR dose to 1.6 W/kg in the general population) produced considerable heat. Even though the cell culture dish was surrounded by water to cool it, many scientists were not convinced that the individual cells did not get heated.
If athermal bioeffects exist why don't we hear the people responsible for RF Safety talk about them? I do not personally know any scientist involved with RF Safety who does not believe that athermal bioeffects exist. However, if you observe carefully you will notice an important distinction: athermal bioeffects have never been shown to affect RF Safety. Since RF Safety experts tend to be concerned with ways that RF can damage tissue, they are unlikely to mention athermal effects. These people don't deny that athermal effects exist, they just don't see any reason to talk about them.
So what are athermal bioeffects? The most commonly evidenced effect these days has made a revolution in medical treatment possible. Magnetic resonance imaging, MRI, has made it possible to see inside the human body to diagnose all kinds of diseases, often before they become uncorrectable problems. MRI works by placing all of the atoms in the body in a very strong static magnetic field (common medical imagers use 1.5 Tesla, or 15000 gauss, magnets), which makes their nuclei align with the magnetic lines of force. When the atoms are exposed to very strong pulses of RF energy, they are knocked out of alignment. Between pulses they regain alignment with the magnetic field and they give off the RF energy that they absorbed, which can be received with an antenna and processed to make images of their distributions. This is a procedure that is performed thousands of times a day; it describes an athermal bioeffect. To date, there has not been any identifiable damage caused by this process. However, note that even though it is possible to make stronger magnets which would produce better images, the magnetic strength used on people has been limited due to concern that the amount of RF required to work with a stronger magnet would cause cellular heating that could be dangerous.
No one denies that athermal bioeffects exist. Some investigators hypothesize that certain athermal effects can lead to disease, though they have been unable to show evidence of this. The mainstream of scientific thought today recognizes that there has not been a single study showing that an athermal effect causes harm to tissue.
Next month I will discuss some of the specific effects that have been investigated and what some people say about them.
| Editor's note: Greg Lapin, N9GL, started working in the RF safety world after spending many years first studying cardiac function imaging and then brain tumor kinetics. He serves as chairman of the ARRL RF safety Committee and as a member of the IEEE Committee on Man and Radiation. A former professor of Biomedical Engineering and Neurology at Northwestern University, Lapin now works as a consulting professional engineer in the electronics industry. He was first licensed while a teenager in 1969 and continues to be fascinated by virtually all aspects of Amateur Radio. One of his many interests is electronic design, and he is the author of Chapter 8, "Analog Signal Theory and Components" in The ARRL Handbook for Radio Amateurs. His non-ham interests include making things grow in his garden and serving as commissioner of the local children's softball league. At other times--when he is not working or helping his kids with their homework--you might find him with the local emergency services agency, climbing his tower, building a new QRP rig, playing with his APRS setup, responding to QSL cards, going off on a DXpedition, or trying to get that "new one." You can reach him by email at g.lapin@ieee.org. |
