I'm a Chemist, so I am constantly amused (sometimes annoyed) at people who panic over "Chemicals" in our environment and what some can do (both known and pure speculated). From my perspective, everything is a chemical. We breathe a chemical mixture of 75% nitrogen, 20% Oxygen, 1% Carbon Dioxide, and a whole bunch of minor other gases. Have you ever seen what happens when human skin is exposed to pure Oxygen? In the short term it will cause severe injury that looks a lot like burns, the skin will crack and get brittle. In the long term there are possibilities of cancerous rapid growth. And Carbon Dioxide? Too much of that and you die of suffocation! And by too much I mean as little as 10% to 20% over a period of a few hours! Why are governments not protecting us from these noxious airborne chemicals??? Oh, I'm talking about normal air, by the way.
As a Chemist often I have told people "I don't believe in zero". To me there is no such thing as absolutely none of any particular material you want to name on this earth. You want to eliminate completely the salt content of water for drinking by distilling it? Even if you use the best possible process to drop the salt content so low you cannot detect any with the most sophisticated lab procedures, I guarantee the remaining concentration is NOT zero. It will be so low you and I really would not care, but it won't be zero. Even in the "perfect vacuum" of outer space where there is "nothing", there really isn't. There are some atoms and molecules spread out really thinly, and we even make use of that in astronomical spectroscopy by looking at the way light from stars is absorbed by gas molecules in the space between the star and us.
The important concept here is that impact on people, good or bad, depends heavily on how much of these "chemicals" are present. A really tiny bit of anything has no effect on us. There are a few things that are so powerful that a small amount can, over a long period of continuous exposure, cause problems for some (but not all) of the people exposed. At high enough concentrations, virtually everything can cause damage, just like the oxygen we need to breathe - at the right concentration (20% in air) it is essential for life, but at very high concentrations it is deadly.
Similar considerations apply to electromagnetic radiation in all its forms. At very low field strengths, so far as we can measure, they have no impact on people. Although the history of man-made microwaves is only 100 years old, the earth has been receiving microwaves from space at extremely low levels since the beginning of the universe. There is a whole field of astronomy based on receiving and analyzing microwave radiation from distant stars. So we know that at very low levels microwaves are harmless to people. And we know that at high enough levels they cause us damage through excessive heating. And obviously somewhere in between there is a transition zone where we can measure an impact on a human body (like minor local heating), but also can show that there is no permanent effect, good or bad. The same holds true for virtually all other forms of electromagnetic waves (or "radiation" to use the more scary term). There are always zones of very low dose where we can measure no impact at all, a little bit higher dose where we can actually measure some temporary impact but nothing lasting, medium dose where there is a statistically non-zero possibility of permanent effect (good or bad), and very high doses that we know will cause harm. The mechanisms by which the permanent impacts happen are highly varied. It just happens that for microwaves, the mechanism is heating of tissues by increasing the rotational energy of the molecules.
All of these factors are well know in the scientific and regulatory fields. Thus, various government and private agencies produce rules about what field strengths should be allowed under specific sets of circumstances, and the rules are all tailored to particular frequency regions. The rules for "radio waves" below, say, 100 MHz will be different from those for microwaves over 1 GHz. All of these are set how? From their known impact on people. Not surprisingly, we consider ourselves the primary life form to protect. So, based on many studies of thousands of people they establish what is the minimum field strength the produces a measurable impact on people that just might be harmful over longer exposure periods. Then they stick in a safety margin, often about 100, and say we believe that this much lower level is safe for continuous exposure with no harm. And realistically, from time to time new studies detect smaller impacts at lower field strengths and the "safe" exposure limits are revised.
For frequencies around 1 GHz, apparently that process has decided that something a bit under 1 watt of transmitter power placed right next to your head is OK. A cell phone in North America can transmit up to 500 mW, or ½ watt - no more. You are allowed to buy, install and use booster amplifiers for cell phones in a car in low-signal areas up to 3 watts power output, but only on condition that the antenna is mounted OUTSIDE the car on the roof, so that the metal roof blocks the radio waves from penetrating into the car where people are. Small-area repeater amplifiers, such as used to provide good cell phone function inside a building, can run at slightly higher power, but only because the transmitting antenna is far from your body so that the actual field strength at your head is much smaller than what your own phone is sending out. Similarly, microwave ovens can be used because, although the field intensity inside them is very high for heating food, the metal box and mesh screen on the door window ensure that what leaks out is well below what anyone would worry about.
So as long as people follow the regulations created this way, we believe we are limiting our risk of harm to such a low level that we are willing to accept that risk in order to benefit from the technology. In this case, we're talking microwave ovens, cell phones, computers, satellite receivers, etc in the 1GHz region, and we all find them quite beneficial. At lower frequencies, still subject to their own regulations, we use radios, TV's, remote controls, personal communicators ("walkie talkies"). At much higher frequencies we use electric heaters that emit infrared radiation. Etc, etc, etc.
Born2bwire raises an important point. VERY much higher frequencies in the far-ultraviolet, X-ray and Gamma-ray region have much different types of interaction with matter, often producing ionized particles with high energies which can harm human tissue in various ways. Some of these effects actually involve permanent changes to fine details of the DNA in some of our cells, resulting in abnormal growth we call cancer. The mechanisms of the effect of these types of radiation and the types of effects they produce are very different from what happens in the radio and microwave region, but the process of measuring those impacts and setting regulations to limit exposure are fundamentally the same. But the difference in types of impact on human tissue are important. There is no evidence that microwaves can cause anything like cancer, neither by their heating effect nor by any other mechanism. "Radiation" is not the same as other radiation, so don't ever make the mistake of assuming all forms have the same impact on people.
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