There is a logical fallacy called the argumentum ad ignorantiam or the argument from ignorance. The essence of this fallacy is that because we don't know something to be definitively false, it must therefore be true. A good example relates to concern about the dangers of electromagnetic frequencies.
There are people who claim with absolute confidence that cell phone and wifi signals cause them severe discomfort even to the point of disability. The problem is called Electromagnetic Hypersensitivity Syndrome (EHS). Many people feel overwhelmed by the rapid pace of technological change. Some may fear the loss of treasured ways of living and communicating. They fear falling behind on the change and losing touch with friends, family, and the larger community. To many people computers are an alienating kind of magic. There is no doubt that some people may attribute a physical illness to exposure to this alienating or stress-causing technology. So is EHS caused by electromagnetic radiation (EMR)? To answer this we need a little background. There are essentially two kinds of electromagnetic radiation that concern us here. Ionizing and non-ionizing.
Ionizing Radiation
Ionizing radiation is electromagnetic waves that have the ability to force electrons to break off other molecules. Ionizing radiation is most familiar in the form of X-rays, and the energy emitted by plutonium, radon gas and other "radioactive" materials. With the exception of smoke detectors, none of the electronic devices you may interact with on a regular basis emit ionizing radiation. In fact the radiation present at natural levels is much higher than what smoke detectors emit. This so-called "background radiation" has been present in varying amounts since just after the Big Bang. Ionizing radiation has been shown to be a cause for many kinds of cancer and also for a portion of the mutations that play a role in evolution. There are even naturally occurring radioactive molecules of carbon and potassium in the human body and in the food we eat. Other sources of radiation include cigarette smoke and other air pollutants.
Non-Ionizing Radiation
Non-ionizing radiation (such as microwaves used to heat food and to transmit cell phone and wifi signals), the heat from a toaster, and light from a candle cannot break molecular bonds. So the question now is what harm can non-ionizing radiation cause? A web search for "dangers of microwaves" comes up with a list of sites that are a veritable treasure trove of logical fallacy and pseudo-science. Examples include reliance on "post hoc" anecdotes (x happened after y, therefore y caused x), single unrepeated studies, broad unproven assumptions (x causes y and y is bad because it certainly seems like it could be bad) and claims that we cannot be absolutely certain of the risks (this is true of virtually everything and is therefore not a meritorious argument). They may also use the questionable cause logical fallacy of using epidemiological claims that are exclusive of other causal factors ("over the past 30 years x has gone up at almost the exact same rate as y, therefore x causes y"). They rely on the circumstantial ad-hominem logical fallacy ("person A has a vested interest in x being true therefore x must be false"). If you remove all the logical fallacies from their arguments, I guarantee you will have nothing left but appeals to emotion and fear. That said, non-ionizing radiation can be dangerous even at low levels, but only in very pedestrian ways you have surely already experienced. For instance, if you sit on a heating pad (infrared radiation) for long enough you will get slow-cooked (this can cause very severe burns). We have all been exposed to high-intensity non-ionizing radiation in the form of burning ourselves on a hot skillet or iron. At the right frequency and intensity electrical current can cause disruption in our nervous system from mild muscular contraction to seizures and heart attack. At a sufficient intensity visible light waves can cause permanent blindness.
Exploring the Problem of EHS
So the question is, does exposure to the low-intensity microwaves that are emitted by cell phones and wifi pose a threat to our health that some people are particularly sensitive to? Are these people the proverbial canary in the coal mine? Or are they misattributing their symptoms to exposure to a specific frequency of radio waves when it may be caused by something else entirely?
I see two ways to approach this problem: first is to determine whether there is a unique characteristic of microwave frequencies that make them different from the radio frequencies that we have been exposed to at much higher levels for much longer than our exposure to the low-level microwaves coming from cell phones and wifi networks. We cannot answer this definitively, but here comes the argumentum ad ignorantiam: just because we cannot be 100% sure that low-intensity microwave frequencies do not cause immediate and intense health problems in certain individuals, does not mean we can assume that they do. We aren't 100% sure about the general theory of relativity, but so far research has confirmed it over and over.
The scientific method is a free and universally accessible system for understanding the workings of the cosmos. The method does not require any special esoteric knowledge nor reliance on obscure ancient wisdom. All you need is your senses, a sharp mind, patience and the willingness to risk hurting your ego if you find your hypothesis to be wrong. Most importantly SCIENCE IS NOT A COLLECTION OF FACTS! You must disabuse yourself of this very wrong assumption. Science is a method of systematic observation that includes mechanisms that take your natural biases and assumptions into account, control for every human's natural tendency to see patterns even where none exist.
Our hypothesis in this case is whether low-intensity non-ionizing radiation is the cause of the symptoms afflicting those who claim EHS. A very useful tool in the workshop of the scientist is the double-blind study. The double blind study is vitally important in identifying the cause of EHS because it is a self-diagnosed disorder (the patient already assumes the cause is low-frequency radio waves specifically from cell phones and wifi networks). Because the researcher may also have a bias in favor of one outcome or another, the double-blinding process helps prevent the researchers or test administrators from influencing the results.
How to Design the Study
Ideally such a study would involve a Faraday cage-enclosed space designed to block out most all electromagnetic radiation. You must have a method to guarantee to the individuals with EHS that no EMR can get through the cage more than what would be natural levels (demonstrate that no cell service or wifi signals can be detected in the cage, maybe?) Then you have two devices: a wireless transmitter and a dummy device that looks and behaves identically to the transmitter but does not emit any radio frequencies (or amounts less than those emitted by a lightbulb or human being). Each device should be marked with an ID so they can be identified during analysis, but the ID should not in any way convey which device it is. Neither the test administrator nor the participant should be able to differentiate the two devices. One at a time, have a participant enter the Faraday cage. Without being able to see if the device has been turned on or not, the participants will be asked to identify when they are being exposed to electromagnetic. Again, the tester should not know if they are turning on a fake or real device either.
A simpler test (given that EHS patients claim it under more "normal" environments) is to just have an low-intensity EMR-emitting device that is either on or off, but is obscured so the patient cannot tell which state it is in. But hey, we don't need to do this test because it has already been done more than 31 times and all the results confirm the hypothesis that EHS is not caused by electromagnetic radio waves.