I hope this thread doesn't blow up too badly. I'll respond here with some actual facts.
First let's discuss what "radiation" is. From Wikipedia's disambiguation page for the word, we see that there are a lot of definitions:
https://en.wikipedia.org/wiki/Radiation_(disambiguation)
The words "radiation" and "radiate" are similar the to words "radius" (as in circles) and "ray" (as in rays of sunshine). They have to do with things going away from a central point. In medicine, pain that "radiates" spreads outward from a central point. The word is used for lots of unrelated things that just happen to have the quality of spreading out from a central point. Keep that fact in mind because, to understand the effects of something, you need to know specifics about it, and not just the fact that it moves outward from a central location and can therefore be referred to as a type of "radiation".
Let's talk about some types of radiation that are harmful to human beings: gamma rays, x-rays, UV radiation, cosmic rays (if you're in space), nuclear radiation, alpha radiation and beta radiation. Gamma rays and x-rays are two words for the same thing (the difference in naming is historical). They, and UV radiation, are composed of high-energy photons. Yes, photons, the same thing as visible light. The difference between the former and the latter is that the former have much higher energy per photon. In fact, they have enough energy that a single photon can break the bonds between atoms and cause chemical changes. That's why they can cause cancer: they can drill holes in your DNA and randomly change the genetic code. Bad stuff.
"Nuclear" radiation is a vague term, but here we'll consider it to be high-speed (i.e. high energy) neutrons and protons. A particle of alpha radiation is the nucleus of a helium atom that is barreling along like a bat out of hell. A beta particle is an electron that is similarly cruising. "But wait!", you say. "My body is made of protons and neutrons and electrons, and I've breathed helium out of a balloon to make my voice high and squeaky. Am I going to die?" Remember, the difference is in how much energy is in the particular particle. If it's moving slowly, it won't hurt you. If it's moving really fast and runs into you, it can do damage. It's the same reason why you can safely hold a bullet in your hand, but would prefer that someone not shoot that same bullet into you with a rifle.
What about radioactive stuff and nuclear fallout? These are made of unstable atoms that decay and shoot out one of the above types of radiation. The shorter the half-life, the shorter the average time for a particular atom to decay, so if you have a bunch of unstable atoms with a short half life, more of them are going to be decaying and spitting out nasties. The longer the half life, the less often these occur. If the half life is very long (like infinite), the atoms are stable and don't decay, and thus don't emit dangerous radiation.
Particles that have enough energy to break chemical bonds are called "ionizing radiation". This is the bad stuff that can hurt you by damaging your DNA and causing cancer. Particles that don't have enough energy to break chemical bonds are called "non-ionizing radiation". They won't give you cancer. They might cause some heat, but as long as you don't get burned, they're really not much different than any other heat source.
So, what's the deal with the radiation from cell phones, microwaves and WiFi? They are radio waves: non-ionizing radiation made of photons, with each photon waaay under the energy limit where it can break chemical bonds. In decreasing order of energy per photon, we have gamma rays and x-rays, UV rays, visible light, infrared radiation, millimeter wave radiation, microwave radiation and then various other names for radio waves. The names "millimeter wave", "microwave" and a bunch of names for different kinds of radio waves are also historical in nature, but they're all types of radio waves (and all of them are photons with different amounts of energy). If gamma rays are bullets and UV rays are pin pricks, photons in the microwave region (where cell phones, microwave ovens and WiFi operate) are big fluffy cotton balls. Toss those at someone all day and it won't hurt them.
"But my microwave oven burns things! I left popcorn in there and it caught fire." Individually the microwave photons don't have much energy, but concentrate enough of them together and they can heat something up. It's just like using a magnifying glass to burn holes in paper. The difference is that it causes heat, and the photons themselves can't break chemical bonds. Do you know what happens if you operate a microwave oven with the door open? I knew a physicist who did this. He said that it was like looking at a camp fire: his face got warm. (He knew what he was doing. Don't you try this.) Again, as long as you don't get burned, it won't cause lasting effects.
"But what about small exposure over a long period of time?" To a heat source? Try this experiment: hold a warm baby next to you for a long time. Did you get cancer? Does central heating cause cancer? Heat is heat.
Here's another freaky factoid: you, due to your nature as a warm blooded creature, are currently emitting tens of watts of infrared radiation at everything around you. Each infrared photon has far more energy than anything produced by your cell phone, WiFi router or your microwave oven. In fact, the total amount of energy you radiate is also more than your phone or router put out (or your microwave oven leaks). Your cell phone and WiFi router each radiate less than one watt of total power. Your microwave oven leaks less than that (though there are several hundred watts inside, so don't climb in).
You? You're emitting tens of watts. Is your personally emitted radiation causing cancer in other people? Is it making them sick?
What about all those studies about cell phones and power lines causing cancer? They're pretty lousy studies. The original power line studies were done epidemiologically, meaning that they didn't actually do controlled experiments, but just looked at cancer statistics, power line locations, etc. In epidemiological studies, it is extremely difficult to control for confusing factors. Power lines tend to run along side of roads, which are polluted by car exhaust. Proximity to power lines can vary according to socioeconomic class, meaning that people with widely varied lifestyles and access to medical care may live at statistically different distances from power lines.
There has not been one credible, decently controlled study that has linked cell phone radiation with cancer or other illness.
https://www.cancer.gov/about-cancer/causes-prevention/risk/radiation/cell-phones-fact-sheetThere has not be one credible, decently controlled study that has linked power line exposure with cancer or other illness.
https://www.cancer.gov/about-cancer/causes-prevention/risk/radiation/electromagnetic-fields-fact-sheetGiven the extreme rise in the number of people using mobile phones over the past few years, if their radio waves caused cancer (even if rarely), we would have seen a massively significant jump in the number of cases of those kinds of cancers. This has not happened. The same goes for the effects of power lines. Instead, the hints and barely significant "bumps" that the studies show are very likely to be normal statistical artifacts in how scientific studies are done.
What about people who claim to be sensitive or allergic to electromagnetic fields? The interesting thing here is that there has not been a single case of someone who claims such sensitivity who can tell whether they are actually in the presence of such fields. Not one person. If you control for confusing factors (noise, visibility of equipment, etc.), they can't tell if someone out of their sight flips a switch and bathes them in radio waves. Do such people have real symptoms? Of course. Are they caused by electromagnetic radiation? No. Such "sensitivity" is psychosomatic.
Anyone who tries to convince a person that they have such a sensitivity is doing them a grave disservice. No competent medical professional will offer such a diagnosis.
CZ