Are you talking about a laser? The mechanism of injury is the same as any RF injury -- localized heating. You stick a soldering iron on your retina, you're going to have permanent damage to your vision. The reason you only need a low power to do damage is because the power is spread out over a very small space, because your cells absorb the energy instead of letting it pass through, and because the retinal cells are extremely sensitive to temperature (a 10C increase is apparently all you need to permanently damage them).
> Today I went out and enjoyed a warming 1000 W/m2 of hundreds of-nm EM radiation all over my body.
Skin cancer from exposure to sunlight kills people. UV light has enough energy to rip electrons off of atoms in your body, causing much damage. ("Ionizing radiation.") From physics, we know where the boundary between ionizing light and non-ionizing light lies (in the middle of the UV spectrum). We know ionizing radiation is dangerous -- we understand the mechanism and the effects. (Notice how much safety equipment is involved in getting a dental x-ray. Minimum dose required to get the necessary image. The technician stands in a separate room lined with lead. You wear a lead vest to avoid exposure anywhere that's not needed for the image.)
We also know when ionization stops happening. The energy carried by a photon is directly related to the wavelength of the light, see https://en.wikipedia.org/wiki/Photon_energy. Below the middle of UV, photons don't have enough energy to ionize anything. Thus begins "non-ionizing radiation", which as far as we know, just burns you. You can, of course, die from burns, so it's not harmless. That is how microwaves cook foods -- water absorbs energy from 2.4GHz photons quite well ("dielectric heating"). The RF energy is converted to heat, and makes you a hot meal. But you sure wouldn't want to have your hand in there when it's on.
Anyway, these effects are well understood. Any "5G is harmful" argument needs to propose a new mechanism for causing damage, or show how a known mechanism works at 5G frequencies. You could show that it turns out we were wrong about ionization (experimentally testable). You could show that certain tissues convert all the energy to heat (again experimentally testable).
Sure, there could be a new mechanism for causing damage that only happens at some random location in the EM spectrum not related to any other effects. Someone has to be the first to discover it. Portal 2 has a good quote: "Marie Curie invented the theory of radioactivity, the treatment of radioactivity, and dying of radioactivity."
It seems unlikely, though. You can do an infinite amount of research without discovering something that doesn't exist. People want fast YouTube downloads more than they want a 0% chance of risk. The risk is already so close to zero that it doesn't matter.
> Look, the #1 post on HN right now is how highly trained, expert engineers accidentally made a bridge (one of the oldest human inventions, around for millennia) create an extremely loud eerie noise currently blanketing San Francisco.
I believe they knew it would make a noise. They just didn't think anyone would care.
A deeper problem is that humans are capable of creating much more complicated systems than nature. Ever debug some code and wonder "how did that ever work?" It's just a lot more complex than basic physics, and so there are a lot more unexpected results.