Those guidelines are all about the heating effects of microwave radiation on flesh, the same way that an animal placed in a microwave oven would be injured or killed. And the FCC has established a limit for exposure of 1.6 W/kg where we can be sure that you aren't anywhere near having parts of you cooked.

Sufficiently intense microwave exposure can be a health hazard but there's nothing subtle about it, like there is with ionizing radiation which can make you sick days or weeks after exposure or give you cancer.

For 5G in particular the radio waves will tend to not penetrate as far into you body. Some militarizes have actually developed non-lethal crowd control weapons that cause a burning sensation in the victims' skins with the same sort of ~100GHz waves used by 5G but at a much, much higher power than the FCC would allow for consumer devices.

Is that 1.6W per kg of exposed tissue or for the whole critter (e.g. 160W for a 100kg manbeast)?

I put my hand on an antenna coupling plate for one of those through the glass CB antennas, and keyed the mic. This CB transmitted at 4W. It truly felt like someone heated the tip of a dagger and stabbed the palm of my hand with it. Unbelievably painful.

The limit is that no particular gram of your flesh can be exposed to more than 1.6W/kg.

I wonder how they determine how much energy is absorbed. In my example above, CB radio operates at ~27MHz. Most folks would say that’s going to pass right through, but damn if it didn’t cook me.

I feel it's pretty obviously per exposed tissue. It's not so much that your mass affects your resistance to it, but that the radiation needs a certain density to cook your hands.