Why has it taken so long for people to talk about using Far-UVC light for disinfecting? If you don't know what I am referring to you can see this simple TED talk https://www.youtube.com/watch?v=YATYsgi3e5A. There are also many articles online about it.
After watching that TED talk, all I could think was, this is so blindingly obvious why haven't we been doing it since the 1950s? Maybe there is no good answer to this question, but maybe there is. It is partly a biology question, but what can you do.
Anyway, it has been known for a long time that high energy electromagnetic radiation kills things. And it has also been known for a long time that shorter wavelengths cannot as easily pass through materials as longer wavelengths. And long ago people designed UV lights for sterilizing things. Why did they just use UV light that was harmful to humans and not stop and think, hey, maybe we should experiment and find the optimal wavelength that can still kill bacteria but is safest for humans and other animals?
Now I know that x-rays and gamma rays are even shorter wavelengths, and yet those are considered harmful to humans. But is that only in high doses? I don't quite get that part, because they are talking about using these far-UVC lights in places where people would be exposed to them day in and day out. Is it just that human skin is opaque to this particular wavelength but transparent again to shorter wavelengths?
Maybe it is just an accident of history that nobody has done this yet. If so I am sorry for asking a non-physics question here. But maybe there is some physics I am missing that makes this an actually clever discovery.
 A: 
The application of UVGI to disinfection has been an accepted practice since the mid-20th century. It has been used primarily in medical sanitation and sterile work facilities. Increasingly it has been employed to sterilize drinking and wastewater, as the holding facilities are enclosed and can be circulated to ensure a higher exposure to the UV. In recent years UVGI has found renewed application in air purifiers

The researches who suggest the use of a  narrow band ultraviolet radiation give references after the year 2000, and their "proof" is published in 2017.

However, the widespread use of germicidal ultraviolet light in public settings has been very limited because conventional UVC light sources are a human health hazard, being both carcinogenic and cataractogenic11,12.
By contrast, we have earlier shown that far-UVC light generated by filtered excimer lamps emitting in the 207 to 222 nm wavelength range, efficiently inactivates drug-resistant bacteria, without apparent harm to exposed mammalian skin13,14,15. The biophysical reason is that, due to its strong absorbance in biological materials, far-UVC light does not have sufficient range to penetrate through even the outer layer (stratum corneum) on the surface of human skin, nor the outer tear layer on the outer surface of the eye, neither of which contain living cells; however, because bacteria and viruses are typically of micron or smaller dimensions, far-UVC light can still efficiently traverse and inactivate them13,14,15.

So this is recent research, testing ranges of UV to find safe for mamals and killer for germs.

In their earlier studies, Brenner’s team demonstrated that far-UVC light was effective at killing MRSA (methicillin-resistant S. aureus) bacteria, a common cause of surgical wound infections but not harm human or mouse skin.

The researchers themselves state:

“If our results are confirmed in other settings, it follows that the use of overhead low-level far-UVC light in public locations would be a safe and efficient method for limiting the transmission and spread of airborne-mediated microbial diseases, such as influenza and tuberculosis,” Brenner said.

So the scientific method dictates replication and checks that really, in contrast to the general UV which brings melanoma to the human skin, this specific range is harmless. I would be cautious on the use for general public to start with, until more studies are done.
The physics is not clear. Why longer wavelengths would be harmful and shorter ones safer is something that needs more research, imo.
from the last link:

UV-C light (100-290nm) is extremely harmful and is almost completely absorbed by Earth's atmosphere. It is commonly used as a disinfectant in food, air, and water to kill microorganisms by destroying their cells' nucleic acids.

How a subset of this can be considered safe is a question mark.
edit after comment:

I am still wondering why this is only now being studied rather than 50+ years ago.

a) nobody thought to look at slices of UV, it was experimentally established that the high range was harmful
b) there was less inter-field interaction (physics-biology)so as to motivate the slicing. Physicists look for detailed spectra in absorption by materials
c) it takes time for propagation of experimental  methods between disciplines.
And yes, "there is still the question of why longer UV light and x-rays and gamma rays are harmful while one in the middle is not."
It is not xrays and gamma rays, but other UV light above and below. The band is right in the middle of the hurtful UV.
d) the main reason: it is  hard to come up with the idea that in the middle of the harmful UV
>UV-C light (100-290nm) is extremely harmful and is almost completely absorbed by Earth's atmosphere. It is commonly used as a disinfectant in food, air, and water to kill microorganisms by destroying their cells' nucleic acids.
the  band of 207 to 222 nm is not penetrating enough the skin and eye, but the ones above and below do, to harmful effect.
It may be true, from some bizarre coincidence biology is full of ( what is living organism but a set of coincidences, as far as physics is concerned).
from the quote above, the claim:

efficiently inactivates drug-resistant bacteria, without apparent harm to exposed mammalian skin13,14,15.

note the word "apparent"  . Have the biology experiments been long enough for  detecting malignancies? the dates of publications do not seem to give enough time for such studies.
I would like to see laboratory experiments scanning dry mammalian (human)  tissue and not penetrating it  to the level of the living cells, an absorption and depth penetration curve in the lab, before trusting myself to environments radiated with this slice of ultraviolet.
