First, we would need to understand a little bit of electromagnetic waves. It’s being described by these beautiful equations
c = 299 792 458 m/s is the speed of light in a vacuum
h = 6.626 070 15 * 10-34 J*s = 4.135 667 696 * 10-15 eV*s is Planck’s Constant
Since, most of the time, all electromagnetic waves can be taken as a constant of speed of light in a vacuum, with the only variables of frequency f and wavelength lambda that are inversely proportional to each other to describe the type of electromagnetic wave.
Since the formation of the entire universe, or currently the best described theory, by the BIG BANG THEORY, it’s being observed that even the events of the remote past could also directly affect the cosmic radiation of the universe, indefinitely.
What we understand is that radiation is moving at the speed of light!
Since the radiation waves are moving at the speed of light, there exist radiation waves that are called ionizing radiation waves which are harmful to the human bodies, directly and indirectly.
Now, from the above graphical representation of the Electromagnetic Spectrum, the current methods of classification of Ionizing Radiation (which is the major source of Health Concerns), usually ranges from 10 eV ~ 33 eV (the Ultraviolet Electromagnetic spectrum) and beyond till Gamma Rays.
The energy of ionizing radiation is between 10 electronvolts (eV) and 33 eV. Typical ionizing subatomic particles include alpha particles, beta particles, and neutrons. These are typically created by radioactive decay, and almost all are energetic enough to ionize.
Which means to say, as long as there are any electromagnetic waves with energy of 10 eV ~ 33 eV (Ultraviolet Type Classification) and above this threshold, there would be ionizing radiation. Now begs the question of What is ionizing radiation?