Why Radio waves are non-ionizing: their photon energy is far too low to knock electrons off atoms, so they cannot break chemical bonds or directly damage DNA the way X-rays, gamma rays, or particles from radioactive decay do. The known hazard from RF is heating of tissue, which is why exposure limits are based on specific absorption rate and power density rather than on cumulative radiation dose.
Watch out Saying RF is perfectly safe is wrong because enough RF power can cause thermal burns and deep tissue heating, which is exactly why the FCC sets MPE limits; RF also travels far beyond a few feet.
RF = non-ionizing: it cooks, it doesn't ionize. Heat hazard, not DNA hazard.