Radiation is harmful to living organisms because it can damage cellular structures and DNA. Ionizing radiation, such as X-rays and gamma rays, has enough energy to remove tightly bound electrons from atoms, creating ions. This can lead to molecular damage, mutations, and disruptions in cellular processes, potentially causing cancer, cell death, or genetic defects. Non-ionizing radiation, like ultraviolet light, can also cause harm by inducing chemical changes in cells, leading to skin damage and increased cancer risk.