The ozone layer blocks ultraviolet (UV) rays. Oxygen molecules (O2) are hit by a high energy photons which split them into two oxygen atoms (photo-dissociation of oxygen). Individual oxygen atoms can then combine with oxygen molecules (O2) to create ozone (O3). The ozone molecules absorb ultraviolet (UV) radiation and in so doing split into an oxygen molecule and an oxygen atom (which can then recombine to create more ozone). Depletion of the ozone layer creating a so called ozone hole is a worry because the UV that reaches the surface can damage DNA and cause skin cancer.