We all know that too much sun can cause skin cancer and accelerate the signs of ageing. What scientists are struggling to figure out is exactly how UVA and UVB penetrate the different layers of skin and what the effect is at a cellular level. Quentin Cooper talks to Dr Mark Birch-Machin from the University of Newcastle about his work in this area. Producer John Watkins. E-MAIL: material.world@bbc.co.uk