Optical micromanipulation studies have solved a puzzle on DNA damage and repair. Such knowledge is crucial for
understanding cancer and ageing. So far it was not understood, why the soft UV component of sunlight, UV-A, causes
the dangerous DNA double strand breaks. The energy of UV-A photons is below 4 eV per photon, too low to directly
cleave the corresponding chemical bonds in DNA. This is occasionally used to claim that artificial sunbeds, which
mainly use UV-A, would not impose a risk on health.
UV-A is only sufficient for induction of single strand breaks. The essential new observation is that, when on the opposite
strand there is another single strand break at a distance of up to 20 base pairs. These two breaks will be converted into a break of the whole double strand with all its known consequences for cancer and ageing. However, in natural sun the effect is counteracted. Simultaneous red light illumination reduces UV induced DNA damages to 1/3. Since sunlight has a red component, skin tanning with natural sun is not as risky as might appear at a first glance.