Science Questions

Do suncreams reduce the energy of each UV photon?

Do suncreams reduce the energy of each photon, thereby taking them below the threshold that could damage your DNA, or do they merely reduce the number that are transmitted through the cream? Peter, via email

There is a chemical in suncream that interacts with, and soaks up, ultraviolet light (the component of sunlight which is known to be linked to skin cancer because it damages DNA.).  It then re-radiates the energy in a less harmful form.

It works in a similar way to a striplight or a TV screen.  The phosphor on the front of a CRT television screen is hit by a high-energy beam, which causes atoms in the phosphor to jump up to a high energy ‘excited’ state.  When these electrons fall back down to a low energy state, they spit out some radiation as visible light.

The same thing happens in suncream; the chemical absorbs the UV radiation, electrons jumps to an excited level and fall again. As they fall, they release the energy in a different part of the electromagnetic spectrum, a part that is not viewed as harmful, usually heat or infrared radiation.

May 2007