If you live in the northern hemisphere, summer is approaching. When the sunshine starts to become intense it may be time to take the sunscreen out of the cupboard. Why do you do this though?
The sunlight itself is not that damaging; instead it is the ultraviolet (UV) light which comes with it that can be damaging to the skin. Visible light and UV light are both types of electromagnetic radiation, but UV has more energy, so it can cause greater ionisation. Ionisation occurs when electrons are knocked off from an atom, causing the atom to be charged (see An-atom-y for information on atomic structure). Therefore UV light is considered to be a type of ionising radiation.
When ionisation occurs in the DNA of a cell, it can cause genetic mutations to occur. If the mutations are extremely severe the cell can undergo uncontrolled cell division, which in essence is the basis of cancer. As a result, overexposure to intense sunshine is an increased risk factor for skin cancer. And you don’t want that, which is why you choose to apply sunscreen.
You also notice that there are different types. Some are labelled SPF 15, some SPF 30, etc. SPF stands for sun protection factor, and is a measure of how good the sunscreen is at protecting you from ionising radiation. SPF 15 blocks 14/15 (93.3%) of the radiation, and SPF 30 blocks 29/30 (96.7%) of the radiation.
The SPF scale is therefore not linear (SPF 30 does not block double SPF 15); this shows that extremely high SPF sunscreen does not offer significantly greater protection than low SPF sunscreen, so don’t be tricked into thinking high SPF is much better. Applying the sunscreen correctly has a far greater effect, as applying half the required level of sunscreen square roots the effective SPF.
A general rule of thumb is that the SPF number indicates how many times longer you can stay out in the sunshine. For example, if your skin burns after 10 minutes, applying SPF 15 sunscreen means that it will burn after 3 hours instead (15 × 10 minutes).
Enjoy the sunshine while you can, but don’t forget about your health while you’re out!