Summer’s almost here, so once again it’s time for you to reach for the sunscreen. For most of us, the default is to grab the bottle with the highest SPF. Bigger is better? But what do those SPF numbers actually mean?
SPF stands for Sun Protection Factor, and the number beside it indicates how well the sunscreen protects skin against sunburn. It is not an indicator of how long you can stay out in the sun, rather, it indicates how much longer it takes untanned skin to start to redden with sunscreen applied compared to how long it takes to start reddening without it.
To work out the SPF of a sunscreen, laboratory tests are carried out on an untanned patch of skin (such as the buttocks) of human volunteers.
Sunscreen is applied liberally to the skin, then the skin is exposed to simulated sunlight via UV lamps.
Measurements are taken of how long it takes the skin to get a minimal burn when covered with sunscreen, and how long it takes to get the same minimal redness without it.
To get the SPF number, a simple formula is used. The number of seconds it takes a patch of skin to slightly redden when covered in sunscreen is divided by the number of seconds it takes to slightly redden when there is no sunscreen applied.
If it took 400 seconds for skin to burn with sunscreen, and 10 seconds to burn without it. 400 is divided by 10, which is 40. The SPF is 40 .
The testing is expensive for the small volume we make in each batch to bring you the freshest natural ingredients. Therefore Frownies is not calling our zinc products by an SPF number value. It is a mild sun block, use caution and reapply often just like any SPF product requires.
What is good to know when it comes to a sunscreen’s SPF. First some UV radiation can always get through to the skin, no matter how high the SPF. No sunscreen can ever block sunburn and associated skin damage completely; sunscreen is only one part of effective sun protection, and not a front-line defence.
Second, SPF is determined by tests in which a liberal amount of sunscreen is applied. That means that, to actually achieve the protection indicated on the label, you need to apply sunscreen thickly enough—and many people don’t. You also need to apply the sunscreen at least every two hours when in strong UV ray light, it can come off through sweat or by being rubbed off by clothing.
Be safe and enjoy your summer!