SPF (Sun Protect Factor) is a measure of how much solar energy is required to produce sunburn on protected skin relative to the amount of solar energy required to produce sunburn on unprotected skin.
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, which is then 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. Say it took 300 seconds for the skin to burn with sunscreen, and 10 seconds to burn without it. 300 is divided by 10, which is 30. The SPF is 30.
Under current Australian regulations, sunscreens must have an SPF significantly higher than 50 in order to be rated at 50+ (the plus means the rating is ‘at least’ the value given). A rating of 51 won’t cut the mustard; the sunscreen needs to have an SPF of 60 or more to be compliant.