SPF stands for Sunscreen Protection Factor. It is measured by determining how long a given person takes to «burn» with and without the sunscreen product on. If it takes 10 times longer with the sunscreen product then that product has a SPF of 10. If it only takes twice as long then that product has an SPF of 2. In reality, the testing labs don’t really burn the test subjects they just give them enough UVR to cause the skin to turn barely red. This minimum dose is called the MED (minimal erythermal dose).