But there’s more to it than this. SPF numbers indicate the amount of time you’re theoretically safe to be out in the sun before you burn.
For example, say it takes 20 minutes for your skin to start turning red without sunscreen. Using an SPF 15 sunscreen would allow you to stay in the sun without getting burned for 15 times longer than you could with no protection on your skin. So, if you wore SPF 15 on your skin, this would be around 300 minutes or 5 hours that you could stay in the sun without burning.
However, it’s really important to know that these protection times vary between people. It really depends on factors such as your skin type and what activities you’re doing. Exercise where you're sweating or swimming where you’re drying yourself with a towel, will decrease the protection time.
How well the sunscreen is applied also matters, it should always be generously rubbed over your skin. Despite the SPF rating, the general rule of thumb is that any sunscreen should be reapplied regularly, every 2 hours.
Of course, the type of sunlight is another factor. We know there is a thinner ozone above countries such as Australia and New Zealand to block UV rays.
The sun exposes your skin to two types of ultraviolet (UV) rays, UVA and UVB. UVA penetrates deeply into the skin and causes wrinkles, age spots and skin cancer. UVB is the main cause of sunburn and skin redness.
When choosing an SPF, it’s also important to know that the SPF scale is not linear. It's a common misperception that SPF 30 provides “double” the level of protection from an SPF 15. When in fact, sunscreens with a very high SPF such as SPF 50 or SPF 75, do not offer significantly greater protection than an SPF 30.
- SPF 15 blocks 93% of UVB rays
- SPF 30 blocks 97% of UVB rays
- SPF 50 blocks 98% of UVB rays
Emma Lewisham recommends wearing an SPF 30 or higher daily. This is based on the advice of skin specialists and organisations such as the American Academy of Dermatology, who say that an SPF 30 is the minimum SPF you should be wearing regularly to prevent skin aging caused by harmful UV rays.