SPF numbers 101
For many people, when they go sunscreen shopping, they get stumped on the SPF numbers. Which one do I pick? 15, 30, or maybe 50? Do not fret. Let me guide you on the SPF numbers game.
How sunscreen works
SPF is an abbreviation for sun protection factor. It protects the skin from the damage of UV rays. There are two UV rays. UVA and UVB. UVB rays cause damage to the outer layer of the skin (dermis) while UVA rays cause damage to the inner layer (epidermis).
The UVA rays cause cell damage and break down collagen. Consequently, it results in photoaging, wrinkles and skin cancer. On the other hand, UVB rays cause sunburn. If left unchecked it can cause skin cancer.
The number on the bottle e.g. SPF 15 translates to the % of UVB rays that are blocked from entering the skin. Most sunscreen will protect you from UVB rays but not UVA rays. Hence, while shopping for a sunscreen it is prudent to buy a broad spectrum sunscreen. It will protect you from UVA rays as well.
The bigger the number the better
The world of sunscreen is ridden with the notion; the higher the number, the better the protection. So many people assume SPF 100 is better than 30. However, this is not the case. Sun protection factor is a theoretical estimate. It gives an estimate of how long you can stay in the sun and not suffer sun damage.
For example, your skin starts to burn after 15 minutes of sun exposure. An SPF 15 promises to protect your skin 15 more times. So when you do the math, it will protect you for 15*15=225 minutes. Therefore, theoretically, one can be in the sun for 225 minutes and not suffer sun damage.
Let’s continue with the mathematical argument. If your skin starts to burn after 30 minutes, the SPF 15 will protect you 15 times more. That will be 450 minutes. Let’s flip it and take SPF 100. The same argument, the sunscreen can protect you 100 times more. That will be 30*100=3000 minutes. It is simply preposterous!
There is a tipping point for SPF. The mechanics of are that; SPF 15 blocks 93% of UVB rays while SPF 30 blocks 97% of UVB rays and SPF 50 blocks 98% of UVB rays. SPF is not linear. Sadly, there has been misleading marketing. It has led consumers to believe sunscreen with bigger SPF numbers provides much more protection. However, this is not the case. Dermatologists recommend using an SPF 15 or SPF 30 sunscreen.
How much to apply
Unfortunately, most people do not use sunscreen properly. Most people use only ¼ of what is needed. The correct use is 2 mg/cm² of skin. That is about 28 grams (1 oz tube) of sunscreen for full body application.
It is important to re-apply after every 2 hours to gain maximum protection. Ultimately, it is best to combine your sunscreen protection with hats, long sleeve shirts, using umbrellas and avoiding the midday sun.