SPF and PA are indices widely used in sunscreens to define the level of protection against UVA and UVB rays.
SPF stands for Sun Protection Factor. It measures how effectively a sunscreen protects the skin from UVB rays, which cause sunburn. For example, SPF 30 theoretically provides 30 times more protection than not using sunscreen at all. Higher SPFs offer slightly more protection, but no sunscreen can block 100% of UVB rays. Common ratings include SPF 15, SPF 30, and SPF 50.
PA stands for Protection Grade of UVA, a rating system used to measure the level of protection a sunscreen offers against UVA rays, which cause ageing and long-term skin damage. They can also increase the risk of skin cancer.
The PA rating is based on the Persistent Pigment Darkening (PPD) method, which measures how long it takes for the skin to tan or darken under UVA exposure compared to how long it takes without any protection. The "+" sign indicates the level of protection; more "+" symbols denote higher protection against UVA rays. It ranges from PA+ to PA++++.
For optimal sun protection, it is advisable to use a sunscreen that offers both a high SPF and a high PA rating to protect against the full spectrum of harmful UV radiation. Dermatologists recommend reapplying sunscreen every two hours or immediately after swimming or sweating.
In Japan and some other Asian countries, SPF 50 and PA++++ are commonly used to achieve optimal sun protection.