SPF, or sun protection factor, is a term used to describe a products’ sun protection qualities. It represents the number of times longer a person who is wearing the protection can stay out in the sun without burning. For example, someone wearing sunscreen rated SPF 30 can expect to spend 30 times longer in the sun before being burnt when compared to having bare skin exposed.
Interestingly, SPF measures sunscreen’s ability to block UV-B rays, the type of UV that is attributed to sunburn. And to make this even more complex, the SPF value does not directly represent the amount of UV light that is blocked, as the scale is non-linear. For example:
-SPF 15 blocks 93% of UV-B rays
– SPF 30 blocks 97% of UV-B rays
– SPF 50 blocks 98% of UV-B rays
While SPF is a good way of representing the relative strength of sun cream, limitations exist. Firstly, UV-B rays are only one of three types of UV rays, the others being UV-A and UV-C. UV-C is not an issue for our health as they never reach the ground and are absorbed in the outer reaches of the atmosphere. UV-A causes aging of the skin and some types of skin cancer, but are not directly considered when the SPF is calculated. New laws in Australia do require the degree of UV-A ray protection to increase with increasing SPF in ‘broad spectrum’ sunscreens.
Despite all of this focus on SPF levels, it is actually the amount of sunscreen applied that largely dictates the level of sun protection achieved. To obtain the SPF as advertised, 1.5 milligrams per square centimeter of skin should be used or about 30 ml for total coverage.
Further, while high (50+) SPF sunscreens do marginally improve UV protection, the false sense of security they can provide often counteract their benefits. People who wear higher SPF sunscreens often spend longer in the sun and wear fewer other forms of protection, such as wide-brimmed hats.
Sunscreens are not the only products considered to have a sun protection factor. Clothes also offer sun protection with some typical SPF values seen in common clothing as follows:
- Nylon stocking: 2 SPF
- Hats SPF 3-6
- Summer clothing – SPF 6.5
- Sun protective clothing – up to SPF 30