Sun Protection Factors
Don't be
confused with SPF ratings.
It is true that SPF 20, for instance, means that you can stay in the sun 20 times longer than without sunscreen before starting to burn. But this approach is now believed to be a factor in the rise in skin cancers although an interesting recent french study seems to counter this concern.
Look at it another way. A sunscreen with SPF 5 covers 80% of sun rays; SPF 10 covers 90%; SPF 20 covers 95%; and SPF 40 covers 97.5%.
Most authorities suggest that SPF 15 is good enough for everyday usage.
Here's something else to bear in mind. SPF was designed to cover UVB protection. It does not guarantee the same protection from UVA rays. SPF numbers can also be misleading in wintertime (for instance on the ski slopes) when the sun is low in the sky. In such cases UVA can become the significant danger
For this reason it's important to use a broad spectrum sunscreen (UVB & UVA), and because eyes are vulnerable, wear UV protection shades as well.
Protection from the mid-day sun (clothing and shelter) and from reflected rays can also help to prevent ageing of the skin and the curse of cancer.
Remember also that over a lifetime, the effect of the UVB & UVA rays that still get through in normal life can be significant. For this reason frequent application is important (e.g, at least every two hours and always after sweating or swimming).
It might also be wise to regard SPF ratings as a rough indicator rather than an accurate measure. In July 2004 a study for "Which?" magazine in the UK concluded that many commercial sunscreens did not meet their published SPF rating, nor did they protect from UVA radiation as they should.
It is fair to say this study was hotly challenged by the industry, but it is worth being aware of a possible problem here.
