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SPF stands for sun protection factor. The SPF protects against UVB radiation. A sunscreen is given a SPF number (of between 4 and 30+) after strict laboratory testing. The testing compares the time it takes for patches of skin with sunscreen to show redness with the time it takes to produce the same amount of skin redness without sunscreen. The higher the SPF number the more protection a sunscreen provides against sunburn. The length of time it can take for skin to burn can depend on a lot of factors:

- person's skin type

- season and geographic location

- time of day

- amount of cloud cover

- nearness to reflective surfaces (such as water, sand and snow)

- correct application of sunscreen

- contact with water, sand, sweat or clothing

- whether the product is past its use-by date.

No sunscreen filters all ultraviolet radiation, so even when using SPF 30+ you are still exposed to 3% of the ultraviolet rays. Broad spectrum filters out both UVA ad UVB. If broad spectrum is not on the label it probably only filters out UVB. Water resistant sunscreen resists being washed off while swimming.

No sunscreen filters all UV radiation. Your skin has its own SPF factor depending on the amount of melanin present.