Sunscreens are intended to protect the skin from the harmful effects of the sun and generally they refer to commercial products that have been specially designed and formulated for this purpose. Sunscreens are available in several forms: cream, spray, gel, topical lotion, that can be applied to the skin; a salve stick that can be applied to the lips, nose, and eyelids; a moisturisering creams and lotions; sunglasses that protect the eyes; clothing that helps sun-protection (NOTE: alot of clothing still allows harmful radiation through so this can be important).
SPF
SPF is an abbreviation for Sun Protection Factor, and is a number that represents how long a topical sunscreen is likely to remain effective on the skin. It is a common misconception that the SPF factor reflects the degree to which the sunscreen protects your skin but it actually refers to how long the protective effects last. The duration of effectiveness can be calculated on an individual basis by simply multiplying the SPF by the length of time it takes for the person to suffer a burn without sunscreen.
SPF 15 - For daily exposure over short periods of time, a sunscreen with 15 SPF is sufficient. Sunscreens and moisturisers with 15 SPF are the least amount of daily protection you should have. For longer periods of exposure, such as spending the afternoon on the beach, you will require a higher SPF, preferably 30 or higher. At high altitudes and the tropics, where the suns rays are strongest, always choose higher sun protection.
SPF 30 – If you have fair skin then you should consider a sunscreen with a SPF of 30+. People with darker skin may use a lower SPF, usually around 15 (the lowest you should use).
SPF 45+ – infants, children and anyone with sun-sensitive skin should use high SPF factor sunscreens and sunblock on both face and body.