The protection a sunscreen offers is affected by its sun protection factor (SPF) rating, whether it is broad spectrum, how evenly and how thickly you apply it, and how long you stay under the sun. The longer the time spent under the sun, the more UV radiation accumulates and the greater the potential for burning.
Even if you are not very active, sunscreen tends to rub off gradually and therefore needs to be reapplied regularly. This applies particularly to children because they are active.
Sunscreens contain either chemical blockers that absorb UV radiation, dispersing it as heat before it can damage the cells; or physical blockers that reflect UV radiation away from the skin. Some sunscreens contain both.
The SPF number is a ranking system that shows how much protection is being offered against UV radiation. The higher the SPF number, the more UV radiation is filtered out, and the greater the protection. SPF gives a general guide to sun protection but does not determine how long it will take for a person to be sunburned. The amount of time it takes to be sunburned depends on the level of UV radiation, and varies according to the time of day, the time of year, the weather, and the person’s skin color.
No matter how high the SPF rating, no sunscreen can screen out all UV radiation. All sunscreens are filters allowing some UV radiation to pass through to the skin. The Cancer Society advises that SPF30+ sunscreen is sufficient for sun protection if applied correctly. A higher SPF sunscreen may give a sense of false assurance regarding the length of time one can spend safely under the sun.