A Sun Protection Factor (SPF) is the rating given to indicate the amount of skin protection a cream or lotion will provide against the sun's ultraviolet rays. A product marked SPF 30 generally allows the user to be in the sun, and avoid sunburn or damage, for 30 times longer than if none was used. The SPF 30 designation is the measurement of light-filtering that the chemicals in the lotion offer. It is very important for users to read the label of any sunscreen to be sure of the amount, and type, of protection it can give.
Ultraviolet (UV) light from the sun has three categories: UVA, UVB and UVC. UVC rays are the shortest and do not break through Earth's ozone layer to reach its surface. This type of light would be considered deadly even with only a small amount of exposure, but since it cannot penetrate the atmosphere, there is no cause for concern about it.
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UVB rays, however, are the kind that create suntanned skin but can, unfortunately, do much more. These rays penetrate the outermost layer of human skin and stimulate cells there to produce melanin, which produces a browned or tanned appearance. While some people consider this attractive, UVB rays also cause sunburn, premature skin aging, and skin cancer, even years later, from unprotected exposure to the sun. UVB rays are strongest in areas where Earth is closest to the sun, such as summer months in the northern hemisphere.
For many years, most sun protection formulas originally blocked only UVB rays because not enough was known and understood about the UVA type. For a long time, UVA rays were considered to be safe for tanning purposes. In recent decades, it has been discovered that UVA rays cause more damage than UVB ones. This is due, in part, to the fact that UVA rays maintain the same strength at all times of year and penetrate the skin more deeply. Other concerns with regard to skin protection can also include infrared and visible rays.
In terms of SPF, a lotion marked SPF 2 will block 50% of UVB rays. This amount jumps to 75% with SPF 4. SPF 30 blocks 97% — and SPF 50 will block 98% — of these rays. Another important related matter to consider is the time protection factor, which is based on how long an average unprotected individual can be exposed to the sun without burning. This length of time is then multiplied by the sun protection factor number of the lotion. A product's label should offer users a guide on how often the lotion, spray, or cream should be applied to offer maximum sunscreen.
SPF 30 lotions may also be marked as waterproof or water-resistant. A waterproof lotion should provide protection for up to 80 minutes in water. A water-resistant type should offer 40 minutes of protection. Various levels of sun protection are also available specifically for use with oily skin or for facial use.
Many types of makeup and facial moisturizers are available with SPF 30 protection. Sunscreens may be marked as having both UVB and UVA protection or as broad-spectrum protection. There are also special articles of clothing designed to offer sun protection, but the rating on them is referred to as Ultraviolet Protection Factor (UPF). UPF ratings range from 15 to 50-plus.