Фактор под 15 не е ефективен, а фактор 30 предпазва на 97% от лъчите, по висок от 30 не води до по-голям процент UV защита, а само удължава времето на предпазване. Формулата е ако изгаряш за 20мин на слънце го умножаваш по фактора напраимер SPF 15 = 300 минути -> следователно с фактор 15 вместо след 20 минути ще се зачервиш (изгориш) след 300 минути. Кремовете с фактор е хубаво да се слагат около 20 минути преди излагането на слънце.
Ето и това е хубаво да прочетеш:
SPF is crucial, but it is only a measurement regarding sunburn (UVB) rays. It is dangerous for your skin to not have UVA protection and many sunscreens do not have ingredients that can provided true full-spectrum (both UVA and UVB) coverage. There are no numbers to tell you about protection from UVA radiation. For that protection you have to check the active ingredient list to see if either zinc oxide, titanium dioxide, avobenzone (which may also be listed as Parsol 1789 or butyl methoxydibenzoylmethane), Mexoryl SX, or Tinosorb (Mexoryl SX and Tinosorb are only available in products sold outside the US). If one of those isn't part of the active ingredient list (it doesn't count if it is just part of the regular or "other" ingredients) you are not applying adequate UVA protection and that is dangerous for your skin.
Oxybenzone (also called benzophenone-3) is a UVA-absorbing sunscreen ingredient that sounds similar to avobenzone, but is not the same. There are several sunscreen ingredients approved for use in the United States for sunburn protection. While benzophenones and oxybenzone do provide some UVA protection, they are not as effective as titanium dioxide, zinc oxide, avobenzone, Mexoryl SX, or Tinosorb.
Общо взето това знам. btw добре е да ползваш крем с фактор 15 през цялата година, дори и да не се излагаш умишлено на слънце.
SPF (sun protection factor) The protection level against UVB rays is determined to all products according to the COLIPA method. SPF = [MED with a protection agent] / [MED without any protection agent] MED (minimal erythemal dose)
SPF EU – determining the protection level ccording to the COLIPA method, valid in Europe SPF USA – a method for determining the protection level valid in the USA
SPF is a ratio between the time when the skin becomes red, despite using a product with protection filters and the time when the skin, which was not treated with a sun care product, becomes slightly red. Example: if the skin becomes red after 5 minutes of sun-exposure, the product with sun protection SPF 15 will protect the skin against sunburn within the interval of 15 x 5 minutes = 75 minutes.
The SPF value tells us many times longer can we expose our skin to sunrays, compared to the natural protection of the skin not using any sun protection product.
Boots Star System / Australian Standard Two methods are used for determining the protection level of the products against UVA rays: Boots Star System in Australian Standard.
The first method makes it possible to calculate the UVA ratio. This is a ratio between UVA and UVB protection and is shown by a star system (Minimum(*), Moderate(**), Good(***), Superior(****), Maximum(*****)).Maximum denotes that the level of protection against UVA rays is the same as the level of protection against UVB rays (SPF).
The second method - the Australian standard shows that the product provides at least 90% protection from UVA rays.
Water Resistant The products are defined the level of their resistance in water according to the COLIPA method. The product is water resistant if the SPF value determined after rinsing with water keeps at least 50 percent of the strength it had prior to contact with water.
Photostable The products carrying the mark photostable contain a combination of organic filters stable in daylight and/or inorganic filters, as well as chosen emulsion components. The results of testing demonstrate the changing of the protection factor’s activity when exposed to daylight.