`ACSindependent Sailing

just for interest sake especially ms. sloth. im sorry for using up so much space. colin ho

Sun protection factor

The SPF (sun protection factor) of a sunscreen is a laboratory measure of the effectiveness of sunscreen; the higher the SPF, the more protection a sunscreen offers against UVB (the ultraviolet radiation that causes sunburn) and UVA (more associated with longer-term skin damage).

SPF is the inverse of the proportion of UVB that penetrates the sunscreen:

$UVB_{penetrating}=\frac{1}{SPF}\quad\quad\quad\quad\quad\quad(1)$

$UVB_{penetrating}+UVB_{blocked}=1\quad\quad(2)$

Substituting equation (1) into (2):

$\frac{1}{SPF}+UVB_{blocked}=1$

$UVB_{blocked}=\frac{(SPF-1)}{SPF}$

Thus a sunscreen with an SPF of x blocks a proportion (x−1)/x of UVB:

Sun Protection Factor	Proportion of UVB blocked
SPF 10	90.0%
SPF 20	95.0%
SPF 30	96.7%
SPF 60	98.3%

In theory, the SPF is a multiplier that can be applied to the time taken to burn. For example, someone who would burn after 12 minutes in the sun would expect to burn after 2 hours (120 min) if protected by a sunscreen with SPF 10. In practice, the protection from a particular sunscreen depends on factors such as:

The skin type of the user.
The amount applied and frequency of re-application.
Activities in which one engages (for example, swimming leads to a loss of sunscreen from the skin).
Time of day and season.
Percentage of UV reflected or scattered by the environment, for example snow or sand.
Amount of sunscreen the skin has absorbed.

The SPF is an imperfect measure of skin damage because invisible damage and skin aging is also caused by the very common ultraviolet type A, which does not cause reddening or pain. Conventional sunscreen does not block UVA as effectively as UVB, and an SPF rating of 30+ may translate to significantly lower levels of UVA protection according to a 2003 study by RAFT trust-funded researchers. Some broad spectrum sunscreens do provide significant UVA protection. According to a 2004 study, UVA also causes DNA damage to cells deep within the skin, increasing the risk of malignant melanomas.

Due to consumer confusion over the real degree and duration of protection offered, labeling restrictions are in force in several countries. In the United States in 1999, the FDA decided to institute the labelling of SPF 30+ for sunscreens offering more protection, and a similar restriction applies in Australia. This was done to discourage companies making unrealistic claims about the level of protection offered (such as "all day protection"), and because an SPF over 30 does not provide significantly better protection.

Adapted from http://en.wikipedia.org/wiki/Sun_protection_factor