SPF (sun protection factor) actually measures that how long a sunscreen can protect your skin from ultraviolet B rays. Ultraviolet B rays (UVB) are the chief cause of sunburn and contribute to skin cancer. SPF does not measure the protection from UVA rays. UVA rays are also dangerous.
Sunscreen, also known as sunblock is a lotion, gel, foam, spray or other topical product that reflects sun’s ultraviolet radiations upto some level and thus helps protect against sunburn.
Relation between UVB rays and skin cancer:
UVB rays destroys the epidermis (the outer layer of skin) where the most common forms of skin cancer can occur. UVB burns the superficial skin while UVA rays penetrate more deeply into the skin and lead to consequences like premature skin aging, wrinkle formation etc. Therefore people with maximum sun exposures like those who works outdoor, athletes in lengthy games etc always need some sort of sunscreen.
SPF for different skin colors:
Sunscreens are found with different SPF numbers i.e SPF 30, SPF 50, SPF 100. These numbers are directed by dermatologists as per the skin color of the individual. Sunscreens with SPF values in the range of 30 to 50 will offer adequate protection, even for the people most sensitive to sunburn.
SPF 15 blocks 93% of UVB radiations
SPF 30 blocks 97% of UVB radiations
SPF 50 blocks 98% of UVB radiations
SPF 100 blocks 99% of UVB radiations
Melanin in the skin absorb UV radiations and divert it away from healthy cells. That’s why melanin is photoprotective and protect the skin from damage. Darker the skin color, more is melanin concentration and less likely an individual will suffers from skin damage, i.e premature aging and skin cancer. This is the advantage of dark skin people over those with light color skin, that they have more melanin which protect their skin from UV radiations. But still dark skin colored individuals will need some protection, they need SPF with somewhat low number. In most people with dark skin, SPF 30 is sufficient along with limited sun exposure.
Now, people with light and fair skin color, sensitive skin, a history of skin cancer, or with photosensitive medications will need a higher SPF.