SPF rating measures protection from UVB which causes sunburn. Supposedly one can stay out longer without getting sunburn.
But I never get sunburn, so I have no way of measuring the effectiveness of UVB protection.
And then there are deeper penetrating UVA rays which produce tanning/damage. I apply the sunscreen generously but I get tanned in the same amount of time as if I had been wearing no sunscreen.
Sometimes I wonder if these sunscreens are just a waste of money in my case.
Sun Protection Factor Explained
SPF measures sunscreen protection from UVB rays, the kind that cause sunburn and contribute to skin cancer.
SPF does not measure how well a sunscreen will protect from UVA rays, which are also damaging and dangerous.
Dermatologists recommend using a SPF15 or SPF30 sunscreen. Higher SPFs don't provide much more protection.
How Tanning Happens
The sun's rays contain two types of ultraviolet radiation that reach your skin: UVA and UVB.
UVB radiation burns the upper layers of skin (the epidermis), causing sunburns.
UVA radiation is what makes people tan. UVA rays penetrate to the lower layers of the epidermis, where they trigger cells called
melanocytes to produce
melanin. Melanin is the brown pigment that causes tanning.
Melanin is the body's way of protecting skin from burning. Darker-skinned people tan more deeply than lighter-skinned people because their melanocytes produce more melanin. But just because a person doesn't burn does not mean that he or she is also protected against skin cancer and other problems.
Tanning Downsides
UVA rays may make you tan, but they can also cause serious damage. That's because UVA rays penetrate deeper into the skin than UVB rays. UVA rays can go all the way through the skin's protective epidermis to the dermis, where blood vessels and nerves are found.
Because of this, UVA rays may damage a person's immune system, making it harder to fight off diseases and leading to illnesses like
melanoma, the most serious type of skin cancer. Melanoma can kill. If it's not found and treated, it can quickly spread from the skin to the body's other organs.
It’s complicated, but to keep it simple, most dermatologists recommend using a SPF 15 or SPF 30 sunscreen.
Why not use a really high Sun Protection Factor? Sunscreens with really high SPFs, such as SPF 75 or SPF 100, do not offer significantly greater protection than SPF 30 and mislead people into thinking they have more protection than they actually do. Additionally, in order to have broad spectrum protection, the UVA protection should be at least 1/3 of the UVB protection. High SPF sunscreens usually offer far greater UVB than UVA protection, thus offering a false sense of full protection.
https://www.badgerbalm.com/s-30-what-is-spf-sunscreen-sun-protection-factor.aspx