There are 2 types of UV rays, UVA and UVB. Simple put, UVA=aging. UVB=burning. UVA rays are longer rays that can reach deeper in your skin, all the way into the dermis. When the UVA reaches this point, it causes cell death, weakens elastin and collagen fibers causing sagging and wrinkling. Elastin and collagen give skin its firm and smooth appearance and is near impossible to rebuild. UVB rays are shorter rays and can damage the epidermis, are stronger then UVA rays, causing sunburn, sun spots, hyperpigmentation, and can damage eyes.
There are 2 types of spf, sunscreen and sunblock. What makes a sunscreen or sunblock? Chemical sunscreen ingredients include, oxybenzone, avobenzone, octisalate and can be quite irritating on skin. Chemical sunscreens absorb ultraviolet rays. Physical sunscreens or Sunblock include, zinc oxide, titanium dioxide and reflect ultraviolet rays. Physical spf is ideal for hyperpigmented skin, so UV rays reflect off the UV barrier. Full Spectrum sunscreens or Broad spectrum protect skin from UVA & UVB rays. A spf of 15 sunscreen blocks 93.3% of UVB rays. Spf 30 sunscreen only blocks 96.9%. Doubling the spf does not double your sun protection factor. Anything higher then spf 30 is not anymore effective. The higher the spf, the more potentially irritating ingredients are added to thicken the concentration. I always say, "double check the ingredients".
When applying spf, apply at least 15 minutes prior to sun exposure so the product has time to shield your skin. There are a ton of spf's out there, liquid, cream, gel, powder, added to makeup, but they are not all sufficient for UV protection. A powder or foundation with spf is not sufficient. A spf should be a layer in itself before makeup. Powder does not cover and surround the skin with protection like a liquid, cream or gel spf can. Lastly no matter what your skin tone, you need to protect your skin from the sun. Darker skin tones have more melanin (skin pigment), but the more the melanin, the more there is to damage. Darker skin is actually more sensitive to the sun the pale skin tones. Melanin is activated by the sun and can cause hyperpigmentation (uneven skintone), as well as damage collagen and elastin. This damage maybe invisible for a while, but it will pop up in later years. Pale skin tones see the instant damage, a sunburn. Because this skin has little melanin, it will burn, and be more susceptible to collagen and elastin damage.
During the summer months, wear a hat, sit under a umbrella, reapply sunscreen if swimming (it will come off, no matter that the label says), and reapply if working out in the sun.
My current favorite is Skinceuticals Physical Fusion UV Defense SPF 50. Specifically great for oily and acne prone skin.