A higher sun protection factor number means more coverage from the sun. However, SPF 30 does not offer double the protection of SPF 15. SPF 15 blocks 93 percent of ultraviolet-B rays, while SPF 30 blocks 97 percent. Higher SPFs like 40 or 50, therefore, may only block 1 percent to 2 percent more UVB radiation.
Ultraviolet-B rays (the ones that cause sunburns) vary in intensity by season and latitude. In the northern hemisphere, they’re strongest during summer months but can be blocked by sunscreen.
Ultraviolet-A rays do not vary with the seasons. They are deeper-penetrating rays that can cause premature wrinkles and skin cancer in the long term. Not all sunscreens block these rays.
Titanium dioxide and zinc oxide
These mineral sunscreen ingredients block both UVA and UVB rays by reflecting or scattering energy rather than absorbing it like chemical sunscreens.
These ultrafine titanium dioxide and zinc oxide particles provide superior sun protection by completely covering the skin. Some public-health watchdog groups fear that these super-small particles may penetrate the skin and lead to cell damage. Most studies on humans, though, have found particles actually clump together, making cellular penetration difficult.