SPF is short for sun protection factor. In sunscreen, SPF helps to block your skin from the sun’s radiation.
The sun emits two types of radiation: UVA and UVB rays. UVA rays contribute to the signs of aging in the skin, like wrinkles and sagging. UVB rays are more carcinogenic and often responsible for sunburns. UVA rays also make UVB rays more reactive, so combined, the two can be deadly.
You’re exposed to harmful radiation from the sun virtually anytime you’re outside or near a window that has sunshine. That radiation has an effect on your skin even if you aren’t prone to sunburns.
SPF works by extending your skin’s natural defenses against the sun’s rays. For example, an SPF of 15 provides about 15 times more protection than just your normal skin without sunscreen. An SPF of 50, then, would provide 50 times more protection than skin without sunscreen. Choosing a broad-spectrum sunscreen means it’s a type of sunscreen that will block out both UVA and UVB rays.