SPF, it’s been in our vocabulary since we were kids and it’s likely that you even have your own particular number that you’re fond of, but what is it anyways and how does it determine how your sunscreen will protect your skin? To figure out SPF, you first need to understand the different UV Rays and how they affect you. 

UVA Rays 
About 95% of UV rays that hit the ground are UVA or Ultraviolet A. They can filter through both windows and clouds. These waves penetrate deeper into your skin and carry most of the responsibility for both your tan and premature aging, leaving behind fine lines and wrinkles, uneven skin tone, and dark spots. Their tanning effects appear instantly, but they can have long term effects as well, including indirect damage to DNA and some skin cancers.

UVB Rays 
UVB or Ultraviolet B Rays are more likely to be absorbed by the ozone and account for only 5% of the UV rays that reach the ground. UVB rays also don’t penetrate windows and are more likely to be filtered by clouds. Unlike UVA rays, They affect the epidermis, or the outermost layer of skin. Directly damaging DNA, they are primarily responsible for sunburn, most skin cancers, and can also contribute to premature aging. Unlike UVA, UVB’s effects are delayed, which is why you don’t usually turn red until after some time spent in the sun. 

When are UV rays strongest?
UV exposure is highest between the hours of 10am and 4pm and in spring and summer, however you’re still exposed during the fall and winter. Exposure is also highest in areas around the equator or at higher altitudes, where the rays have less distance to travel before hitting the ground.

What does SPF mean? 
SPF stands for sun protection factor, and it is a measurement of how your sunscreen will protect your skin from UVB rays. The number itself indicates how long it’ll take the sun to redden your skin with use compared to exposure without it. To get the SPF number, you divide the number of seconds it takes the skin to redden when wearing sunscreen by the number of seconds it takes without sunscreen. SPF 15, for example, would allow you stay in the sun 15x longer without getting red than without sunscreen. This is only a rough estimate and is affected by the amount used. The more liberal the usage, the better the protection. It’s important to note that no sunscreen offers 100% protection from UVB rays. SPF 15 blocks about 93% of UVB rays, SPF 30 blocks about 97%, SPF 50 blocks about 98% and SPF 100 blocks about 99%. If you want protection from both UVB and UVA rays, you’ll want to invest in a broad spectrum sunscreen. 

Things to know 
You should apply 30 minutes before heading outside to give the active ingredients time to absorb into your skin. When spending extended amounts of time in the sun, you should reapply every two hours and immediately after swimming or sweating. It’s important to make sure the product is not past its expiry date and to liberally apply (at least one ounce) to ensure effectiveness. It is recommended that everyone wears sunscreen, regardless of whether or not you burn in the sun.