There's a lot of information out there regarding SPF, which is why we wanted to break it down for you to best answer some common questions that everyone should be aware and continuously reminded of.
How does sunscreen work?
Sunscreen absorbs or reflects UV light and is divided into three wavelength bands, which are ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC) rays. Only UVA and UVB light actually reach the earth.
Physical Sunscreen
Minerals such as zinc oxide or titanium dioxide, act as a physical sunblock. They reflect UV rays, similar to how white paint reflects light. The white-colored noses on beach-goers in the 1980s and 1990s were due to these compounds; because manufacturers make these particles much smaller now, we don't see the visible white.
Chemical Sunscreen
Chemical sunscreens often contain organic chemicals, with names such as avobenzone or oxybenzone. Instead of physically deflecting UV light, these molecules absorb UV radiation through their chemical bonds. As the bonds absorb UV radiation, the components of the sunscreen slowly break down and release heat.
What does the SPF number stand for?
The SPF number stands for
Sun Protection Factor, and refers to how well the sunscreen protects against one type of UV radiation, called UVB (it may be helpful to think B for burning). UVB rays cause sunburn and several types of
skin cancer. UVA radiation, penetrates deeper into the skin and can cause premature wrinkling, age spots and can also heighten the risk for
some skin cancers. Sunscreen lotions labeled broad-spectrum block against both UVA and UVB, but currently there is no standard for listing UVA blocking power.
We recommend using sunscreen with an SPF between 30+ (SPF ratings higher than 50 have not been proven to be more effective than SPF 50). A sunscreen with an SPF of 30 protects against about 97 percent of UVB rays, and one with an SPF of 50 protects against 98 percent of rays, according to the Mayo Clinic.
No SPF can block 100 percent of UV rays.
Because some UV radiation still gets through the sunscreen and into your skin, the SPF number refers to roughly how long it will take for a person's skin to turn red. Sunscreen with an SPF of 15 will prevent your skin from getting red for approximately 15 times longer than usual (so if you start to burn in 10 minutes, sunscreen with SPF 15 will prevent burning for about 150 minutes, or 2.5 hours), according to the American Academy of Dermatology. We highly recommend always re-applying sunscreen every 2 hours no matter the SPF.
What is the correct way to apply sunscreen?
Here are the basic rules: The minimum SPF you should use is an SPF 30. If you're using a Chemical Sunscreen, be sure to wait at least 20 minutes to go out into the sun after applying. Physical Sunscreen works immediately. Again, you should
 apply generously every two hours.
Coat your entire body with enough product to fill a shot glass (1fl oz.) or if you're using a spray, continuously spray it for at least thirty seconds until you have an even sheen over your exposed skin.
If you don't have someone else to apply a cream sunscreen, many spray sunscreens are conveniently designed to reach the back or other tough to reach areas by spraying upside down.
Why not just cover up with clothing?
Clothing certainly helps to block out the sun's rays, but not the way sunscreen does. Also, not all clothing is created equal as the shirts and shorts that work best must have a tight weave.
In any case, because sunscreen is not an end-all solution, we strongly recommend also using a hat and sunglasses, clothing and shade to protect your skin.
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