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  • Susan Busch NP

The Science Behind Sunscreen

Updated: Jun 6, 2019

Are you baffled by SPF, Broad Spectrum, Ultraviolet UVB and UBA rays? If you answered YES then you are like most people. Keep reading to unlock the mystery and be prepared to select the best sunscreen that meets your needs.


What is SPF?

The simple answer is: SPF stands for Sun Protection Factor, which is the measure of protection from the sun’s UVB (burning) rays.


What do SPF numbers mean?

The numbers on your tube of sunscreen indicate how long the sun’s UV radiation would take to burn your skin when using that product exactly as directed. For instance, SPF 30 would take you 30 times longer to burn than if you weren’t wearing any sunscreen at all. Each number will also protect your skin from a certain percentage of the sun’s UVB (burning) rays

See here for the breakdown of SPF numbers:


SPF 30 – Blocks 97 percent of UVB rays

SPF 50 – Blocks 98 percent of UVB rays


The American Academy of Dermatology recommends using SPF 30 or above and reapply every 2 hours.


What is the difference between UVA and UVB rays?

UV stands for “ultraviolet,” and exposure to ultraviolet radiation is a major risk factor for most skin cancers. Sunlight is the main source of UV radiation, but tanning beds can also mimic this form of light and cause skin cancer and other kinds of skin damage.


There are two main types of UV rays: UVB rays are the are the ones that are typically associated sunburns and skin cancer. UVA rays are the ones responsible for aging, fine lines, dark spots and all those little things we don’t like about our skin (along with skin cancer).


We need protection against both UVB and UVA and that is why it is important to select Broad Spectrum when you are buying sunscreen It protects against both UVB and UVA.


How much sunscreen do I need to apply daily?

You’ll need one-half of a teaspoon (about a nickel-sized dollop) for your face and neck, and one ounce (a shot-glass-full) for your entire body. It is essential that you reapply every 2 hours.


What is the difference between chemical and physical sunscreen?

A chemical sunscreen protects you by absorbing the sun’s rays. It may contain one or more of many possible active ingredients, including oxybenzone or avobenzene.


A physical sunscreen protects you by deflecting the sun’s rays. Contains the active ingredients titanium dioxide and/or zinc oxide.


Some sunscreens use both types of active ingredients. Physical SPF usually works best for those with sensitive or acne-prone skin.


What is the difference between water resistance and very water resistance?

Water resistant means the sunscreen stays effective for 40 minutes in the water. Reapplication necessary after 40 minutes.


Very water resistant means the sunscreen stays effective for 80 minutes in the water. Reapplication necessary after 80 minutes.


What is most important while selecting a sunscreen?

· SPF 30 or greater

· Broad-spectrum protection

· Water resistance


Always remember to reapply every 2 hours.


Sue's Recommendations

-La Roche-Posay Anthelios Mineral Sunscreen-Gentle Lotion, SPF 50


-Neutrogena Sheer Zinc Dry-Touch Sunscreen Lotion, SPF 50



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