SPF: The 3 Letters We See Everywhere
What does SPF mean for our skin? We’ve seen it written virtually everywhere— on lotions, sunscreens, chap sticks, and even clothes. The textbook definition of SPF, which stands for Sun Protection Factor, is the degree to which a sunscreen protects the skin from the direct ultraviolet rays of the sun. So the next question is, how does one read and understand the rules of SPF?
First off, a huge myth has been circulating about SPF since the early days, and that is, it directly relates to the amount of time a person can be exposed to the sun. For example, consumers believe that if they apply sunscreen with an SPF  15, then they are allowed to stay out 15 times longer without getting sunburn.

This is definitely not true because sun exposure is not constant.  The intensity of the sun increases and decreases depending on the time of day, which makes it hard to rely on an SPF number to prevent skin damage. Obviously, various factors come into play that determines how much sun a person is exposed to such as skin type, amount of sunscreen applied, and reapplication frequency.   Therefor SPF does not reflect time you can spend in the sun, but rather, is an imperfect and relative measure of how much protection is provided by particular sunscreen lotions.
The second question that itches our brains is why do we need SPF?   We have been taught that the sun is friendly; that is provides us with warmth and is a good source of Vitamin D.   On the other hand, the sun is the source of ultraviolet  radiation, which can lead to permanent skin damage and cancer.

There are 3 different types of UV based on their wavelengths: UVC, UVB, and UVA. The 2nd and 3rd type,
UVB and UVA, have been proven to be very damaging to skin.  They are known to cause wrinkles, lowered immunity, aging skin conditions, and lastly, skin cancer.
A main functionality of sunscreen or sun block is to reflect or absorb these UV rays and prevent them from entering our bodies, protecting us from sunburn and other skin damage.  According to the EPA, a sunscreen with an SPF factor of 15 will screen 93% of the UVB radiation from reaching the skin while an SPF of 30 only increases the screening effect to 97%.   As you can see in the attached graphic,  lowering the SPF below 12 brings a significant drop in protection.  On the flip side, moving beyond an SPF of 20 brings almost no gain in skin protection.  You can read more about sunscreen and SPF in the  epa sunscreen guide .

How does it work?

The physical compounds titanium dioxide and zinc oxide reflect, scatter, and absorb both UVA and UVB rays. These ingredients, produced through chemical processes, do not typically cause allergic reactions. Using new technology, the particle sizes of zinc oxide and titanium dioxide have been reduced, making them more transparent without losing their ability to screen UV.  Some broad spectrum sunscreens   contain a number of chemical ingredients that also absorb UVA  radiation such as   UVA-absorbing avobenzone or a benzophenone (such as dioxybenzone, oxybenzone, or sulisobenzone)

In rare cases, these sunscreens with PABA or other benzophenones may cause  skin reactions, including acne, burning, blisters, dryness, itching, rash, redness, stinging, swelling, and tightening of the skin. Consult a physician if these symptoms occur.  Some sunscreens also contain alcohol, fragrances, or preservatives, and should be avoided if you have skin allergies.  You can read more about sunscreens in our sunscreen features here.