How Exactly Does Sunscreen Work?

by | May 15, 2018


Here in St. Louis we’ve basically skipped over spring right into summer. Thus, it’s time to dig out the sunscreen. Ever wondered how sunscreen works? It’s pretty crazy if you think about it. Go without it and you might be sporting a killer sunburn. Slather on some  SPF45 and you are protected. How does sunscreen do that?

As you are aware, there is more to light than what we can see.  The visible light spectrum begins at red and ends at violet.  Infra-red light exists below red and ultra-violet light exists above violet.  As you move from infra-red through the visible light spectrum to ultra-violet the light photons have shorter and shorter wavelengths and higher and higher energy.

The high energy photons found in ultra-violet (UV) light is what damages your skin when we sunburn.  When these high energy UV photons strike your skin they generate free radicals and can also directly damage your DNA.  Over the short term, this UV-induced damage can produce a painful sunburn.  Over the long-term it causes premature aging of the skin, as well as millions of new cases of skin cancer each year.  Tanning is the skin’s natural response to UV exposure and is the result of increased production of skin pigment (i.e., melanin), which gives skin a golden brown color.  Melanin absorbs ultraviolet light and dissipates it as heat and thus acts as the body’s defense mechanism. (Melanin is also what makes those of African descent darker than Caucasians – their added melanin provides additional protection – up to an estimated SPF 13 or so – but black people can and do get sunburns too.)

The UV rays that we are exposed to on the earth’s surface consist of UVA and UVB photons.  The shorter UVB rays don’t penetrate deeply into the skin but they have higher energy and cause significant damage to DNA and are the primary cause of sunburn and skin cancer.  The longer wavelength UVA rays penetrate the deeper layers of skin where they produce free radicals which cause premature aging and immunologic problems.

Sunscreen acts like a very thin bulletproof vest, stopping UV photons before they can reach the skin and cause damage. Suncreens work by physically and chemically blocking UV rays.  Most sunscreens have a combination of physical and chemical blockers.

Physical blockers of UV rays are the simplest type of sunscreens, and are typically composed of inorganic oxides, such as zinc oxide (ZnO) and titanium dioxide (TiO2). These blockers rely on their physical properties to block UV light.  Particles of physical blockers have very high refractive indexes, which is a material’s ability to change the direction of light as it passes from one medium to another. Therefore, physical blockers do just as they describe: they physically block, or more specifically reflect and scatter UV light away from the skin.

Chemical blockers, such as the para-aminobenzoic acid (PABA) family, are typically organic molecules. They act in a more complicated manner than physical blockers. With a molecular structure that is specifically designed to absorb UV light, chemical blockers in effect “grab” the UV rays before they penetrate into the skin cells.  UV light that is absorbed by a chemical blocker is converted to a longer wavelength/lower energy form (IR light) which is harmlessly radiated away from the skin, thereby decreasing potential negative effects posed by UV light.

The term SPF stands for “sun protection factor.”  Products with a higher SPF allow fewer of the photons that can cause sunburn to strike the skin.  In simple terms, you can view a SPF 10 sunscreen as allowing 10 out of every 100 photons to reach the skin and SPF 20 as allowing only 5 out of every 100 photons to reach the skin.  Because sunburn is primarily a UVB effect, it is possible for a sunscreen to deliver high SPF while allowing a significant percentage of the UVA photons to reach the skin.  So, it is important to pick a sunscreen that blocks both types of UV rays effectively.

As to tanning beds – the UV output varies from bed to bed, but they generally contain less UVB and significantly more UVA rays than does natural sunlight.  This leads to less sunburning and more tanning.  In the long term, however, the UVA rays take their toll on skin . . .

A typical white t-shirt has an SPF of 3.  Colored shirts have a higher SPF, but unless you are wearing a shirt designed to block UV rays it is doubtful that you have protection greater than an SPF of 10.  Wet clothing is less protective than dry clothing.

Pay attention to the quality of sunscreen you buy.  A recent study found that about 40% of the top selling sunscreens on Amazon don’t meet the recommended guidelines of the American Academy of Dermatology. How to select a sunscreen:


Which Sunscreen to use?  A survey of 1,572 dermatologists conducted at the 2013 American Academy of Dermatology Annual Meeting revealed the following sunscreen recommendations as to brand:

  1. Neutrogena
  2. Aveeno
  3. La Roche-Posay
  4. Elta
  5. Vanicream
  6. Coppertone
  7. Blue Lizard
  8. Eucerin
  9. Solbar
  10. Fallene

How much to use?  Recent reports indicate that although the public may be using sunscreen, it is being incorrectly applied and is therefore not as effective as it should be. According to Dr. Darrell Rigel of the American Academy of Dermatology, “the average person requires one ounce of sunscreen, enough to fill a shot glass, to adequately cover the exposed areas of the body,” but the majority of people don’t use enough sunscreen to receive the level of protection that is indicated on the package. Many people also fail to apply sunscreen at least 15-30 minutes before going outdoors in order to allow it to be completely absorbed into the skin, and they neglect to re-apply it every two hours or after swimming.” He went on to say that people should wear sunscreen every day, all-year round. This is true even on cloudy days, since 80 percent of the sun’s rays can penetrate clouds. It is also a little known fact that daily sun exposure, like the kind received inadvertently while driving a car or taking a walk, accounts for the majority of a person’s lifetime exposure, rather than time spent at the beach or pool. The American Academy of Dermatology recommends that everyone follow these sun protection guidelines: Avoid outdoor activities between 10 a.m. and 4 p.m. when the sun’s rays are the strongest; Seek shade whenever possible; Wear sun-protective clothing and accessories, such as wide-brimmed hats and sunglasses; Follow the “Shadow Rule” – if your shadow is shorter than you are, the sun’s damaging rays are at their strongest and you are likely to sunburn; and Avoid tanning beds.

What about sunglasses?  Sunglasses standards for lenses place limits solely on UVB and UVA rays, but bear in mind that both the standards and labeling are voluntary, not mandatory. According to these standards, sunglasses must block at least 70% of UVB and at least 60% of UVA.  To best protect your eyes, look for sunglasses that provide at least 98% protection from both UVA and UVB rays. UVC rays are blocked automatically since they are absorbed in the atmosphere and do not reach the earth. Some of the higher-priced products with polycarbonate lenses can claim to block 100% of the UV rays.


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