Sunscreens 101. The basics. |

Sunscreens 101. The basics.

Photoaging is the long-term consequence of sun exposure. While not threatening to life, it does reduce quality of life. Repeated unprotected exposure to UV light leads to premature wrinkling, sagging, a leathery texture and hyperpigmentation (“aging spots”). UV light produces DNA damage that may also lead to mutations in genes involved in the development of skin cancer. Therefore, along with other sun safety strategies, sunscreens that absorb or block UV rays serve an important protective function.

BFT Reminder: 

No sunscreen provides 100% protection from all harmful UV rays. Practice safe sun exposure habits: wear hats, long sleeve and long leg garments, and avoid unprotected sun exposure whenever possible.  Apply sunscreen liberally on exposed skin and remember that sand, snow, and water intensify exposure because of reflected as well as direct radiation. Clouds are only partial protection and significant UV penetration can occur. Sun exposure is cumulative – your skin remembers forever.


UVA and UVB Radiation – What’s the difference?


It is estimated that 80% or more of skin aging is due to photoaging caused by UVA and UVB radiation from the sun. No other cause of skin aging comes close.

UVB light is able to penetrate the outermost layer (epidermis) of the skin and is considered a direct DNA mutagen that damages elastin and collagen, resulting in superficial wrinkling of the skin.  While the epidermis does not contain blood vessels or nerve ending, it is the layer where melanocytes and basal cells are located. When exposed to UVB rays, melanocytes produce melanin, the pigment in abundance in dark skinned people, and the reason lighter skin individuals develop suntans. Freckles and dark spots, two additional signs of photoaging, are also caused by accumulations of melanin. Over time, UVB rays can induce cancer or precancerous lesions.

UVA light penetrates deeper into the skin, making UVA damaging to the both the epidermis and dermis.  As the thicker, denser layer comprised of collagen, elastin, and extrafibrillar matrix – the structural support system of the skin – damage to the dermis results in more pronounced wrinkles. Because of the presence of blood vessels in the dermis, UVA exposure can cause dilated or broken blood vessels, most commonly visible on the nose and cheeks. UVA also damages DNA indirectly through the generation of reactive oxygen species (ROS) which includes superoxide anion, peroxide and singlet oxygen. These ROS damage cellular DNA as well as lipids and proteins.

Acute overexposure to the sun results in the inflammation and vasodilatation we recognize as sunburn, caused by a panoply of cellular signals and messengers called cytokines including   NF-κB, TNF-α, interleukins 1 and 6, VEGF, and others. But chronic exposure is also damaging, whether or not there are episodes of redness and pain. The prudent precaution is to always use sun protection when outdoors, whether working, relaxing, or engaging in any other activity.

Myth Busting – “It’s good to get a little color”

A tan is healthy.

Not true. A suntan is not a sign of health – it shows that your skin has been damaged. Sun damage is not reversible and usually happens before you can see or feel it.

  • A tan protects you from sun damage.

Not true. Overexposure to the sun increases your risk of skin cancer and causes premature wrinkling

  • People with dark skin are already protected

Not true. Everyone, regardless of skin type, needs protection from the sun.

  • Staying in the shade prevents burning

Not true. Even if it’s cloudy, surfaces such as water, sand, concrete, and snow reflect the sun’s rays on to your skin. Clouds filter about 80 percent of the ultraviolet rays, but UV can penetrate thin clouds, fog and haze.

  • You will not sunburn while swimming.

Not true. The sun’s rays penetrate under water. Radiation penetrates deeper into the skin when it is wet. Wear a t-shirt and hat while in the water. Use a water resistant sunscreen and re-apply it frequently and liberally.

  • Glass protects you from ultraviolet rays.

Not true. UVA rays can penetrate glass and produce some tanning, both immediate and delayed. This can cause premature aging and skin cancer.

Sunscreen or Sunblock? What’s the Difference?

All sun creams and lotions contain one or both of two types of ingredients to combat UV radiation – physical barriers or chemical absorbants. Barrier types (sunblock) contain microscopic particles that block, diffuse, reflect, and scatter UV light. These products are usually thick, white, and may leave a visible residue on the skin until washed or rinsed off. They are usually well tolerated and recommended for people with sensitive skin.

Chemical absorbents (sunscreens) are molecules that absorb UV light, thereby preventing it from penetrating into the skin. Some may also serve to scatter light as well. Unlike sunblock, sunscreen is often clear and can be applied as a liquid, lotion, or spray. More people are sensitive to this type of sun protection. More and more sun protection products are including a combination of both types of UV protectant.

For years, there have been effective UVB sunscreens but not so for harmful UVA rays. Ingredients such as octylcrylene and benzophenones improved UVA defenses considerably. In 1998, the FDA approved the chemical Avobenzone (Parsol 1789) which is effective against all UVA rays.

The terms sunblock and sunscreen are now nearly interchangeable. Many products contain a combination of chemical and physical protectants and are classified as “Broad-Spectrum” because of their effectiveness against both UVA and UVB rays.

The best sunblocks combine both chemical and physical active ingredients. Dermatologists routinely recommend sunblocks that contain either a physical blocking agent or avobenzone (Parsol®1789) in combination with other chemicals.  However, in the USA, combinations of avobenzone and physical sunscreens are not permitted. Avobenzone has been reported to be unstable when contained in formulations with physical sunscreens. Surface coating of pigment has sometimes been shown to increase its stability.

Sunscreens designed for infants and children or those with sensitive skin are often based on titanium dioxide and/or zinc oxide. These minerals are less likely to cause skin irritation than the chemical absorbers.

What is SPF? 

The SPF rating system was developed in 1962 to measure the capacity of a sunscreen to block UVB radiation. The current SPF rating system continues to apply to UVB rays only, since they are the rays that cause sunburn. There is no approved Sun Protection Factor rating system for UVA rays.

The FDA and other scientists are working on this since UVA is such a huge factor in skin damage. Although UVA rays do not cause sunburn, they penetrate deeper than UVB radiation and cause bigger problems – skin cancer and wrinkles within the dermis.

How does the SPF system work?

The Sun-protection factor system measures the length of time a sunscreen will protect your skin from reddening / burning from UVB rays, compared to how long your skin would take to redden/ burn without sunscreen protection.

If it takes about 20 minutes without sunscreen for your skin to become reddish and start to burn, theoretically by using a sunscreen with an SPF-15 it should prevent the reddening/ burning of the skin 15 times longer, or about 5 hours.

A valid question is do the higher numbers truly afford that much more protection? That depends.

A major consideration is whether or not enough was put on in the first place. Most people use sun protection too sparingly so the SPF number may not be valid. Slather it on thick to make sure the protection is effective. The second consideration is how often sun protection is reapplied, especially after getting wet.

Remember – the SPF system pertains to exposure to the sunburn causing UVB rays. A higher number does not necessarily imply adequate protection against UVA radiation.

As the numbers below demonstrate, there is very little difference in added protection, as the sun protection factor goes higher. That is not to say, however, that it is not worth it. Even small amounts of repeated exposure can add up to a great deal of sun damage.

SPF-15-30-50 Guide:

SPF 15 means 1/15 of the UVB rays get through to your skin – blocking about 93%.

SPF 30 means 1/30 of the UVB rays get through to your skin – blocking about 97%.

SPF 50 means 1/50 of the UVB rays get through to your skin – blocking about 98%.

FDA Oversight of Sunblocks and Suncreens

  • New FDA labeling regulations go into effect this year.
  • Products previously labeled “broad spectrum” may or may not protect against UVA. The new rule reserves the “broad spectrum” claim only for products that protect against UVA and UVB.
  • The old “SPF” designation will show how well a product protects against UVB rays. Products with the new “broad spectrum” label will have to pass a test showing that they protect against UVA.
  • Products can claim to protect against sun-related premature skin aging if it has the broad-spectrum designation.
  • Sunscreen labels now will be able to claim that a product protects against skin cancer if it has an SPF rating of 15 or higher. Products with SPF rating under 15 must state they do not protect against skin cancer.
  • The highest permitted rating will be “50+” as there is no convincing data that SPF levels higher than 50 are meaningful.
  • Claims a product is “water resistant” are permitted; claims of “water proof” are not.
  • Water resistant products must specify whether protection is for 40 or 80 minutes after swimming or heavy perspiration.
  • Product not water resistant must carry a warning to that effect
  • No product may claim protection for more than two hours unless proof is submitted.


Approved Ingredients

A total of 17 active ingredients are approved by the FDA for use in sunscreens and sunblocks in the United States. Among them are:

Avabenzone ( Parsol 1789) – many newer products contain Parsol 1789 and are highly effective and appear less irritating.

Benzophenones (Oxybenzone, Dioxybenzone) have been used in sunscreens for 50 years and are known to cause irritations.

Cinnamates (cinoxate, ethylhexyl p-methoxycinnamate, octocrylene, octyl methoxycinnamate) are a frequently used UVB absorber. They are often found in liquid foundations that have an SPF factor.

Ecamsule (Mexoryl SX) an organic compound that is a broad spectrum UV absorbant. It is exclusive to L’Oréal and its brands.

PABA, (para-aminobenzoic acid) was used primarily in the early 1970s and was the first widely available true sunscreen. PABA is a frequent cause of allergic reactions and not commonly used. Related chemicals are used but promotion of PABA-free sunscreens is common.

Salicylates (ethylhexyl salicylate, homosalate, octyl salicylate) are weak UVB absorbers and they are generally used in combination with other UV filters.

Titanium Dioxide is found in almost every sunscreen. It is a blocker of UV light and has strong absorbing capabilities.

Zinc Oxide absorbs both UVA and UVB rays and can be used in ointments, creams, and lotions to protect against sunburn and other skin damage. It is the broadest spectrum UVA and UVB absorber that is approved for use in sunscreen by the FDA.


Questions & Controversies

Sunscreens are not given a clean bill of health by everyone. Certain critics are concerned that the very same phenomenon of UV absorption used to provide sun protection, ruptures double bonds within the molecule of active ingredients and results in free radical generation. Free  radicals, in and of themselves, are recognized as agents of DNA and other cellular constituent damage, and are incriminated in both aging and carcinogenesis.

Another concern relates to purported estrogenic effects of some sunscreen chemicals. Because the compounds are quite soluble in lipids, significant amount may be absorbed through the skin and into the body, allegedly affecting sexual function and development – in adults, in children, and in the developing fetus.  This controversy is ongoing and unlikely to be resolved any time soon.

There is no free lunch.  Risk / benefit analysis must come into play in making the decision as to whether or not to use sunscreens that contain chemical UV absorbents. If in doubt, the option to opt for particulate barrier type protection is always there.

This is a topic of discussion for another posting.


  1. lyonella says:

    And then there is Tinosorb, a hybrid chemical/physical ingredient:

    “Tinosorb S Is Photostable

    Tinosorb S (also called Bemotrizinol) is an oil soluble ingredient that is capable of absorbing UVA and UVB rays, resulting in broad UV spectrum protection for the skin. One of the benefits of Tinosorb S is that it is a photostable compound. When combined with other sunscreen ingredients, it helps to avert molecule degradation caused by the photons present in the sun’s UV rays. This photostability enhances the sunscreen’s effectiveness in protecting the skin.

    Tinosorb M Provides Stabilization

    Tinosorb M (also called Bisoctrizole) is a water soluble ingredient that is added to sunscreens to provide additional UV absorption. Like Tinosorb S, this ingredient can absorb both UVA and UVB rays. It also stabilizes other sunscreen compounds that absorb UV rays. This organic ingredient uses a unique blend of particulates to offer 3 powerful actions. By absorbing, reflecting and scattering the sun’s UV rays, Tinosorb M provides high-level broad spectrum coverage of UVA and UVB rays. It is also naturally stable and not susceptible to significant photodegradation (degradation upon exposure to sunlight).

    Complementary UV Absorption And Reflection

    When used together, Tinosorb S and Tinosorb M provide coverage across a wide spectrum of UV radiation. Sunscreens that include both active ingredients can protect the skin by acting as a filtering agent for UVA and UVB rays. The ability of Tinosorb S to absorb UV rays while preventing the photodegradation of other ingredients complements the triple-action qualities of Tinosorb M (absorbing, scattering and reflecting UVA and UVB rays).

    Effective Tinosorb Sunscreens

    The high efficiency and complementary qualities of Tinosorb S and Tinosorb M provide concentrated protection from broadband UV radiation. For those who have ultra-sensitive skin that is sun-intolerant, the following Tinosorb sunscreens should be considered:

    Bioderma Photoderm Max SPF 50+ Lait for Face and Body – contains both Tinosorb S and Tinosorb M. Can be used on pale skin and provides maximum protection from the sun’s intense UV rays. It’s especially useful in extreme conditions such as the mountains or tropics.

    Bioderma Photoderm Max Fluide SPF 50+ for Face – a lighter textured formulation than the lait which contains both

    Tinosorb S and Tinosorb M. It’s formulated to be used on skin that is particularly sensitive to sun exposure.
    Avene Creme SPF 50+ – with Tinosorb M and Tinosorb S to deliver high UVA and UVB protection for sensitive dry to very dry skin types.”

    Tinosorb is only avail in Europe for now tho.

  2. Drgeorge says:

    Tinosorb S and Tinosorb M have been in the regulatory pipeline for a while.

    The regulatory hurdles to get new sunscreens approved in the U.S. have been relaxed from prior years when it was handled in the manner of a new drug application – an extremely expensive and time consuming process. It is still more rigorous than elsewhere. In Europe, the list of approved useful UV-A and UV-B sunscreens numbers 28. The list in Australia has 26 sunscreens and Canada has 21. The U.S. has 17.

    While the approval process for a new drug is expensive and long, annual sales can be in the hundreds of millions of dollars, making the process cost-effective for a new cancer drug or antibiotic. A blockbuster sunscreen ingredient would only have sales of about $10 million. The total U.S. market for sunscreen lotions and potions at the producer level came in at about $640 million a year (2005) whereas the annual U.S. pharmaceutical industry sales exceed $150 billion.

  3. weezy says:

    Thank You Lyonella!
    Tinosorb S and Tinosorb M are the only sunscreen ingredients that I have encountered that keep my very fair, red headed, and freckled face child from burning. Unfortunately I can’t get it in this country. I discovered it in Montreal and stocked up while there, but sunscreens have a shelf life and there’s only so much stocking up one can do.
    I find it interesting that countries that have a universal healthcare plan for their citizens have deemed it important to make the most up to date and effective sunscreen/sunblock ingredients available for them.

  4. Jenny says:

    Are Tinosorb S and Tinosorb M absorbed through the skin into the bloodstream?

    What are your thoughts on nano particles in sunscreen Dr G and Dr J?

  5. Drgeorge says:

    There is very little if any sunscreen that makes it through the stratum corneum to the dermal / epidermal junction which is where vascular access can first theoretically occur. Electron microsopic studies show that not more than 0.03% of of zinc oxide nanoparticles penetrated the stratum corneum and none was detected in the viable epidermis, the deepest layer just above the dermis. Another study showed no titaniium dioxode and negligible Tinosorb penetrated into viiable tissue. Practically all remains topical and was removed by application of adhesive tape (one of the methods used in the study) of so no need to worry. Slather away…

    Ref: Skin Penetration of Sunscreen Nanoparticles: In-vitro Assessment of a Novel Micronized Zinc Oxide Formulation. Skin Pharmacology and Physiology 2007, Vol. 20, No. 3

    In vitro Percutaneous Absorption and in vivo Stratum Corneum Distribution of an Organic and a Mineral SunscreenlSkin Pharmacol Physiol 2007;20:10-20

  6. Janis says:

    Hi Docs,

    Will you explain how the following two sunscreens can be SPF 50. One has 19% zinc oxide and the other only has 5%. Does the Z-Cote in the Skinceuticals make a difference?

    Skinceuticals Physical Fusion SPF50
    6% Titanium Dioxide, 5% Zinc Oxide (Z-Cote). Water, Dimethicone, Isododecane, C12-15 Alkyl Benzoate, Undecane, Triethylhexanoin, Isohexadecane, Nylon-12, Capylyl Methicone, Butyloctyl Salicylate, Phencythl Benzoate, Styrene/acrylates Copolymer, Silica, Tridecane, Dicapylyl, Carbonate, Dicapylyl Ether, Talc, Dimethicone/PEG-10-15 Crosspolymer, Aluminum Stearate, Pentylene Glycol, Peg-9 Polydeimthcylsiloxyethnyld Iethicone, Alumina, Polyhyroxystearaic Acid, Phenoxyethanol, Magnesium Sulfate, Capylyl Glycol, Iron Oxides, PEG-8 Laurate, Disteradimonium Hectorite, Triethoxycapylylsilane, Tocopherol, Propylenen Carbonate, Artemia Extract, Benzoic Acid, C9-15 Flouroalcohol Phosphate, Peg-9, D46941/2.

    Chanel SPF50
    Active Ingredients: Zinc Oxide 19%, Octinoxate 7.5% Ingredients: Cyclopentasiloxane, Water, Alcohol Denat., Isononyl Isononanoate, Dipropylene Glycol, Glycerin, Neopentyl Glycol Dicaprate, Polymethylsilsequioxane, Polymethyl Methacrylate, Dimethicone, Triethoxycaprylylsilane, Peg-9 Polydimethylsiloxyethyl Dimethicone, Propylene Glycol, Tocopheryl Acetate, Butylene Glycol, Parfum (Fragrance), Hydrolyszed Wheat Protein, Mannitol, Dimethicone/Vinyl Dimethicone Crosspolymer, Methyparaben, Glycogen, Panax Ginseng Root Extract, Aesculus Hippocastanum (Horse Chestnut Seed Extract), Tilia Cordata Wood Extract, Trimethoxycaprylylsilane, Sodium Citrate, Sodium Hyaluronate, Chlorphenesin, Faex (Yeast Extract), Bht, Peg-40 Hydrogenated Castor Oil, Glycyrrhiza Glabra (Licorice) Root Extract, Phenoxyethanol, Calcium Panthothenate, Biotin, Butylparaben, Ehtylparaben, Propylparaben, Titanium Dioxide

  7. drgeorge says:

    Janis, interesting question. The answer comes down to what are the specific active ingredients in each product since the SPF factor results from their cumulative effect.

    Sunscreen products prevent penetration of UV light into the skin by either scattering or reflecting the light (mineral based products incorporating titanium dioxide and/or zinc oxide) or by absorbing the radiation into specific types of molecular bonds within a chemical, such as octinoxate (in the Chanel product), which the literature suggests may have endocrine disruptor traits: “This common sunscreen ingredient is shown to have hormone-mimicking effects on laboratory animals. This chemical is found in humans, including mothers’ milk samples.” Mineral based products have become more and more popular because they are inert and manufacturing techniques enable smaller particle sizes, which has greatly eliminated the white, pasty look associated with earlier products.

    TiO2 is more effective in UVB (290-320 nm) and ZnO in the UVA (320-400 nm) spectra, the combination of these particles provides broad-band UV protection, which may explain the SPF50 rating for the Skinceuticals product which contains both minerals, although the ZnO is at a lower concentration than the Chanel product.

    As an aside, as smaller particle sizes in the “nano” range have been used in sunscreen products, concern has been raised about possible absorption below the skin surface, and potentially into deeper tissues. A 2009 study published in Skin Pharmacology and Physiology determined that the stratum corneum is an effective barrier that prevents percutaneous absorption. Another review article published in 2010 concluded that ”Based on the current weight of evidence of all available data, the risk for humans from the use of nano-structured titanium dioxide (TiO(2)) or zinc oxide (ZnO) currently used in cosmetic preparations or sunscreens is considered negligible.”

    Hope this helps.

  8. Sophia says:

    Very interesting! Could I ask you, what SPF is enough in everyday life to prevent photoaging and rosacea from worsening? I currently use SPF 30 only and worry that this is not enough. Hard to find a physical 50 though.

    • drgeorge says:

      Thanks, Sophia. The issue of what SPF to use is not as simple as one might think. Theoretically, the MED (minimal erythemal dose) is the length of time one may stay in the sun before absorbing enough solar energy to produce redness. This, however, varies tremendously among skin types. A famous dermatologist, Dr. Thomas Fitzpatrick, developed a grading scale in 1975 that categorizes skin according to it propensity to burn vs. tan. Fitzpatrick 1 individuals are very light skinned, typically have Scandanavian or British Isles heritage, and tend to burn very rapidly with little sun exposure. Type 6 individuals have very dark skin and seldom if ever burn. Types 2 to 5 lie somewhere between these two extremes.

      Sun protection, for lighter skin types is imperative to prevent photoaging and potential malignant changes in the skin. So if your MED is 10 minutes without protection, then a SPF 30 would protect you for 300 minutes, and one with a SPF 50 for 500 minutes. There are other caveats, however. It is now recognized that no sun protection product lasts all day without applying more, regardless of SPF number. And, the amount applied is also very important.

      If you apply a thin coat of a SPF 50 product, and a thick coat of a SPF 30 product, the protection is likely to be fairly equal between the two. For these reasons, recommendations are to reapply liberally and often. Of course, that advice often falls on deaf ears with college kids in the Florida sun over Spring break. That may be a major reason most experts say most skin damage form solar radiation occurs during the first two decades of life.

      Also, be aware that there is now some focus on the infrared “A” part of the spectrum. These are shorter lenght infrared wave lengths that can penetrate to dermal layers of the skin.

      So to answer you question: Buy products with higher SPF numbers and use them liberally and often. Some references you may want to look at below.

      Photoprotection beyond Ultraviolet Radiationa – Effective Sun Protection Has to include protection against Infrared A Radiation-induced Skin Damage. Skin Parmacol Physiol 2010;23:15-17

      The influence of the amount of sunscreen applied and its sun protection factor (SPF): evaluation of two sunscreen including the same ingredients at different concentrations. Photodermatology, Photoimmunolgy & Phtomedicine: Aug 2009

      Photoprotection: a Review of the Current and Future Technologies. Dermatologic Therapy;Jan-Feb 2010

  9. Sophia says:

    Thank you very much for your reply, it is very helpul and I really appreciate your help!!

  10. John says:

    Thank you for sharing this! I have a quick question. You wrote, “No product may claim protection for more than two hours unless proof is submitted”. Does this mean that if I apply my sunscreen at say, 8 am, sunprotection is only effective up until 10 am? That would then mean, I have to reapply my sunscreen once again? What if I’m using a highly stable sunscreen that includes Tinosorb S, Tinsorb M, etc. Please and thank you. I really enjoy your blog and all its information!

    • drgeorge says:

      The regulations that dictate what can and cannot be claimed on sunscreen labels state that for non-water resistant products, sunscreen should be reapplied every two hours. For products that claim to be water resistant (either 40 minutes or 80 minutes may be claimed depending on what tests have been performed), product should be reapplied immediately after towel drying, and thereafter every two hours.

      The URL at the bottom takes you to the official FDA regulations regarding sunscreens.

  11. Ed says:

    Why do chemical sunscreen ingredients lose their efficacy over time? Is it because they absorb sunlight and turn it into heat and once they’ve done this job they are no longer active? Or do they become ineffective 2 hours after putting the sunscreen on the skin due to time regardless of sun exposure?

    I agree with you that there are benefits to both chemical and physical ingredients.

    So, I have an idea that I would like to run by you: using chemical sunscreen first and layering on physical sunblock on top throughout the day.

    I reason that multiple layers of physical sunblock throughout the day will reflect the rays and protect my skin from UVA and B penetrating into my skin, and the chemical sunscreen underneath will protect me from any possible rays that may penetrate the physical sunblock into my skin. Of course, this would all be unnecessary if chemical sunscreens lose their efficacy over time after two hours regardless of sun exposure.

    What do you think? Is this an effective way to protect my skin? Is it over-excessive (which I don’t think you can be to protect yourself)? Is it unsound or unnecessary?

    • drgeorge says:

      Interesting suggestion about applying physical sunblock over chemical ones to gain long-lasting protection. Actually, the reason that there are suggestions to apply sunscreen liberally and often has to do with the fact that it just doesn’t remain in place for long, and particularly if there is water exposure or significant perspiration. The chemical double bonds between carbons are what absorb the UV in chemical sunscreens,and they do not “tire” or “fatigue”.

      The recommendation to reapply every two hours is a guideline that even dermatologist have a hard time following. Published research below.

      J Am Acad Dermatol. 2001 Dec;45(6):882-5.
      When should sunscreen be reapplied?

      “A common recommendation by many public health agencies is to reapply sunscreen every 2 to 3 hours. Is this recommendation effective in minimizing ultraviolet exposure of the skin during time in the sun? Objective: The purpose of this article is to determine how the time of sunscreen reapplication affects the solar ultraviolet exposure of the skin. Methods: A mathematical model was derived that took into account typical amounts of sunscreen application and sunscreen substantivity to determine how these factors, when combined with the time of sunscreen reapplication, influence the photoprotection provided by sunscreen during exposure for several hours around mid day in strong sunshine. Results: Using a sunscreen that is readily removed from the skin achieves little in the way of sun protection, no matter when it is reapplied. For sunscreens that bind moderately or well to skin, typical of modern waterproof or water-resistant products, the lowest skin exposure results from early reapplication into the sun exposure period, and not at 2 to 3 hours, after initial application. Typically reapplication of sunscreen at 20 minutes results in 60% to 85% of the ultraviolet exposure that would be received if sunscreen were reapplied at 2 hours. Conclusions: Advice given to sunscreen users should be to apply sunscreen liberally to exposed sites 15 to 30 minutes before going out into the sun, followed by reapplication of sunscreen to exposed sites 15 to 30 minutes after sun exposure begins. Further reapplication is necessary after vigorous activity that could remove sunscreen, such as swimming, toweling, or excessive sweating and rubbing.”

      J Drugs Dermatol. 2018 Jan 1;17(1):116-117.
      Realistic Sunscreen Durability: A Randomized, Double-blinded, Controlled Clinical Study.

      Studies show that sunscreen under real-life conditions is often not reapplied and/or applied insufficiently. This study investigated the durability of 2 current sunscreens with different SPF protection over an 8-hour period under simulated real-life conditions.

      Participants (n=24) were randomized into two study groups utilizing either 2 mg/cm2 (FDA testing concentration) or 1 mg/cm2 (real-life application levels) of sunscreen. Two current SPF 15 and 70 sunscreens were applied to test spots on each participant’s back. SPF values were obtained at baseline, 3.5, and 8 hours after initial application, during which subjects completed 30 minutes of moderate exercise followed by 80 minutes of water exposure.

      Participants in both dose study groups revealed only a 15-40% overall decrease in their SPF protection 8 hours after application. The study group that received half the FDA test concentration of sunscreen achieved approximately half or less the labeled SPF. At 8 hours, the test sites that received SPF 70 maintained an average SPF greater than 64 (2 mg/cm2 application) and 26 (1 mg/cm2 application). Similarly, the SPF 15 product test sites revealed an in vivo protection of 13 (2 mg/cm2) and 7 (1 mg/cm2).

      This study demonstrates that current sunscreens may be durable on skin even following significant exercise and water exposure, suggesting that reapplication intervals may be longer than currently recommended. In addition, the higher SPF sunscreen maintained a skin cancer-protective level of SPF following extended use.

      J Drugs Dermatol. 2018;17(1):116-117.


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