Why do we need sunscreens?
As I discussed in my previous blog post on anti-aging, the ultraviolet radiation (UVR) from the sun is one of the most potent causes of premature skin aging. This phenomenon is called photoaging, and this commonly manifests as deep wrinkles, uneven skin texture and skin hyperpigmentation (dark spots). Younger patients wont often develop these symptoms despite high sun exposure, but the damage builds up over time, appearing as one grows older. Regular sunscreen use can protect the skin from UV damage and is a great tool to retard skin aging. Some studies suggest that it may even reverse some skin aging because it protects the skin from further damage and allows the skin to repair itself. This is why I consider sunscreens as very valuable anti-aging products. It is also important to know that UV radiation can also cause skin cancers, and sunscreens are very helpful in their prevention. In addition, there are some skin care products that have active ingredients that make the skin more sensitive to the sun (e.g. tretinoin), so you need to apply sunscreens to protect your skin further.
All about UV radiation: A is for Aging, B is for Burn
Ultraviolet radiation (UVR) from the sun has three types, each with specific wavelengths: UVA (320-400 nanometers [nm]), UVB (290-320nm) and UVC (100-280nm).
UVC has the shortest wavelength of the three and does not penetrate the earth’s atmosphere. It is mostly blocked by the ozone layer.
UVB has longer waves than UVC, but shorter waves than UVA. It accounts for 5% of the UVR reaching the earth’s surface. UVB is able to penetrate the epidermis (upper layers of the skin), and is the main UVR responsible for sunburn. It is strongly linked to malignant melanoma and basal cell carcinoma risk (types of skin cancer). UVB intensity varies by season, location, and time of day (usually peaks at 10AM – 3PM). Stronger exposures of UVB are documented in high altitudes and on reflective surfaces (these surfaces can bounce back ~80% of the UVB rays). UVB does not significantly penetrate glass.
UVA radiation has the longest wavelength of the UVR types, and accounts for up to 95% of UVR reaching the earth’s surface. UVA penetrates deeper into the skin, causing changes in the dermis (the layer below the epidermis). UVA is primarily responsible for photoaging. Unlike UVB, UVA is present with relatively equal intensity during all daylight hours throughout the year, and can penetrate clouds and glass.
What is SPF?
SPF, short for Sun Protection factor, is mainly a “Sunburn” protection factor. It describes how long it will take for UVR to redden/burn skin when using a sunscreen, compared to how long skin would take to redden/burn without it. As abovementioned, sunburns are primarily caused by UVB rays. As such, SPF mainly refers to protection from UVB. A sunscreen with a high SPF will help block UVB rays and prevent the skin from developing sunburn, and by extension damage that can cause skin cancer. The SPF does not measure protection from UVA.
SPF testing of sunscreens is performed on human volunteers in a laboratory. The sunscreen being tested is applied at 2 mg/cm2 (equivalent to around ¼ teaspoon for the face, or a shot glass full for the entire body). A lamp is shone on bare skin and skin covered with sunscreen, and the time taken for redness (erythema) to develop are incorporated into this formula to calculate SPF:
In simpler terms, someone using a sunscreen with an SPF of 50 will take 50 times longer to redden/burn than without sunscreen. It is important to know that sunscreens are often tested with higher amounts than is used in real life — in reality, only ¼ – ½ of recommended amount is applied. To achieve the SPF in sunscreens, it is essential to apply adequate amounts of product.
A higher SPF sunscreen will always give more protection as long as you apply an adequate amount. An SPF 15 sunscreen will let in twice as much UV as an SPF 30 sunscreen (if the same amount is applied). Studies have shown that SPF 15 sunscreen filters out ~93% of UVB, SPF 30 sunscreen filters out ~97% of UVB, while SPF 50 sunscreen filters out ~98% of UVB. Because measurement of UVB protection in sunscreens with SPF higher than 50 aren’t as reliable, some countries don’t allow higher SPFs to be labelled (these may be labeled SPF50+).
UVA protection
For UVA protection, look for the phrase “broad spectrum” sunscreen or the UVA circle logo. This means that the UVA protection is at least a third of the labelled SPF. Some sunscreens, depending on the country, have a specific numerical UVA protection factor (UVAPF), similar to SPF numbers. Examples arePPD (commonly used in Europe: works like SPF and is expressed in numbers), PA (commonly used in Asia: ranges from PA+ to PA++++ depending on PPD values), and the Boots Star rating (commonly used in the UK: ranges from one star to 5 stars depending on UVA protection as a percentage compared to SPF).
How do sunscreens work?
Sunscreen filters are the active ingredients in sunscreens. These filters coat the skin and form a protective barrier on surface to absorb or reflect UV radiation. Most sunscreens have two to six filters. These filters should be able to withstand degradation after absorption of UVR (they must be photostable). Some filters are photounstable but are stabilized by other filters.
What are the different types of sunscreen filters?
Traditionally, sunscreen filters were categorized as Chemical and Physical filters. The former was thought to act as “sponges”, absorbing UVR and releasing it as heat, while the latter was often described as “mirrors”, reflecting UVR off the skin. Recently, filters are more accurately categorized as Organic and Inorganic filters. Organic filters are carbon-based, meaning these filters are made of compounds that contain carbon (note: organic sunscreen does not refer to production via organic farming). On the other hand, inorganic filters do not contain carbon. Instead, they are made up of a metal bonded to a/n oxygen molecule/s. There are only two inorganic filters: titanium dioxide and zinc oxide. All other available sunscreen filters are organic.
Both organic and inorganic filters work by absorbing UV and converting into heat (the traditional definition of a Chemical filter), although inorganic filters also have the ability to scatter and reflect about 5-10% of UV. In that sense, inorganic filters are both “Physical and Chemical” in their mechanism of action.
There are advantages and disadvantages for both types. Organic filters often give higher protection, especially against UVA rays. These filters tend to stay on skin longer and resist being rubbed off. Some organic filters are photounstable (e.g. avobenzone and octinoxate), but are stabilized by other filters in combination. Select organic filters commonly cause allergic and irritant reactions (avobenzone, octocrylene, oxybenzone, avobenzone and PABA).
Inorganic sunscreens are often prescribed for sensitive skin (babies, allergy- or rosacea-prone individuals). However, these sunscreens are more likely to leave a “white cast” on the face. Their formulas tend to be greasier and thicker than organic sunscreens, and as such may have higher likelihood of clogging pores. Fortunately, inorganic filters can be ground up into smaller particles to avoid forming an opaque white layer (e.g. microparticles [100–2500nm] or nanoparticles [<100nm].
Sunscreens and skin types
Like any skin care product, the sunscreen selection must be based on your skin type. Here are some tips on selecting the right product based on your skin care needs:
Dry skin
- Will often tolerate sunscreen as these products often have thick textures
- Best suited for: Generally any sunscreen but better to apply emollients underneath, or use an SPF-rated moisturizer
Allergy- and rosacea-prone skin
- Sunscreen allergies are quite common, especially in children.
- Some occur after sun exposure (photoallergic reactions).
- Avoid products containing fragrances, preservatives and alcohol.
- Avoid octocrylene, oxybenzone, avobenzone and PABA (filters that cause the most reactions).
- Best suited for: Inorganic sunscreens or organic sunscreens without the abovementioned filters
Acne-prone skin
- Avoid greasy sunscreens (often marketed as “creams”), as these may exacerbate breakouts.
- Some acne medications increase sun sensitivity, so rigorous daily sun protection is important.
- Best suited for: Gel formulas (usually contain alcohol): more drying and less likely to aggravate acne
- If patient is acne- AND allergy-prone, a light lotion may be better than a gel formulation.
Aging and hyperpigmented skin
- Daily use of SPF 30 and up + frequent reapplication
- Best suited for: Broad spectrum sunscreens with high UVA protection
What should I look for in a sunscreen?
This is my suggested Sunscreen Checklist. To find the sunscreen most suitable for you, knowledge on sunscreen active ingredients as well as your skin type and accompanying skin issues are essential. It is helpful to consult a board-certified dermatologist to find the right sunscreen for you.
- Provides both UVA and UVB protection
- Photostable filter/combination of filters
- Wearable texture (no/minimal white cast, not too greasy)
- Suitable for your skin type
- Low potential of causing skin reactions
- Water resistant
How much sunscreen should I use?
This is a helpful guide in applying the right amount:
- ½ teaspoon on face and neck
- ½ teaspoon on each arm
- 1 teaspoon on each leg, the front and the back of torso
How and when should I apply and reapply sunscreen?
Sunscreens, regardless if they are made of organic, inorganic or a combination of both filters, should be applied 15 to 20 minutes before sun exposure. This allows the sunscreen to form an even layer on the surface on the skin and dry itself down to avoid removal. An adequate amount of sunscreen should be spread on all exposed areas as evenly as possible. If applying one thick layer is difficult, you can apply two or more thin layers over each other instead. Sunscreen is best applied before make-up and after other skin care products (e.g. toner, moisturizer, serum, etc). Please remember that UVA can penetrate through glass even though UVB cannot, so you still need sunscreen (with UVA coverage) if a lot of sunlight is getting through indoors. Reapply sunscreen every 2 hours if you are exposed to a lot of sun, and after getting wet from swimming/bathing.
Do “oral sunscreens” work?
Oral supplements marketed as oral sunscreens do not provide high protection against UVR. In fact, these products are estimated to only have an SPF of 5. If used, these should only be an adjunct to topical (applied on the skin) sunscreens.
With summer just around the corner, please remember to not only use sunscreen, but to also do other protective measures against UV radiation such as lip protection (use an SPF-rated lip balm), sun protective clothing (hats, sunglasses, clothing), and sun avoidance (particularly in times of peak sun exposure).
References:
- Hughes MCB et al., Sunscreen and prevention of skin aging: a randomized trial, Ann Intern Med 2013, 158, 781–790. DOI: 10.7326/0003-4819-158-11-201306040-00002
- B Petersen & HC Wulk, Application of sunscreen – theory and reality, Photodermatol Photoimmunol Photomed 2014, 30, 96-101.
- U Osterwalder & B Herzog, The long way towards the ideal sunscreen – where we stand and what still needs to be done, Photochem Photobiol Sci 2010, 9, 470-481.
- Cole C, Shyr T, Ou-Yang H, Metal oxide sunscreens protect skin by absorption, not by reflection or scattering, Photodermatol Photoimmunol Photomed 2016, 32, 5–10. DOI: 10.1111/phpp.12214
- Yap FH, Chua HC, Tait CP, Active sunscreen ingredients in Australia, Australas J Dermatol 2017, 58, e160–e170. DOI: 10.1111/ajd.12597
- The LabMuffin Guide to Basic Skin Care
- http://www.bad.org.uk/for-the-public/skin-cancer/sunscreen-fact-sheet#uva-and-uvb
- https://www.aad.org/media/stats/prevention-and-care/sunscreen-faqs
Skincerely yours,
Dr. Mara Evangelista-Huber
