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by Dr.Ying-Chin Wu

5-minute crash course to sun protection standards

UV rays are everywhere, and considering how UVB gives you sunburn and how UVA causes skin to age, it is essential to incorporate sun protection into your skincare routine, because black spots and wrinkles can be very difficult to reverse once developed, no matter how hard you try. The importance of choosing the right sunscreen cannot be overstated! However, there is more to choosing sunscreen than reading labels alone, because there are several different measurements of how well a product protects you from the sun: some indicate protection strength while others indicate spectrum or evenness of protection. So how should consumers interpret these ratings? In the following article, we have summarized the relevant facts on sun protection to give you a good understanding in just 5 minutes!  

UV ray - the invisible threat

Before choosing a sunscreen, let us learn more about ultraviolet (UV) radiation. UV ray exists in many different wavelengths, and there are three wavelengths that cause harm to the skin:

UVC: With a wavelength of 100-280nm, it is the most powerful but the least penetrating type. Virtually all UVC is blocked by the Earth's atmosphere and is unable to reach ground surface, except in the case of ozone depletion where UVC can penetrate and reach ground surface.

UVB: With a wavelength ranging from 280nm to 320nm, UVB makes up 5% of the radiation that reaches ground surface. It causes redness, burn, and darkening to the skin, and it is able to penetrate the epidermis but cannot reach the dermis.

UVA2 (shorter-wavelength UVA): With a wavelength ranging from 320nm to 340nm, UVA2 makes up 20% of the radiation that reaches ground surface. It penetrates glass and clothes deep into the epidermis, and causes tan, redness, age spots, and loss of translucency to the skin.

UVA1 (long-wavelength UVA): With a wavelength ranging from 340nm to 400nm, UVA1 makes up 75% of the radiation that reaches ground surface. It penetrates glass and clothes and directly damages the dermis, causing inflammation, loosening, wrinkles, and dark spots to the skin. This is also the type of UV ray that some sunscreens struggle with.

What are the advantages of physical and chemical sunscreens?

Sunscreens can be distinguished between physical and chemical characteristics depending on the ingredients and approaches used to achieve sun protection. Sunscreens with all-physical, all-chemical, as well as hybrid formulas are currently available on the market, each claiming to serve a different purpose, and the following is an explanation of the differences

Physical sunscreen:  works by forming a physical shield that reflects and scatters UV rays, predominantly using titanium dioxide and zinc oxide. This approach is safe, stable, and non-irritating to the skin, which makes it suitable for post-surgery, pregnant women, children, and people with sensitive skin. Its disadvantages are that the solutions tend to leave white residues on the face or weigh the skin down, but as technologies progress, sunscreen powders can be produced in finer grounds to reduce density and appear more transparent, while being able to reflect and scatter UV rays more effectively.

Chemical sunscreen: uses chemical agents to absorb and convert UV ray energy into heat, thereby reducing harm to the skin. Chemical sunscreens tend to have lighter and more refreshing texture; new-generation ingredients such as Tinosorb M, Tinosorb S, and DHHB are all stable and effective against UVA. The downside is that some chemical agents can become unstable once the compound is broken down by sunlight, causing irritation to the skin or producing the effect of environmental hormone. 

Hybrid sunscreen: since neither approach is perfect, most sunscreens in the market are made with a mixture of physical and chemical ingredients, offering the best of two worlds for better protection and more refreshing texture.

Now that we have established the basics of sun protection, let's take a look at how sunscreens are rated using three popular indicators: strength, spectrum, and evenness. Knowing what the numbers mean will help protect you from sun tan and aging! 

Summary of 3 popular sun protection indicators

Indicator #1: Sun protection strength

SPF (Sun Protection Factor)

SPF measures how well a sunscreen blocks UVB. The test method involves rubbing sunscreen on one side of the skin while leaving the other side unprotected, and measuring the time it takes for the skin to develop redness. If it takes one hour to for your skin to burn with protection on it versus 10 minutes on unprotected skin, the sunscreen will have a SPF rating of 6 (60/10=6), meaning that it delays the time to redness by 6 times. Similarly, SPF50 means that the product is capable of delaying time to redness by 50 times. Obviously, the actual time to redness may vary depending on individual skin conditions and the strength of UV ray at the time. It is recommended to choose products with SPF rating of no less than 20 for general use, and no less than 50 for more intensive exposures.    

Note: Applying SPF50+ (the highest SPF rating) once every two hours does not fully protect you from the sun. According to tests conducted by Australian Radiation Protection and Nuclear Safety Agency, SPF30 blocks 96.7% of UV ray while SPF50 blocks 98% of UV ray, meaning that there is always at least 2% of UV ray that reaches your skin.

PPD  (Persistent Pigment Darkening)

Sunburn recovers, but aging is difficult to reverse, which is why it is very important to choose sunscreens that protect against UVA. PPD is an European standard for UVA protection. It is measured in ways similar to SPF, by applying product to one side of the skin and not the other side, and measuring the duration of light exposure it takes to develop more than 24 hours of sustained darkening. Assuming that an average person takes 10 minutes to tan, a PPD10 sunscreen would delay the time it takes to tan to 10 times, or 100 minutes.

PA (Protection Grade of UV-A)
This standard is developed by Japan Cosmetic Industry Association. PA simplifies the presentation of PPD, using plus symbols (+) to indicate protection against tan and aging. The more plus symbols, the stronger the protection, and a product can have up to 4 plus symbols. It is recommended to choose products with PA+++ (PPD8-16) for general use, and PA++++ (PPD16 and above) for more intensive exposures.  

Indicator #2: Spectrum of protection

UV rays of longer wavelength, such as UVA1, cause more damage to the skin and are more difficult to protect against, therefore it is important to take note the broadest spectrum of protection offered by sunscreen, not only for short-wavelength UVA2.

Critical Wavelength

UV rays of longer wavelength are harder to block, which is why although most sunscreens are effective against UVB, the protection diminishes as wavelength increases. The wavelength beyond which a product loses a significant degree of protection is determined as the product's critical wavelength. Critical wavelength represents the spectrum of protection for a particular sunscreen; a higher value indicates protection against a broader spectrum of UV ray, and better protection against UVA1. It is desirable to choose a sunscreen with critical wavelength of 370nm and above, or ideally 380nm and above. Currently, U.S. regulations allow products with critical wavelength of 370nm and above to be labeled "Broad Spectrum." 


Indicator #3: Evenness of protection

A sunscreen is not considered to offer comprehensive protection if it works better at blocking UVB but less so against UVA. A good sunscreen should work equally well against both UV rays, and the two ratings below are used to measure the evenness of protection for a sunscreen.

Boots Star Rating

This is a UK standard that measures UVA protection based on the same concept as the EU standard, albeit in greater level of detail. The standard awards one star if a product offers UVA protection at 20%-40% strength of UVB, two stars for offering UVA protection at 40%-60% strength of UVB, three stars for 60%-80% strength, four stars for 80%-90% strength, and five stars for 90%-100% strength. Generally speaking, three stars would suffice for work or short duration of outdoor exposure such as shopping, whereas four stars are suitable for long duration of outdoor activities. 

High SPF and broad spectrum for total protection against UVA and UVB

Additional tips for choosing sunscreen: refreshing texture, non-stickiness, and no white residue

As explained above, each country has different standard on sunscreen protection, and aside from choosing products that offer broad spectrum protection, Boots Star Rating may also be a useful indicator. When choosing sunscreen, it makes sense to begin with your needs and desired texture. For everyday use and work, a product rated SPF30 would suffice. Meanwhile, you may also opt for sunscreens that are refreshing, non-sticky, no white cast, and do not work against your makeup. Skin tone correcting is a good feature to look for in sunscreens, too! Most importantly, be sure to re-apply sunscreen frequently, whichever product you choose. For long duration outdoor exposure or beach activities, re-apply every 1-2 hours for total protection.