Below, I explain the theory and background behind the numbers shown on my individual sunscreen reviews, as well as explain the testing procedure that I use for each sunscreen.

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Why and how do I test sunscreen?

When searching the internet for sunscreen effectiveness, I quickly realized that trying to find any objective standardized results were impossible to find. Opinions upon opinions from everything from the feel, to how effective a particular sunscreen was was legion, but honestly…were not very useful. After some thought, I decided upon using the global solar UV index as a framework for testing one sunscreen against another.

Why did you choose to use the UV index as the key differentiator?

Sunscreen itself is a strange thing to test as there are no real formulation standards as to what make up a sunscreen. There are active ingredients for sure, but even those vary depending on chemical or physical ultraviolet blockers selected. To compound matters even further, many sunscreens add moisturizers, alcohols, fragrances, stabilizers, and other conditioners to its formula. This complicates matters as I can’t test sunscreen against another based on standard formulations alone.

Instead, I chose to test how well a particular sunscreen blocks UVA and UVB with the end result being the UV Index reading after the ultraviolet radiation has passed through the sunscreen. Using the UV index has the benefit of being easily understood, gives a general idea of how much harmful UV light is blocked, and it combines both UVA and UVB protection in one value. It also has the added benefit of allowing a person to quickly determine if a sunscreen could provide protection against a daily UVI value found on an app such as apple weather (more on this later).

Why reinvent the wheel and not test SPF?

Did you know that a sunscreen SPF rating only applies to the blocking of the UVB spectrum? I didn’t know that either until I began this sunscreen testing journey. After finding this out, a sunscreens SPF rating means even less to me. Since the UVB radiation that reaches earth only accounts for ~5% of the total UV radiation that reaches earths surface (with UVA accounting for the remaining portion), evaluating just SPF does not provide full protection results.

What rating system is used for UVA?

Since were on the subject of per spectrum UV blocking ratings, I found there is actually no real standardized rating system which grades a sunscreens effectiveness against UVA radiation. There is however, something called the ‘Photoprotection grades against UVA’ which was developed for cosmetics in Japan. However, its messaging is pretty terrible, and its not included on most sunscreen labeling. Most sunscreen labeling mentions the phrase “broad spectrum” if it protects against UVA, but does not include the extent to which it actually protects against it. If your sunscreen does protect against UVA and lists something such as PA+, PA++, or PA+++, it refers to the Japanese grading system for UVA photoprotection. This rating system leaves much to be desired, but it is better than nothing.

BTW, the PA rating system is based on the Persistent pigment darkening system or PPD (which isn’t even that great of a rating system to begin with). The PPD system does not test Skin Type I (the skin type most sensitive to solar damage), and its clinical significance is not clear (source).

So why not just test SPF for UVB, and PA grades for UVA?

My answer is why?

SPF is confusing and PA grades are too general. There is also something far better in existence already. The Global Solar UV Index is easily understood and has a better to understand linearity which corresponds to both UVA and UVB intensity.

The global solar UV Index describes the amount of solar ultraviolet radiation that has reached the earths surface. It is easy to understand as the higher the number, the greater the potential for skin and eye damage from the suns harmful UV radiation. This is important as the cumulative exposure to UV radiation is related to damage (such as skin cancer) from the harmful effects of the sun.

Don’t get me wrong, I do still test UVA and UVB blocking of a sunscreen, but I show the results of the blocking ability as a percentage rather than SPF or PA ratings. Personally, I place more value on how well a sunscreen will drop the UV Index as it makes a lot more sense. When shown as a percentage of how much UVI is blocked when dealing with the sun, it is much more intuitive.

Still Confused?

Lets make it even simpler…just say your apple weather app shows a UV Index of 10 for the day, and my test results of a given sunscreen show that it drops the UVI by 97%. Its just a quick estimation to then understand that the sunscreen is good to drop your ultraviolet exposure to less than 1.

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My sunscreen testing procedure – hands on experimenting

The procedure I use to test sunscreen is as follows:

1. I take a baseline reading of just using a microscope slide and slide cover (no sunscreen applied).
2. I apply sunscreen to the microscope slide and apply a coverslip using a magnetic vice for 5 minutes
3. I place the slide into the machine, and run a test for the duration of 2 hours.
4. The machine takes a reading at the 15 and 30 minute interval, then again at the 1 hour and 2 hour interval. After 2 hours, the test ends.
5. I remove the sunscreen slide and again take a baseline reading with just the microscope slide and slide cover (no sunscreen). The before and after calibration readings are used to calculate an average value which I can then compute the UV blocking percentage of the particular sunscreen being tested.

How do you apply the sunscreen to the microscope glass?

I use a Magnetic Vice. A what!?

The hardest aspect about building testing equipment is being able to consistently reproduce environments from one test to the next. Being that sunscreen formulations are inconsistent (viscosity, thickness, active ingredient content, etc.), I needed a way to apply sunscreen to the microscope slide in the most consistent manner possible to eliminate any errant results.

Enter the magnetic vice.

Creating consistent layers of sunscreen

By using the force of magnets and a properly designed retaining platform (to reduce movement) I could apply the same even pressure on the sample to create a thin layer of sunscreen time and time again. Even when checking FDA guidelines for how one should test SPF using solar simulators, they just advise smearing the sunscreen onto the surface by hand. Its not really accurate or consistent. I believe my method is far superior.

Dispensing consistent amounts

I also chose to use volume rather than the weight as this is how most people apply sunscreen (squirt a dollop the size of a quarter etc). To ensure the same quantity of sunscreen was applied for each test, I used a 3d printed measurement jig which allows sunscreen to be filled into an open cylindrical space of 3mm x 3mm. After the sunscreen fills the cylindrical cavity, the excess is wiped flush using a metal pick to square off the open-ended top and bottom. A 3D printed plunger of the same diameter as the cylinder is used to push the sunscreen out and onto the center of a microscope slide, much like a syringe.

Using time and pressure to create the sunscreen sample

After the sunscreen sample is applied to the glass microscope slide, a cover slip is placed over it and the top magnetic pressure cover is placed onto the assembly for a total of 5 minutes. These 5 minutes allow for constant pressure to disperse the sunscreen as even as possible between the cover slip and slide.

The Results

After the sunscreen has spent two hours in the testing machine, I am emailed results which give me values the sensor has taken at the 15m, 30m, 1 hour, and 2 hour intervals. I then take the average of the entire testing duration for the UVA, UVB, and UV Index readings. Using the calibration readings without sunscreen applied to the microscope slide from before and after the test, I compare the UVA, UVB, and UV index that has reached the VEML6075 UV sensor with sunscreen applied and compute the ultraviolet blocking capabilities in percentages.

The results look like this:

The values in blue show how much UVA, UVB, and UVI has been blocked in percentage. The values to the far right (the greyish-green color) show the values for UVA, UVB, and UVI during testing without sunscreen applied to the glass, as well as values after sunscreen has been applied. As you can see, the sunscreen at the bottom section has the capability to block up to 99% of both UVA and UVB radiation. The sunscreen in the section above however seems to only block 81% UVA and only blocks 76% UVB radiation. FYI: Both sunscreens were labeled as “broad spectrum.”