Developing the perfect sunscreen

As a formulation chemist, sunscreens are one of the most challenging products to get right. With so many options on the market, how do you decide which product is going to be suitable for you? The texture, the finish, the protection - we all have different skin with different needs, and the product that works best for you is unlikely to work best for your partner, sister or best friend. 

I’ve spent a number of years developing sunscreens for a variety of brands. From formulating them in the lab for one of the leading sun care brands in the UK, to testing their UV protection in-house at large corporations, to working with innovative suppliers across Asia, Europe and the US to find and develop the best textures - and I can testify that it’s no mean feat! Sunscreens are more complex than most other skincare categories - they undergo more testing and have more requirements to comply with (not to mention the regulations vary significantly between regions, so trying to develop one for more than one market is extra complex.)

How is sunscreen formulated? 

The best sunscreens protect against both UVB and UVA rays, which is referred to as broad-spectrum protection. In order to do this, you need to include either a blend of different UV filters (common), or a single broad-spectrum filter (less common, only seen in mineral products.) Until the 80s, we only knew about UVB rays (which result in sunburn), but since understanding the impact of UVA rays on the skin, the regulations were changed to include this protection as well. 

On top of UVA and UVB rays, the best sunscreens today usually offer protection against part of the visible light spectrum as well, which can still impact melasma and hyperpigmentation. Formulas that also include antioxidants for additional protection get our gold star!

How do the different types of sunscreen protection work?

There are three types of filter systems that are used in sun care; 

• Mineral
• Synthetic
• A blend of Synthetic and Mineral 

There are just two mineral filters; zinc oxide and titanium dioxide, so mineral-only formulations usually use a combination of these, or just one. These are white mineral powders that are able to protect against the full spectrum of UVA and UVB. Mineral formulations are great for sensitive skin, but due to their nature, they can leave a white cast on the skin. 

Synthetic formulations (also referred to as chemical filters) are a bit more complex. It takes a bit of mastery to balance the level of filters to ensure coverage across the full UVA and UVB spectrum. On the most part, these types of filters are great for creating clear formulas that work well for all skin tones, but certain filters can be more problematic for sensitive skin and eyes. 

Best sunscreens for darker skin tones

For darker skin tones, ideally you would avoid formulations with high levels of physical powders. This is what can cause a whitening effect on the skin. This is most common with physical sun filters, zinc oxide and titanium dioxide, but there are also chemical filters that can have this effect. One common example is Methylene Bis-Benzotriazolyl Tetramethylbutylphenol. This does depend on the level used in the formulation, but if you want to be on the safe side it’s best avoided. 

Mattifying powders such as silica are common in sunscreens, and at high doses can also cause a whitening effect, although this is less common.  

Renude Recommendation: Thank you, Farmer Sun Project Light Essence 

Best sunscreen for sensitive skin

If you have sensitive skin, it can make finding the right sunscreen a bit harder. There are certain filters which, depending on the dosage, can irritate the skin and eyes. This can commonly manifest as skin redness, itching or watery eyes. 

If this applies to you, there are two options that can help to navigate the choices. The first is to opt for a mineral-only formulation. Mineral filters tend to be more gentle on the skin and eyes, so are a great choice for more sensitive skin. 

The second option is to look for a formulation which has been tested as hypoallergenic. The term “hypoallergenic” is used to indicate that a product has been tested on a group of people with sensitive skin and the number of reactions has been below a specific threshold. There is no standardised test protocol for this, but typically products are tested on a panel containing people with self-perceived sensitive skin, as well as those with hypersensitive skin. Eucerin, Bioderma and SVR Laboratories are some examples of brands that do this type of tolerance testing. 

Renude Recommendation: SVR Sensifine AR Anti-Redness Cream SPF 50 

How texture is achieved in Suncare

Sunscreens are notoriously difficult to balance both high-level protection and a great texture. Whilst innovations have improved this greatly over the last decade, it’s still a hard nut to crack! 

The challenge is that most synthetic filters are oil-based or oil-soluble, meaning that oils need to be included in the formula to disperse the sun filters to give you an even coating on the skin. So, you’re starting with an oily texture off the bat, and your best tools to improve this are 1) adding silicones, which are very volatile so can make the product feel more lightweight, and 2) adding mattifying powders like silica which help to absorb excess oil, but if used in high concentrations can also leave a white cast on the skin.

Innovative brands like Evy Technology are approaching this in a different way, using proprietary mousse technology. They use a range of synthetic filters and film formers dispersed via an aerosol to ensure high protection and good coverage. As the formulation is a mousse, the formula doesn’t feel as heavy, although it does take a few minutes to dry down on the skin after application. 

Renude Recommendation: EVY Technology Daily Defence Mousse SPF50 

How sunscreens are tested and approved

In Europe, sunscreens must undergo a variety of tests before they can launch. They undergo all the usual tests for skincare, which are:

• Stability testing - the formula is packaged in small glass jars and left in ovens at a variety of temperatures (20/30/40) typically run for 3 months to test how the formulation fares. 
• Compatibility testing - this tests the final formulation in the final packaging to see whether there are any interactions between the materials. Again this typically runs for 3 months. 
• Micro testing - microbes are introduced into the formula to stress test the preservative system. If microbes can grow, the preservatives need adjusting. This usually runs for 1 month.
• Patch testing - not compulsory, but this is where the final formulation is applied to the skin and occluded for 24 H to understand whether it causes a reaction. 

Sunscreens also have to undergo the following additional tests:

• In-vitro testing - This is where a specific amount of product is applied to a set area of a test substrate and artificial UV light is applied. The amount of UV that is able to penetrate through the sunscreen is assessed to give a SPF prediction, but this test alone is not enough to claim a sun protection factor.
• In vivo testing for UVB protection - this is carried out on at least 10 human volunteers. A specific amount of sunscreen is applied to an area of the skin usually on the back. This is then irradiated with an artificial source of UV light. There is a control section of the skin which does not have any product on. Each section is exposed to the light until it starts to show erythema (sunburn). A calculation is then made to determine the SPF level of the formulation. If it takes 30X the dose of UV exposure to cause sunburn in the treated area vs. the untreated area, that would give the product an SPF30. There are ethical concerns about this method of testing and regulatory bodies are looking at replacing this with fully in vitro models over the next few years. 
• Critical wavelength test - This tests the ratio is UVB: UVA protection. The legal requirement in Europe is that the level of UVA protection must be at least ⅓ of the UVB protection (listed as the SPF factor). The Boots star rating system goes a step further by grading this level - with 5* being the highest level of UVA protection at over 90% of the UVB. 

It is a long process, and you can expect to fail several times before you get to a formula that passes all of these tests, taking years to finalise. Especially if you are looking to include active ingredients or complex fragrances into the formula which can be harder to stabilise altogether. 

How sunscreens are tested in the US

In the US, sunscreens are classified as an OTC drug by the FDA. In the UK, in terms of regulation, we have cosmetics (which sunscreens fall into), and drugs/prescriptions. In the US OTC drugs are an additional third class of product which sits in between cosmetics and drugs, and include sunscreens and certain acne products amongst other things. The testing protocol is even more extensive to launch a sunscreen into the US, with two years of testing being required before launch. 

Why sunscreens need to be reapplied every 2 hours 

Official guidance from the British Association of Dermatologists (amongst other governing bodies), is to reapply sun protection every 2 hours, or after swimming or excessive sweating. 

Part of the reason for the two-hour rule is that sun filters get used up when doing their job of protecting the skin from sunlight. Some of them get used up more quickly, including Avobenzone, which is one of the most common UVA filters used in synthetic sunscreens. This is thought to have been completely used up after 2 hours of application, so this is one driver for this guidance. 

We also know that product moves around the face over time - sweating, movement etc. can all result in our skin not being evenly protected. Reapplying every 2 hours gives us confidence that our skin will be adequately protected at all times.