Beyond the SPF Number: How to Check a Sunscreen’s True Stability

You diligently apply a sunscreen labeled ‘SPF 50’ and ‘Broad Spectrum’ before heading out for the day. You trust the label. But by the time you re-emerge into the midday sun after a few hours, is the UVA protection you paid for still fully active on your skin? The answer depends on a factor rarely mentioned on the front of the bottle: photostability.

One of the most common and effective UVA filters used globally is Avobenzone. It provides excellent protection against the UVA rays responsible for photoaging. However, Avobenzone has a well-documented weakness: when formulated on its own, it can degrade under UV exposure. This means a sunscreen can be correctly labeled and provide its stated protection upon application, yet offer genuinely less protection a couple of hours later, even before you’ve sweat it off. The key to its endurance lies with other ingredients in the formula, the photostabilisers.

What is Photostability (and Why It Matters)

Photostability is a measure of a chemical’s ability to remain intact after absorbing UV radiation. Think of a UV filter as a molecule that intercepts UV rays before they can damage your skin. When it absorbs that energy, it enters a temporary, high-energy ‘excited’ state.

• A photostable filter can efficiently release this absorbed energy and return to its original form, ready to absorb more UV rays.
• A photounstable filter, when excited by UV energy, may instead break down or change its chemical structure. When this happens, it can no longer function as a UV filter.

This degradation is a chemical process that happens within the sunscreen film on your skin as it’s exposed to sunlight. It’s a critical factor in a sunscreen’s real-world performance. While daily broad-spectrum SPF is a non-negotiable strategy for preventing the breakdown of collagen, the substance that keeps skin firm, its effectiveness throughout the day hinges on the stability of its filters.

The Unsung Heroes: Meet the Photostabilisers

Because the instability of certain filters like Avobenzone is well understood by formulators, they are rarely used alone. Instead, they are paired with other ingredients called photostabilisers. These ingredients act as helpers, protecting the less stable filters from degrading.

At a chemical level, these stabilisers work by ‘quenching’ the excited filter molecule. They essentially take on the excess energy from the UV-absorbing filter, allowing it to return to its stable ground state without breaking apart. This allows the primary filter to continue doing its job for longer.

Some of the most common and effective photostabilisers you might see in a sunscreen formula include:

• Octocrylene: This is a very common UV filter in its own right that is also widely used to stabilise Avobenzone.
• Bemotrizinol: A more modern, highly stable broad-spectrum filter. You’ll find it on an INCI list under its chemical name, Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine.
• Other modern filters, which are inherently photostable, also contribute to the overall stability of a formula.

The presence of one or more of these ingredients alongside Avobenzone is a strong positive signal about the thought that went into the formulation’s endurance.

How to Read a Sunscreen INCI List for Stability Clues

Brands are not required to publish their photostability data, so we have to look for clues in the ingredient list (INCI). While this method doesn’t provide certainty, it offers a more educated way to assess a product. Here is a simple, three-step process for your next sunscreen audit.

Step 1: Locate the UVA Filter Scan the INCI list for Butyl Methoxydibenzoylmethane. This is the official INCI name for Avobenzone. If you see it, you know the product is using this filter for its UVA protection.

Step 2: Hunt for a Stabilising Partner Now, scan the entire list for the stabilisers mentioned above. Look specifically for Octocrylene or Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine.

Step 3: Consider the Order Ingredients on an INCI list are listed in descending order by concentration, down to 1%. If you see a known stabiliser like Octocrylene listed before or very close to Avobenzone, it suggests it’s present in a significant enough quantity to perform its stabilising function effectively. If it appears far down the list, after the 1% line (often marked by preservatives or fragrance components), its role as a stabiliser may be less impactful.

Remember, this is an interpretation based on formulation principles, not a definitive lab test. The overall vehicle and combination of other filters also play a major role. However, it’s a far better indicator of a sunscreen’s likely endurance than the SPF number alone.

Conclusion

The SPF number on a sunscreen tells you about its UVB protection at the moment of application, but it doesn’t tell you the whole story. The true test of a broad-spectrum sunscreen is whether its UVA filters remain stable and effective throughout your time in the sun. Understanding photostability and learning to spot key stabilising ingredients like Octocrylene or Bemotrizinol on an INCI list empowers you to look beyond marketing claims and make a more informed choice about your sun protection.

Want to know whether your sunscreen’s filter blend holds up on paper? Paste the INCI list into the free Sakura SkinPro analyser and get an auditable breakdown — no account required to start.