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Sun filters & SPF: Everything You Need to Know to Formulate and Choose Better
- May 13
- 8 min read
The sun is a wonderful ally for health, morale and vitamin D synthesis.But it can also become a silent enemy if we fail to protect ourselves properly. UV rays, SPF, chemical or mineral filters, environmental controversies... Today's suncare products are the subject of much debate. Whether you're a consumer or a cosmetics formulation professional, this article sheds some light on sun filters and how they work, to help you understand them better, choose them better and formulate them better.
I. Why protecting your skin from the sun is vital
The sun's rays that reach the Earth's surface include ultraviolet (UV) rays, which fall into three categories: UVC, UVB and UVA. UVC, which is very energetic, is fortunately filtered by the ozone layer and does not reach the Earth's surface. UVB, on the other hand, penetrates the superficial layers of the epidermis. It is UVB that causes sunburn and, in the long term, can lead to mutations in cellular DNA. UVA rays penetrate deeper into the skin. Although painless, they are formidable: they play an active role in skin ageing, the appearance of pigmentation spots, the breakdown of collagen fibres... and are also implicated in certain skin cancers¹.
Things to remember: UVB rays cause sunburn, while UVA rays accelerate skin aging. Both types damage the skin and increase cancer risk.
Repeated, unprotected exposure can therefore have serious consequences. It encourages the appearance of early wrinkles, loss of firmness, pigmentary changes, but above all, it increases the risk of developing carcinoma or melanoma. Given these facts, sun protection cannot be seen as a simple cosmetic or seasonal gesture. It is a public health issue. The World Health Organisation actively recommends the use of suncare products, as well as wearing protective clothing and hats, and limiting sun exposure to the hottest hours¹.
In this context, it's essential to understand what the different indices on the bottles measure. The SPF (Sun Protection Factor) is an index that measures the level of protection against UVB rays. For example, a product with an SPF of 30 theoretically increases the time it takes for the skin to redden by 30 times compared to unprotected skin.
Did you know? Did you know? To achieve the stated efficacy, you must apply 2 mg of product per cm² of skin. In reality, most people apply 2 to 4 times less, which dramatically reduces effectiveness.
In addition, some creams also specify their effectiveness against UVA rays, using a specific logo (UVA in a circle) or, in Asian countries, a rating ranging from PA+ to PA++++.
II. Understanding the different families of sun filters
Sun filters are the active ingredients used to protect the skin against UV rays. There are two main categories: organic filters, often called ‘chemical filters’, and mineral filters, sometimes called ‘physical’. Each has its own specific features, advantages and limitations.
Organic filters are synthetic molecules capable of absorbing UV rays and converting them into heat. They are generally very effective and produce light, transparent textures that are pleasant to apply. Among the best known are octocrylene, avobenzone and homosalate. However, some of these filters raise safety issues. Octocrylene, for example, is now being discussed for its potential to transform into benzophenone, a compound suspected of disrupting the endocrine system⁴.
Mineral filters, meanwhile, act by reflecting or diffracting light. They are often recommended for sensitive skin, as they are generally better tolerated. Titanium dioxide and zinc oxide are the two main mineral filters used.
They are available in both nano and non-nano forms, they offer excellent light stability. However, they can leave a white film on the skin, which remains an obstacle for some consumers, particularly those with dark or matt skin.
Did you know? Non-nano titanium dioxide is very effective, but difficult to disperse. This is one of the reasons why mineral sunscreens are often “paler”.
Another trend is the use of so-called ‘natural’ ingredients such as shea butter, raspberry oil or coconut oil, touted for their protective properties. It is important to emphasise here that these products have a very low natural SPF, often less than 10, and can in no way replace genuine approved sun protection.
III. Deciphering regulatory information and the importance of packaging
The labelling of suncare products complies with precise standards. In Europe, displaying the SPF is compulsory, as are clear warnings about use. The UVA logo surrounded by a circle guarantees that the product offers protection against UVA equivalent to at least a third of the SPF, in accordance with European regulations.
Other terms can lead to confusion. For example, ‘water resistant’ means that the product retains some of its effectiveness after immersion, but does not mean that it is completely waterproof. This claim is subject to standardised criteria (notably ISO 24444).
Remember: ‘Water resistant’ does not mean ‘waterproof’. It means that the product retains some of its effectiveness after 2 20-minute baths.
Packaging also plays a crucial role in terms of education and correct use. A clear, informative bottle, which indicates, for example, the recommended quantity, precautions for use and the actual duration of protection, helps raise public awareness about photoprotection.
IV. Formulating a suncare product: a balancing act for laboratories
Developing a high-performance suncare product is a complex operation, as much a matter of science as of cosmetic art. It is a permanent compromise between efficacy, stability, sensoriality, safety and regulations.
Did you know? In vivo SPF tests are carried out on the skin of volunteers with a precise application of 2 mg/cm¹. The standard used is ISO 24444:2019.
The first step is to choose the right filters, combining them in a coherent way to provide broad-spectrum protection (UVB + UVA), while ensuring their stability over time and under exposure to the sun's rays. Some filters are photostable, while others require stabilisers. The interactions between filters, preservatives, fragrances and texturing agents must be carefully studied.
Next comes work on the galenic formulation. Consumers expect a texture that is fluid, non-greasy, invisible and pleasant to apply. This represents a real challenge, especially with non-nano mineral filters, which are more difficult to disperse.
Finally, measuring SPF cannot be improvised. It is based on standardised tests, carried out in vivo on volunteers (standard ISO 24444:2019) or in vitro for the UVA part (ISO 24443:2021)¹. These tests cost money, take time and require rigorous planning.
Please note: According to European recommendation 2006/646/EC, a sunscreen product must protect against both UVA and UVB rays, and must undergo tests to assess :
SPF (tested in vivo, ISO 24444),
The UVA index (tested in vitro or in vivo, ISO 24443),
The critical wavelength, which assesses whether the protection extends well into the UVA zone.
Minimum requirements include:
A minimum SPF of 6,
UVA protection of at least 1/3 of the SPF indicated on the label,
A critical wavelength greater than 370 nm (guarantee of broad-spectrum protection). (standard ISO 24444:2019) or in vitro for the UVA part (ISO 24443:2021). These tests cost money, take time and require careful planning.
It is also essential to remember that filters can degrade over time. A cream that has been open for more than a year or exposed to heat can lose its effectiveness. Respecting the expiry date is an essential precaution.
V. Human health: between protection and scientific controversy
Some sunscreens raise concerns about their long-term safety. Octocrylene, for example, can be transformed into benzophenone, a molecule that is considered to cause concern for the hormonal system⁴. Studies have also shown that some filters, notably oxybenzone, are capable of crossing the skin barrier and ending up in the blood, urine and even breast milk⁸ . However, the levels detected remain, according to the health authorities, well below toxicity thresholds.
Did you know? Despite the presence of UV filters in the body, the levels measured remain well below the thresholds deemed toxic by the health authorities.
The issue of nanoparticles, particularly in the case of titanium dioxide or zinc oxide, comes up regularly. Current studies indicate that these particles, in cream form, do not penetrate beyond the superficial layers of healthy skin⁷. The main risk lies in inhalation, which is why these filters are not recommended in sprays.
VI. Ecological impact: when sunscreen threatens the oceans
Sun filters don't just protect our skin: they also end up in aquatic ecosystems, particularly through swimming. Numerous studies have highlighted the impact of certain filters on coral reefs, in particular oxybenzone and octinoxate. These substances can cause coral bleaching, alter their DNA or disrupt the reproduction of larvae⁷.
In response to these risks, some regions such as Hawaii, Palau and the Virgin Islands have banned certain filters from products sold locally. The “reef-safe” label has emerged, but it is not governed by official regulations, which opens the door to greenwashing.
More environmentally-friendly alternatives include non-nano mineral filters, or certain new-generation molecules developed to combine effectiveness and eco-responsibility. Research is also moving towards bio-based filters, derived from plants or algae, but their development is still in progress.
VII. Regulations: a strict framework… that varies from country to country
In Europe, sunscreen products are considered to be cosmetics, subject to Regulation (EC) No 1223/2009. They must therefore meet all the requirements in terms of labelling and safety, and be assessed via a Product Information File (PIF). Europe currently authorises 29 UV filters, each with a maximum concentration.
In the United States, sunscreens are classified as over-the-counter (OTC) medicines, which implies different requirements, particularly in terms of testing. Australia also adopts a very strict approach, with controls similar to those for medicines. Finally, Japan classifies these products as quasi-drugs.
In the EU, claims such as “total protection” or “100% screen” are prohibited as they are considered misleading.
VIII. Choosing or formulating a sunscreen product : key points to remember
For the consumer, there are five criteria that should guide the purchase of a suncare product:
Opt for an SPF adapted to your phototype (at least 30).
Check for the UVA logo
Follow the application recommendations (quantity, frequency, etc.)
Make sure the product is fresh
If possible, avoid controversial filters or those with a high ecological impact.
For formulation professionals, it is crucial to select filters that are effective, compatible with each other, stable and meet sensorial expectations. Taking into account environmental issues and monitoring regulations have become essential. Finally, undesirable effects must be reported as part of cosmetovigilance, in particular via mechanisms such as Vigicare.
Bonus: 3 preconceived ideas to forget
“I've got a tan, I don't need cream any more”
False. A tan is a reaction to UV rays.
“I can mix two SPF creams to increase their effect”
“Coconut oil protects naturally”
Sources
¹ World Health Organization, UV Radiation & Health - Recommendations for sun protection, WHO, 2023.
² International Journal of Cosmetic Science, Natural Oils as Sun Protectants: Myth or Reality? scientific review, 2016.
³ Scientific Committee on Consumer Safety (SCCS), Opinion on Octocrylene (SCCS/1627/21), European Commission, December 2020.
⁴ Schlumpf et al, Detection of UV filters in human breast milk, Chemosphere, Vol. 44, Issue 4, 2001.
⁵ Matta et al, Effect of Sunscreen Application on Plasma Concentration of Sunscreen Active Ingredients, JAMA - Journal of the American Medical Association, May 2019.
⁶ Downs et al, Toxicopathological effects of oxybenzone on coral planulae and cultured primary cells and its environmental contamination in Hawaii and the US Virgin Islands, Archives of Environmental Contamination and Toxicology, 2016.
⁷ Scientific Committee on Consumer Safety (SCCS), Opinion on Titanium Dioxide (nano form) (SCCS/1518/13), European Commission, April 2014.
Regulation (EC) N°1223/2009, European regulation on cosmetic products, European Parliament and Council, 30 November 2009.
CosIng - European cosmetic ingredients database, European Commission.
ISO 24444:2019, Cosmetics - Sun protection test methods - In vivo determination of the sun protection factor (SPF), International Organization for Standardization.
ISO 24443:2021, Cosmetics - In vitro determination of UVA protection, International Organization for Standardization.
⁸ Camille Le Bihan, Oxybenzone Monograph, 2 December 2015.
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