In the early days of the science of sunscreen formulation and in the area of combining sunscreen actives, we talked about something called “the synergistic effect.” This was generally recognized as the combining of UV actives such that the sum SPF, when tested in vivo, was greater than the total contribution to the SPF if each active was individually clinically determined.
Many early UV actives exhibited low efficiency. For example, the SPF 4 Reference Sunscreen required for FDA Monograph1 testing contained only Homosalate at 8% level i.e. an efficiency of 0.5.
Both the earlier FDA Monograph version and the current version2 include a requirement that “The concentration of each active ingredient must be sufficient to contribute a minimum SPF of not less than 2 to the finished product. The finished product must have a minimum SPF of not less than the number of sunscreen active ingredients used in the combination multiplied by 2.”
According to the Merriam-Webster Dictionary, a synergist is “something (such as a chemical or a muscle) that enhances the effectiveness of an active agent.”
A quick AI search gives a somewhat differing response. Synergy is “the interaction of two or more agents or forces so that their combined effect is greater than the sum of their individual effects.”
Synergistic UV Actives
Increasing the SPF by synergy is a well-recognized process for many formulators. An example is the addition of titanium dioxide to classical organics-based actives. This example appears to date back to the time when formulations moved from SPF 15 limits to SPF 30 and beyond as higher efficacy targets.
This beneficial process can certainly be modelled using in-silico tools. Figure 1 is indicative of this opportunity for potential synergy and is only limited by the white mask effect somewhat inherent in this official UV active. In this example, the synergy models as an increase of 14 SPF units (i.e. 33/19 = 74% increase in SPF yield).
Figure 1 - Synergistic effect of organics with titanium dioxideOnline Simulator Courtesy of DSM
More recently, solid forms of the triazone class of UV actives have been shown to provide impressive SPF boosting impact. These are widely used in most of the world but restricted in the USA. Only very small amounts – even inclusion in the order of a few percent having a significant SPF impact when in combination with TiO2 or ZnO (see Figure 2).
Figure 2 - Synergistic effect of newer-generation organics with addition of titanium dioxide.Online Simulator Courtesy of BASF
Here, we can observe a very significant impact, elevating the filter efficiency by over 50%. In this modelling, it is not apparent that the primary boosting effect is related to the physical structure of the emulsion and as a consequence of the several of the organic filters being present as nano-particle solids. The synergistic effect is then the result of the physical form of the combination of organics and inorganics.
Now let’s turn to what appears to be the currently in-vogue term “booster.” Here again, Merriam-Webster teaches that this is “an auxiliary device for increasing force, power, pressure, or effectiveness” or “a substance or dose used to renew or increase the effect of a drug or immunizing agent.”
So, is “synergy” synonymous with “boosting?” On the one hand, if we look within the context of increasing effectiveness, then the two terms might be considered as interchangeable.However, in jurisdictions such as the United States, Canada, and Australia, sunscreens are categorized as drugs. Therefore, the suggestion of 'increasing the effect of a drug' implies that the additions being made are non-drug in nature.
A metaphor for consideration of this might be found in everyday analgesics, such as the combination of paracetamol with opioids3. Here, the evidence is that “Experimental preclinical and clinical evidence supports that paracetamol augments significantly the analgesic effects of anti-inflammatory, opioid and anti-neuropathic drugs in different clinical settings.”
Is synergy then the norm in formulating in any case? For a sunscreen formulation, we are dealing with multiple biological effects, both short-term (erythema) and long-term (cellular damage) with the added challenge of targeting the 290 nm to 400 nm UV “broad” spectrum. So, is it normal practice to combine individual UV actives in the least because of this broad-spectrum requirement of efficacy? It could thus be argued that we are in an area of science where, with the possible exception of zinc oxide, a single chemical entity cannot really do the job for us.
As our primary objective is to maximize sun protection efficacy, then is it not reasonable that we utilize every tool available to us as formulators?
Solubility Enhancers
Particularly for UV actives which are solids, the role of appropriate solubilizers cannot be ignored. The largest group of these is probably fatty esters. Without full stabilization for shelf-life solubility, these actives will not exhibit their SPF contribution. This issue can often only become apparent well post-manufacture. Major suppliers of these actives now include recommendations and even incorporate them in their in-silico tools with guidance for the suitability and concentration levels.
Physical Boosters – Structural Formers
Structural additives are utilized for the purpose of enhancing SPF by impacting positively on the pathlength that photons have to pass through the sunscreen film. Examples of these include glass beads, nano size micro-encapsulations and micro-encapsulated actives. Due to changes in environmental best practices, the issue of micro-plastics have impacted their use to some degree, as some fit the definition of nano, microplastic or both4.
However, a number of substances are still promoted for the purpose of supporting this physical effect on formulations. Examples are Copernicia Cerifera Wax (and) Oryza Sativa Wax, Hydrogenated Soy Polyglycerides, Lignin5 and cellulose-based ingredients.
Chemical Boosters
This is a seemingly growing class of SPF boosters, and one where attention is being drawn to the need for more transparent disclosure. They are currently offered for use with both organic and inorganic UV filters. Many examples of these excipients are specifically marketed by ingredient suppliers for the purpose of raising the SPF of sunscreen formulations. Some of these exhibit an SPF in their own right and some have very similar chemical structures to the recognized chromaphore-containing molecules approved for sunscreen inclusions.
Measurement of direct SPF contribution for some of these is set out below in Figure 3. Calculated as 100% strength, in vitro SPF determination of these indicates the very significant contribution of some, whilst others have actually been incorrectly misrepresented as contributors – for example fixed oils6.
Figure 3 - A review of some excipients commonly encorporated into sunscreensCourtesy of SciPharm Pty Ltd
Pseudo-actives
It has recently become apparent that the question of potential mis-representation of these excipients has come to the attention of regulators. Are some of these, in effect, SPF actives?
Many zinc oxide-based brands currently make the claim of being “mineral only” but then rely on chemical boosters to enhance SPF. A recent study conducted at the University of NSW, Sydney Australia7 examined 143 sunscreens claiming only zinc oxide as their active and registered with the regulator, Therapeutic Goods Administration (TGA). The review reported that more than one-third contained additional UV-contributing excipients and, of these, 25% included butyloctyl salicylate and ethylhexyl methoxycrylene.
Is this then a mis-representation or should it be treated as a valid formulation enhancement?
Conclusion
To summarize, “boosters” might best be classified to fit into the following categories.
· Synergistic UV Actives
· Solubility Enhancers
· Physical Boosters – Structural Formers
· Chemical Boosters
· Pseudo-actives
Singularly, or in combination, there is no doubt that all five approaches are valuable as tools for supporting the efficacy of sunscreens. Is there then, the possibility that we should be concerned at the legal implications of misrepresentation of label claims for this final category, from both regulatory and consumer protection viewpoints?
This is certainly a consideration in those jurisdictions where there is no requirement to comprehensively declare the full list of excipient ingredients of the formulation on the product packaging. It is further aggravated by advertising claims which imply that the product is based on inorganic actives.
References
- Sunscreen Drug Products For Over-The-Counter Human Use; Final Monograph. A Rule by the Health and Human Services Department, and the Food and Drug Administration. Publication date: May 21, 1999. Effective date: May 21, 2001 https://www.govinfo.gov/content/pkg/FR-1999-05-21/pdf/99-12853.pdf#page=22
- U.S. Food and Drug Administration Final Administrative Order (OTC000006) Over-the-Counter Monograph M020: Sunscreen Drug Products for Over-the-Counter Human Use (Posted September 24, 2021) https://www.accessdata.fda.gov/drugsatfda_docs/omuf/monographs/OTCMonograph_M020-SunscreenDrugProductsforOTCHumanUse09242021.pdf
- Freo U. Paracetamol for Multimodal Analgesia. Pain Management Vol.12, 2022 – Issue 6 https://doi.org/10.2217/pmt-2021-0116
- Glisovic, S., Pesic, D., Stojiljkovic, E. et al. Emerging technologies and safety concerns: a condensed review of environmental life cycle risks in the nano-world. Int. J. Environ. Sci. Technol. 14, 2301–2320 (2017). https://doi.org/10.1007/s13762-017-1367-2
- Widsten P., Tamminen T., Liitiä T. Natural Sunscreens Based on Nanoparticles of Modified Kraft Lignin (CatLignin) ACS Omega 2020 5 (22), 13438-13446. DOI: 10.1021/acsomega.0c01742
- Staton J, Dennyson C, Orban G. Inhouse data: Dermatest Australia – presentation at New Zealand Society of Cosmetic Chemist Conference Workshop 2016
- Wang A. UNSW Chem. School https://www.theguardian.com/australia-news/2025/nov/29/mineral-sunscreens-australia-identical-to-others The Guardian 23rd Nov 2025”
