On its own, the powder was useless as a sunscreen. A Thai team at Prince of Songkla University measured it plainly: formulations “containing 10% HAp alone, regardless of source, exhibited negligible UV protection (SPF = 1.03–1.13), failing to meet the Thai Industrial Standard requirement (TIS 2804–2017: SPF > 15).” If hydroxyapatite were being sold as a UV filter, that line would end the story.
So what happened in the blend?
The same failed powder was then combined with 5% Tinosorb® S, an organic filter that alone managed SPF around 11. Together: “the SPF values for SKJ- and TG-HAp hybrid systems reached approximately 29–30 (~180% increase relative to 5% TS). Notably, the SM-HAp system exhibited a higher SPF of 32.19 ± 1.46 (205% enhancement).” The mineral contributed almost nothing alone and tripled the result in combination — the study confirms “that HAp did not function as an intrinsic UV absorber.” The working mechanism is optical: particles scattering light through the film, stretching each photon’s path so the organic filter gets more chances to absorb it.
Then the detail that matters for specification. Refining the winning powder — milling it “from irregular submicron aggregates (430 ± 140 nm) to a nanoscale distribution of 117.39 ± 61 nm” — lifted SPF “from 32.19 ± 1.46 to 35.15 ± 1.44,” and the accompanying UVAPF gain “was sufficient to elevate the protection grade from PA+++ to PA++++.” A label-grade improvement, bought entirely with particle-size control.
There is a sourcing lesson in which powder won, too. Three fish-bone sources were compared and they did not perform identically as boosters — the spread between them was measured in whole SPF points. Feedstock, in other words, is a formulation variable. For a scattering application, everything that shapes particle geometry upstream — species, cleaning, calcination, milling — surfaces downstream as label grade, which is a more direct line from raw-material consistency to shelf claim than most ingredients ever show.
What specification should a buyer pay for?
Not “UV protection” — the mineral has none to sell. The value is as an SPF booster: a scattering aid that lets a formulator hit a target SPF with less of the expensive organic filter, or clear a marketing threshold the filter alone misses. And because the mechanism is optical, the specification that carries the value is physical, not chemical: particle-size distribution, aggregation state, dispersion behaviour in the film. Two lots with identical purity and different aggregate sizes are, for this application, different products — one is PA++++, one is not.
The limits travel with the claim: every figure here is in vitro, measured on PMMA plates, and the authors state that for regulatory approval and commercial labelling, in vivo testing remains the global standard. The material was fish-bone-derived, from Thai seafood processing waste — another biogenic calcium stream finding a second market.
The lesson generalises past sunscreen. When an ingredient’s job is physical, lot-to-lot consistency in the physical numbers is the whole purchase — and a certificate that reports only chemistry is describing a different product from the one you are buying.
Sources
Sookchoo P, Benjakul S, Prodpran T, Amnuaikit T, et al. “Synergistic Effects of Hydroxyapatite Derived from Fish Bone and Tinosorb® S on the UV Protection Performance of Sunscreen.” Cosmetics, 2026;13(2):71 (open access) — mdpi.com/2079-9284/13/2/71.