April 2023 • Vol. 29 • Issue 4
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Sunscreen Formulations – Emphasis on Inorganic Sunscreens …Hani Fares
ver since the FDA published their proposed monograph ruling in February 2019 recognizing titanium dioxide and zinc oxide as the only Category I (Safe and Effective) sunscreens, a cascade of reformulations of most sunscreens products on the U.S. market took place. Inorganic sunscreen formulations are now center-stage and are slowly replacing organic sunscreen formulations. In fact, the trend started in 2018 when the state of Hawaii proposed a ban on octinoxate and oxybenzone, stating that these two sunscreens have a negative effect on coral reefs. Now that the ban is in effect, another bill is proposing to ban sunscreens containing octocrylene and avobenzone for the same reasons. It is true that many regulatory bodies including the FDA did not support the Hawaiian ban, and the Personal Care Product Council (PCPC) is addressing the proposed monograph rulings. All these actions might lead to uncertain outcomes. In fact, in a few years U.S. consumers might be limited to the use of products only containing inorganic sunscreens (with the exception of ensulizole and ecamsule). There is some hope that certain Time and Extent (T&E) molecules are being reviewed by the FDA and may be approved for launch. Bemotrizinol is a front-runner and its use in formulation is quite good as it protects skin from both UVB and UVA radiation.
n Selecting the Right Inorganic Sunscreen
Zinc oxide and titanium dioxide not only refract light but also absorb it. The refractive index of titanium dioxide is 2.8 whereas that of zinc oxide is only 2.0. This makes titanium dioxide much more effective at scattering light when delivered from a formulation. In terms of absorption, zinc oxide and titanium dioxide have conductance bands around 3.4 and 3.1 eV, respectively. This makes zinc oxide a bit more efficient in protecting against UVA rays and titanium dioxide more efficient at shielding UVB rays. As particle size decreases there is a much more pronounced blue shift due to a change in the band-gap width. For example, a 0.15 eV blue shift has been reported for 4.7 nm particle size titanium dioxide compared to bulk. Keep in mind, when particles become smaller than their optimal light scattering size (typically half their wavelength) they become much more transparent. For example, zinc oxide becomes transparent below 200 nm whereas titanium dioxide becomes transparent at sizes around 10-20 nm. This makes formulating with zinc oxide much easier to achieve transparent formulations but harder to reach high SPF due to its performance in the UVB region. Sometimes the so-called “boosters” can help many formulators resort to using salicylates as UVB boosters in their formulations. Butyl octyl salicylate is not an approved sunscreen in the U.S. but many formulators use it to boost their inorganic sunscreen SPF while (continued on Page 6) claiming no organic sunscreens added.
S U S TA I N A B L E A C T I V E I N G R E D I E N T S ...see page 5 for more information.
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APRIL 4th