Routine and appropriate use of sunscreens has been demonstrated to protect against the adverse effects of sun exposure such as skin cancer, photoaging (skin aging resulting from sunlight overexposure), and other health problems [1,2]. Two types of sunscreen are available on the market: chemical and mineral (also referred to as "physical").
Chemical sunscreens use chemical active ingredients (filters) that work by penetrating the skin and absorbing ultraviolet (UV) radiation. The resulting energy is then dissipated into heat which is released from the skin . Chemical sunscreens are commonly used by consumers since they are cosmetically appealing and spread easily with a transparent finish. Most chemical filters provide a limited light-range protection (often classified as UVAI [340-400 nm], UVAII [320-340 nm] or UVB [290 -320 nm] filters). These filters usually need to be combined in a sunscreen to ensure broad-spectrum protection from both UVA and UVB rays.
Some chemical filters are considered photounstable and break down when exposed to UV radiation. This photochemical instability leads to the formation of reactive byproducts (free radicals) and to a decrease in sun protection, especially in the UVA spectrum. Certain filters, such as Avobenzone, present higher photochemical instability than others . To stabilize photoreactive agents, most chemical sunscreens use different filters in conjunction. In any case, the stability of a product is difficult to predict since it also depends on many other formulation factors . It is therefore important to apply sunscreen regularly to keep skin protected from UV rays.
Chemical vs. Mineral Sunscreen:
It is important to apply sunscreen regularly to keep skin protected from UV rays.
Several chemical filters also have a documented allergenic potential . For example, commonly used oxybenzone, avobenzone, or octinoxate can trigger irritation or allergic reactions. However, taking into account the population exposed to these filters, the rate remains relatively low .
Concerns have been expressed regarding the systemic absorption of chemical filters (oxybenzone, homosalate, octinoxate,…) and their potential to act as hormonal disruptors in animals and in vitro (laboratory setting) [2,5-10]. While more research needs to be performed in this field, current available data does not demonstrate significant endocrine disruption in humans with topical application . It should be further noted that some experts have mentioned that children may be more affected than adults by some agents. Children don't eliminate drugs as well as adults and might be more sensitive to small levels of hormone perturbations .
Mineral (Physical) Sunscreen
Mineral sunscreens use mineral zinc-oxide and/or titanium-dioxide UV filters. Older physical sunscreens used large mineral particles leaving a white cast on the skin which made them not very appealing to consumers. There are now more advanced physical sunscreens that have a thinner consistency, are pleasant to use, offer better protection, and go on completely sheer. Newer physical sunscreens are formulated with micronized (smaller size, < 200 um) minerals and work mainly by absorbing light [1,11, 12] but also by scattering UV rays away from the skin .
Titanium diode is effective at filtering mainly UVB rays and some UVA light (UVA protection decreases with particle size). Zinc oxide offers protection throughout the full UVB and UVA range for all particles sizes . Studies have shown that micronized zinc oxide and titanium dioxide do not penetrate the skin barriers and filter UV rays by remaining in the upper skin layers [2,13,14].
Titanium dioxide to a much greater extent than zinc oxide has the potential to be photoreactive and generate free radicals. In sunscreens, titanium dioxide is therefore generally coated with aluminum oxide or silica to decrease its photoreactivity (which still remains higher than that of zinc oxide) [15,16]. However, to affect the living tissues, titanium dioxide would have to penetrate the skin which has been demonstrated to not be the case with physical filters.
Physical sunscreens also have a lower rate of photoallergic reactions  and zinc oxide has even been shown to reduce cutaneous irritation and redness .
Why Use Zinc-Oxide Mineral Sunscreen?
At THOYA, we only use zinc-oxide mineral UV filter in our sunscreens. Zinc oxide remains a safe and a naturally-derived physical active ingredient approved by Health Canada that provides effective sun protection against both UVA and UVB rays . It is also a skin protectant / anti-irritant that does not penetrate the skin barriers. Zinc oxide is likely to be less whitening than titanium dioxide (for the same particle size) since its refractive index is smaller.
Zinc-oxide mineral sunscreen is recommended for all skin types including hypersensitive skin and for children 6 months and older.
Elizabeth Hartinger, Ph.D., M.A.Sc., B.Eng.
 Burnett ME, Wang SQ. Current Sunscreen Controversies: A Critical Review. Photodermatol Photoimmunol Photomed. 2011 Apr; 27(2):58-67.
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 Gonzalez H, Tarras-Wahlberg N, Strömdahl B, Juzeniene A, Moan J, Larkö O, Rosén A, Wennberg AM. Photostability of Commercial Sunscreens Upon Sun Exposure and Irradiation by Ultraviolet Lamps. BMC Dermatol. 2007 Feb 26;7:1.
 Schlumpf M, Cotton B, Conscience M, Haller V, Steinmann B, Lichtensteiger W. In Vitro and In Vivo Estrogenicity of UV Screens. Environ Health Perspect. 2001;109(3):239-244.
 Janjua NR, Mogensen B, Andersson AM, Petersen JH, Henriksen M, Skakkebaek NE, Wulf HC. Systemic Absorption of the Sunscreens Benzophenone-3, Octyl-Methoxycinnamate, and 3-(4-Methyl-Benzylidene) Camphor After Whole-body topical application and reproductive hormone levels in humans. J Invest Dermatol. 2004 Jul;123(1):57-61.
 Sarveiya V1, Risk S, Benson HA. Liquid Chromatographic Assay for Common Sunscreen Agents: Application to In Vivo Assessment of Skin Penetration and Systemic Absorption in Human Volunteers. J Chromatogr B Analyt Technol Biomed Life Sci. 2004 Apr 25;803(2):225-31.
 Egerton TA, Tooley IR. UV Absorption and Scattering Properties of Inorganic‐Based Sunscreens. International Journal of Cosmetic Science. 2012;35(2): 117-122.
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 Cross SE, Innes B, Roberts MS, Tsuzuki T, Robertson TA, McCormick P. Human Skin Penetration of Sunscreen Nanoparticles: In Vitro Assessment of a Novel Micronized Zinc Oxide Formulation. Skin Pharmacol Physiol. 2007;20(3):148-54.
 Maier H, Schauberger G, Brunnhofer K, Hönigsmann H. Change of Ultraviolet Absorbance of Sunscreens by Exposure to Solar-Simulated Radiation. J Invest Dermatol. 2001 Aug;117(2):256-62.
 Baldwin S1, Odio MR, Haines SL, O'Connor RJ, Englehart JS, Lane AT. Skin Benefits from Continuous Topical Administration of a Zinc Oxide/Petrolatum Formulation by a Novel Disposable Diaper. J Eur Acad Dermatol Venereol. 2001 Sep;15 Suppl 1:5-11.
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