Intermediate Physics for Medicine and Biology: Sunscreen

In Chapter 14 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss ultraviolet light, and specifically sunscreen.

Protection from the sun certainly reduces erythema [sunburn] and probably reduces skin cancer. Protection is most important in childhood years, both because children receive three times the annual sun exposure of adults and because the skin of children is more susceptible to cancer-causing changes. The simple sun protection factor (SPF) alone is not an adequate measure of effectiveness, because it is based on erythema, which is caused mainly by UVB [ultraviolet B light, with wavelengths from 280 to 315 nm]. Some sunscreens do not adequately protect against UVA radiation [315-400 nm]. Buka (2004) reviews both sunscreens and insect repellents for children. He finds several products that adequately block both UVA and UVB. Look for a sunscreen labeled “broad spectrum” or with at least three stars in a UVA rating system. An adequate amount must be used: for children he recommends 1 fluid ounce (30 ml) per application of a product with SPF of 15 or more. The desired application of sunscreen is 2 mg cm⁻². Typical applications are about half this amount. It has been suggested that one make two applications (Teramura et al.2012) or use a sunscreen with a very high SPF (Hao et al.2012).

This week the Food and Drug Administration has issued a proposed update to sunscreen rules.

This significant action is aimed at bringing nonprescription, over-the-counter (OTC) sunscreens that are marketed without FDA-approved applications up to date with the latest science to better ensure consumers have access to safe and effective preventative sun care options. Among its provisions, the proposal addresses sunscreen active ingredient safety, dosage forms, and sun protection factor (SPF) and broad-spectrum requirements. It also proposes updates to how products are labeled to make it easier for consumers to identify key product information.

“Broad spectrum sunscreens with SPF values of at least 15 are critical to the arsenal of tools for preventing skin cancer and protecting the skin from damage caused by the sun’s rays, yet some of the essential requirements for these preventive tools haven’t been updated in decades. Since the initial evaluation of these products, we know much more about the effects of the sun and about sunscreen’s absorption through the skin. Sunscreen usage has changed, with more people using these products more frequently and in larger amounts. At the same time, sunscreen formulations have evolved as companies innovated. Today’s action is an important step in the FDA’s ongoing efforts to take into account modern science to ensure the safety and effectiveness of sunscreens,” said FDA Commissioner Scott Gottlieb, M.D. “The proposal we’ve put forward would improve quality, safety and efficacy of the sunscreens Americans use every day. We will continue to work with industry, consumers and public health stakeholders to ensure that we’re striking the right balance. To further advance these goals, we’re also working toward comprehensive OTC reform, which will help foster OTC product innovation as well as facilitate changes necessary for the FDA to keep pace with evolving science and new safety data.”

The agency is issuing this proposed rule to put into effect final monograph regulations for OTC sunscreen drug products as required by the Sunscreen Innovation Act. OTC monographs establish conditions under which the FDA permits certain OTC drugs to be marketed without approved new drug applications because they are generally recognized as safe and effective (GRASE) and not misbranded. Over the last twenty years, new scientific evidence has helped to shape the FDA’s perspective on the conditions, including active ingredients and dosage forms, under which sunscreens could be considered GRASE.

The feds then get specific.

Of the 16 currently marketed active ingredients, two ingredients – zinc oxide and titanium dioxide – are GRASE for use in sunscreens; two ingredients – PABA and trolamine salicylate – are not GRASE for use in sunscreens due to safety issues. There are 12 ingredients for which there are insufficient safety data to make a positive GRASE determination at this time.

Latha et al. (Sunscreening Agents: A Review, The Journal of Clinical and Aesthetic Dermatology, Volume 6, Pages 16-26, 2013) discuss the mechanism of sunscreen action (references removed).

Sunscreening agents contain titanium dioxide (TiO₂), kaolin, talc, zinc oxide (ZnO), calcium carbonate, and magnesium oxide. Newer chemical compounds, such as bemotrizinol, avobenzone, bisoctizole [sic], benzophenone-3 (BZ-3, oxybenzone), and octocrylene, are broad-spectrum agents and are effective against a broad range of solar spectrum both in experimental models and outdoor settings. Ecamsule (terephthalylidene dicamphor sulphonic acid), dometrizole trisiloxane [sic], bemotrizinol, and bisoctrizole are considered organic UVA sunscreening agents... Commercial preparations available in the market include a combination of these agents to cover a wide range of UV rays.

Composition and mechanism of action of sunscreening agents vary from exerting their action through blocking, reflecting, and scattering sunlight. Chemical sunscreens absorb high-energy UV rays, and physical blockers reflect or scatter light. Multiple organic compounds are usually incorporated into chemical sunscreening agents to achieve protection against a range of the UV spectrum. Inorganic particulates may scatter the microparticles in the upper layers of skin, thereby increasing the optical pathway of photons, leading to absorption of more photons and enhancing the sun protection factor (SPF), resulting in high efficiency of the compound.

Researchers are postulating that the generation of sunlight-induced free radicals causes changes in skin; use of sunscreens reduces these free radicals on the skin, suggesting the antioxidant property. Broad-spectrum agents have been found to prevent UVA radiation-induced gene expression in vitro in reconstructed skin and in human skin in vivo.