Physical UVA+UVB sunscreen/sunblock: Zinc Oxide

Generic name: Zinc oxide; Zn0

Brand(s): Various

Type: Physical

Range of UV spectrum covered: UVA (both UVA-1 and UVA-2), UVB

Wavelengths covered: good uniform coverage between 290-400 nm, and some coverage up to 700 nm

Stability:
Regular zinc oxide is highly stable under most conditions; minimal reactivity may occur, especially in direct sunlight. However, zinc oxide nanoparticles are far more reactive/catalytic than regular zinc oxide powder and might promote the formation of harmful free radicals when exposed to sunlight. Most manufacturers of zinc oxide nanoparticles coat them to reduce or eliminate such effects.

Summary:
Regular zinc oxide is one of the most broadly effective, established and safe sun blocking ingredients available. Its major disadvantage is unsightly whitish tint it creates. Nanoparticle forms are less unsightly but their potential risks are unclear and need to be more thoroughly researched. Also, it is difficult to achieve high SPF (UVB protection measure) with zinc oxide alone. Therefore, in high SPF sunscreens, it is generally combined with chemical UVB blockers.

Details:
Zinc oxide is a physical sun blocking agent that work primarily by reflecting/scattering ultraviolet light. It has a broad range of effectiveness, covering UVB as well as both short (320-340 nm, a.k.a UVA-2) and long (340-400 nm, a.k.a. UVA-1). Regular zinc oxide also blocks visible light up to wavelengths 700 nm, whereas zinc nanoparticles block only up to 380 - 400 nm, depending on the specific formulation. As a single ingredient, zinc oxide is the broadest range sunscreen on the market.

Zinc oxide has a long history of safe use. It is not irritating and compatible with sensitive skin. In fact, zinc oxide is a skin protectant and anti-irritant, and is widely used in treating various forms of dermatitis/skin irritation, including diaper rash.

The main complaint about zinc oxide-based sunscreens is that thy may leave unsightly white residue. In fact, the concentrations of ordinary forms zinc oxide required for high degree of protection against the full range of UVA and UVB inevitably produce some whitish tint. This problem has been partly addressed by the advent of zinc oxide nanoparticles as a sunscreen agent (one popular brand is Z-cote). Zinc oxide nanoparticles have different optical properties and tend to produce much less whitish tint than regular powdered zinc oxide. Yet, early research indicates that zinc oxide nanoparticles retain the capacity to protect from UVA and UVB light.

The safety of zinc oxide nanoparticles remains a controversial subject. When exposed to sunlight, zinc oxide nanoparticles may promote the generation of free radicals and increase the risk of mutations. However, this effect has been shown only in cell culture: it may not be relevant to topical use as long as zinc oxide nonoparticles do not penetrate stratum corneum (the outer skin layer consisting mostly of dead cells). Furthermore, manufacturers typically cover zinc oxide nanoparticles with special coating (e.g. Z-cote is based on zinc oxide nanoparticles coated with dimethicone), which increases the stability and reduces photoreactivity of the particles. Finally, there is a question whether zinc oxide nonopaticles could penetrate the skin, reach systemic circulation and accumulate in body organs. At present, there is no clear evidence that they do, but research of that issue has been insufficient. Furthermore, a recent UCLA study demonstrating DNA damage in mice from ingestion of titanium oxide nanoparticles raises new concerns about the risks of this technology. For more details see our article on the potential risks associated with nanoparticles in skin care.

Virtually no single sun blocking ingredient can provide complete, broad protection alone. In that respect, zinc oxide is arguably closer to the ideal than most others. Still, when used alone, it is insufficient if high SPF (UVB protection measure) is required. Hence, in commercial sunscreens, zinc oxide is usually combined with chemical UVB blockers. Sometimes it is also combined with other physical (titanium dioxide) and/or chemical UVA blockers.