Zinc oxide is an inorganic compound with the formula ZnO. ZnO is a white powder that is insoluble in water and is widely used as an additive in numerous materials and products including rubbers, plastics, ceramics, glass, cement, lubricants, foods, and as a sunblock.
For material science applications, zinc oxide has a high refractive index, high thermal conductivity, binding, antibacterial and UV-protection properties.
Zinc oxide can be used in ointments, creams, and lotions to protect against sunburn and other damage to the skin caused by UV light. It is the broadest spectrum UVA and UVB absorber that is approved for use by the FDA and is completely photostable. As stated in an article titled ‘Microfine zinc oxide (Z-Cote) as a photostyable UVA/UVB sunblock agent' the suitability of microfine zinc oxide as a broad-spectrum photo protective agent was assessed by examining the properties considered important in suncreens: attenuation spectrum, sun protection factor (SPF) contribution, photostability, and photoreactivity.
Results: Microfine zinc oxide attenuates throughout the UVR spectrum, including UVA I. It is photostable and does not react with organic sunscreens under irradiation. Conclusion: Microfine zinc oxide is an effective and safe sunblock that provides broad-spectrum UV protection, including protection from long-wavelength UVA.
In 1999, the FDA issued a notice of proposed rule making that amended the tentative final monograph (proposed rule) for over-the counter (OTC) sunscreen drug products. ‘Sunscreen Drug Products for Over-the-Counter Human Use' states the following:
The agency discussed a study submitted to the Panel using zinc oxide alone and in combination with phenyl salicylate, another sunscreen ingredient (58 FR 28194 at 28213). The study was designed to measure the ability of zinc oxide (15 to 33.3 percent) to absorb ultraviolet (UV) radiation over a broad range of wavelengths.
In the proposed rule, the agency also discussed the public health significance of ultraviolet A (UVA) radiation and the characteristics and proposed labeling of OTC sunscreen drug products that claim to provide protection from UVA radiation. One comment measured the spectral absorbance of three formulations: (1) 4 percent zinc oxide, (2) 25 percent zinc oxide, and (3) 2 percent oxybenzone.
Albino hairless mouse stratum corneum/epidermis samples were prepared by mechanical removal of the dermis using a dulled razor
blade. The samples were cut into 1-inch circles and maintained in a hydrated state by floating the samples (dermal side down) on a water bath. The absorbance of each skin sample was measured and recorded. Ten microliters (L) of sunscreen were applied to the skin substrate, allowed to dry for 15 minutes, and the absorbance measured. The absorbance of each sunscreen treated sample was subtracted from the absorbance of the skin (without sunscreen) to yield the absorbance of the sunscreen. Five replicate measurements for each sunscreen formula were averaged and plotted with standard deviations at each 10 nm.
The spectral absorbance plots established that
...zinc oxide has a relatively flat and broad absorbance curve from 250 nm through 370 nm with a sharp drop in absorbance beyond 370 nm and extending into the visible spectrum. Comparison of the measurements of the 4 percent zinc oxide with 25 percent zinc oxide showed that the magnitude of absorbance is related to the amount of zinc oxide in the formulation.
These measurements adequately demonstrated that zinc oxide
...absorbs radiation between 290 and 380 nm and, thus, support effectiveness.
Another comment included the results of in vitro testing of a formulation containing 15 percent zinc oxide in a stable emulsion. The transmittance data supported the premise that
...zinc oxide can protect against UV radiation, including both UVB and UVA.
One comment included a spectral profile of attenuation for zinc oxide alone in a cosmetic formulation and from 1:1 and 3:1 combinations of zinc oxide and titanium dioxide. These spectral profiles of zinc oxide in various formulations demonstrated that
...zinc oxide as a single ingredient can provide protection in both the UVB and UVA spectral regions.
Recent scientific advances in understanding the photochemistry and photobiology of sunscreen drug products have raised many issues regarding sunscreen active ingredients, including zinc oxide and titanium dioxide. Because zinc oxide and titanium dioxide have many similar physical characteristics and may be used in combination in OTC sunscreen drug products, the following discussion addresses both ingredients.
There has been renewed interest in using physical sunscreens, i.e., zinc oxide and titanium dioxide, in sunscreen formulations because these ingredients may confer protection for a broad range of the UV radiation spectrum. Some manufacturers have developed ultrafine forms of these ingredients in the range of 0.02 to 0.10 microns that are transparent on the skin, may offer both UVA and UVB protection, and are esthetically pleasing…
Sunscreens have been generally classified as chemical (organic) or physical (inorganic) depending on whether they absorb specific UV radiation wavelengths or reflect and scatter UV radiation.
Zinc oxide and titanium dioxide have been described as physical sunscreen ingredients that provide protection from UV radiation through reflection and scattering. However, new data and information indicate that they also absorb UV radiation as well as scatter visible light.
Various authors have shown that these ingredients exhibit a semiconductor optical absorption gap meaning they absorb most radiation at wavelengths shorter than the gap (approx. 380nm) and scatter radiation at wavelengths longer than the gap.
When zinc oxide and titanium dioxide are irradiated with light containing energy greater than the band gap (approximately 3 electron volts), an electron from the valence band can be excited to the conduction band, thus creating an electron-hole pair.
There are many formulation variables that may affect the photocatalytic capability of zinc oxide and titanium dioxide.
Such variables include mineral components, particle size, surface area, crystalline structure, particle coatings, pH of the medium, differences in the refractive index of the medium, and other components in the formulation.
Although the FDA continues to evaluate data and information for the purpose of proposing a monograph method for determining UVA radiation protection, it nonetheless finds
there is ample data demonstrating that zinc oxide provides protection against UVA radiation.
As it relates to the frequency one should apply sunscreen, a common recommendation by many public health agencies is to reapply sunscreen every two to three hours. Is this recommendation effective in minimizing ultraviolet exposure of the skin during time in the sun? ’When should sunscreen be applied?’ studied how the time of sunscreen reapplication affects the solar ultraviolet exposure of the skin.
A mathematical model was derived that took into account typical amounts of sunscreen application and sunscreen substantivity to determine how these factors, when combined with the time of sunscreen reapplication, influence the photoprotection provided by sunscreen during exposure for several hours around mid day in strong sunshine.
Results of the study were as follows:
Using a sunscreen that is readily removed from the skin achieves little in the way of sun protection, no matter when it is reapplied. For sunscreens that bind moderately or well to skin, typical of modern waterproof or water-resistant products, the lowest skin exposure results from early reapplication into the sun exposure period, and not at 2 to 3 hours, after initial application. Typically reapplication of sunscreen at 20 minutes results in 60% to 85% of the ultraviolet exposure that would be received if sunscreen were reapplied at 2 hours.
The concluding statement advises sunscreen users
…to apply sunscreen liberally to exposed sites 15 to 30 minutes before going out into the sun, followed by reapplication of sunscreen to exposed sites 15 to 30 minutes after sun exposure begins. Further reapplication is necessary after vigorous activity that could remove sunscreen, such as swimming, toweling, or excessive sweating and rubbing.
Another abstract, ‘A noninvasive objective measure of sunscreen use and reapplication’, studied whether a noninvasive swabbing technique can detect sunscreen use for up to 6 hours, and whether the technique can detect reapplication of sunscreen.
Thirty volunteer office workers were randomly assigned to have one of a variety of sunscreens applied using recommended application techniques, and half were randomly assigned to have sunscreen reapplied after 3 hours. Alcohol-based swabs were used to obtain a sample from participants' arm at 20 minutes, and hourly from 1 to 6 hours post-application. Absorption readings were analyzed using an UV-visible spectrophotometer…The “swabbing technique” was consistently able to distinguish the sunscreen from control swabs for up to 6 hours. The absorption readings between 20 minutes and 6 hours were significantly higher than control swabs. There were no differences between the group that had sunscreen reapplied and the group that did not.The swabbing technique was consistently able to distinguish the sunscreen from control swabs for up to 6 hours. The absorption readings between 20 minutes and 6 hours were significantly higher than control swabs. There were no differences between the group that had sunscreen reapplied and the group that did not.
The study concluded with evidence indicating that the sunscreen swabbing technique is
an effective noninvasive method for detecting a variety of sunscreen products in adults over a 6-hour period. No differences in absorption readings were found with sunscreen reapplication. This procedure will be a useful adjunct to other objective measures of sun protection and UV radiation exposure, resulting in a more accurate picture of the sun protection habits of individuals.
Moreover, the Skin Cancer Foundation answers several relevant questions in an article titled ‘Sunscreens Explained’.
How much Sunscreen Should I Use and How Often Should I Put it On?
To ensure that you get the full SPF of a sunscreen, you need to apply 1 oz – about a shot glass full. Studies show that most people apply only half to a quarter of that amount, which means the actual SPF they have on their body is lower than advertised. During a long day at the beach, one person should use around one half to one quarter of an 8 oz. bottle. Sunscreens should be applied 30 minutes before sun exposure to allow the ingredients to fully bind to the skin. Reapplication of sunscreen is just as important as putting it on in the first place, so reapply the same amount every two hours. Sunscreens should also be reapplied immediately after swimming, toweling off, or sweating a great deal.
Finally, to address your question: “Why do I need to reapply a zinc sun blocker if it has not been exposed to sun or sweat?”; The International Agency for Research on Cancer’s ‘IARC Handbook of Cancer Prevention Volume 5,’ explains in Chapter 6: Other beneficial effects of sunscreens, that there are other potential beneficial effects of sunscreens that are not related to the prevention of skin cancer include prevention of painful sunburns, photodamage and photoageing UVR-induced provocation of certain cutaneous diseases, and photoimmune suppression.
Use of sunscreens can prevent skin diseases from progressing acutely after exposure to the sun; these diseases include cutaneous lupus erythematosus…and reactivation of herpes labialis… The other potential benefits of sunscreens are related to the type and duration of exposure to UVR. Prevention of photodamage and photoageing, which are related to cumulative exposure to UVR, in countries where solar irradiance is intense throughout the year requires daily, longterm sun protection… Prevention of acute flares of cutaneous diseases, which may be related to episodic exposure to UVR, requires anticipatory use of sun protection.
Under certain circumstances, diseases of various etiologies can be aggravated by sunlight in people who on other occasions may react normally.
These diseases include lupus erythematosus, lichen planus and herpes simplex. The disease most frequently recognized as requiring careful photoprotection from both UVB and UVA is lupus erythematosus in the discoid, systemic and subacute forms…The available evidence suggests that regular use of sunscreens reduces morbidity from both cutaneous and systemic lupus erythematous.
Therefore, if you are unsure about whether it is necessary to wear sunscreen indoors, you may want to consider your medical history and potential predisposition toward developing a disease that may increase the need for more intensive "sun-blocking" protective measures. However, it is always advisable to consult with your primary care physician and/or dermatologist with any personal health-related questions or concerns.