The Protective Role of Zinc Oxide
Zinc and zinc compounds play an important role in human health. Zinc deficiency can manifest itself in many ways, including growth retardation, decreased immune function, skin distrubances, gastrointestional dysfunction, and various blood disorders.
Although zinc has been recognized as an indespensable element for animals since 1934, the essentiality of zinc to humans was documented much later in 1963. Since then, it has become well known that zinc deficiency is common throughout the world, especially in developing countries. Currently, zinc is known to be required by approximately 200 enzymes in the human.
Zinc containing compounds are also of great benefit to humans. For example, some zinc containing compounds are well known antimicrobials. Zinc oxide, too, has many benefits including its use as a broad-spectrum sunblock.
Zinc oxide has been used as a dermatotherapeutic agent for over 300 years, first as a component of calamine and then on its own in various preparations.
In the "Phamacopea Londinensis" published in 1618 for the London College of Physicians, Lapis Calaminaris is mentioned as a component in three of forty-two therapeutic ointments. In the U.S. Dispendary of 1883, calamine is mentioned and recommended for use as "a mild astrigent and exsiccant in excoriations and ulcerations".
Calamine was originally a naturally occuring mixture of zinc containing compounds which was used as is or heated and pulverized into a powder which was then applied to the skin. What we know as calamine today is actually "neo-calamine", a mixtuure of zinc oxide and iron oxides. This mixture was standardized in 1947 with the publication of the reciepe in that years National Formulary in the United States.
Today, zinc oxide is still widely used as a topical therapeutic product. In fact, it may be the most commonnly used topically applied product of all time. One of the more important uses for zinc oxide is as a sunscreen. Why is zinc oxide ideally suited for this purpose?
Ultraviolet Radiation UVR
The earth is continuously showered with solor radiation: the electromagnetic energy emitted by the sun. This energy is the source of all that we know and yet, like many other good things, too much can be harmful. Among the spectrum of radiation that the sun emits, Infrared, Visable, and Ultraviolet Radiation command our attention.
The solor spectrum is divided into various portions by wavelength, which is measured in nanometers (nm). UVR covers from 200 to 400 nanometers. UVR is divided into UVC (200-290nm), UVB (290-320 nm) and UVA (320-400 nm). UVA is further categorized as UVA I (340-400 nm) and UVA II (320-340 nm), also called long and short UVA respectively. UVR is generally credited with most of the biologically significant sequela of sun exposure like sunburn, skin cancer, and visible aging.
UVC, also known as Germicidal UV, is very toxic. As its name implies, it is lethal to many microorgansims as well as to most plant life. In addition, it is carcinogenic to humans. Fortunately, virtually all UVC is filtered out by the ozone layer.
UVB makes up about 18% of the solar UV spectrum (prior to attenuation by the earth's atmosphere) but only about 1% of the UVR that reaches the earth's surface, because it is largely filtered out by the ozone layer. Despite its relatively low presence, however, UVB is associated with much of the damage caused to humans by sun exposure. Traditionally, UVB was credited as being the sole cause of sunburn and various skin cancers. Although still considered a major cause of sunburn, UVB is no longer thought to be acting alone with respect to skin cancer. It also seems likely that other wavelengths (UVA) will be found involved in tumor formation, and prehaps, in some cases, even as the primary agent.
UVA makes up about 75% of the solar UV spectrum but about 99% of the earthly spectrum. This is because UVA is largely unaffected by the ozone layer. Much more abundant, UVA is also less energetic that UVB and thus is thought to be biologically less significant. UVA is the major cause of skin darkening (tanning) and aging. In addition to the sun, there are some man-made sources of UV exposure. They include welding arcs, germicidal lamps, some laboratory equipment and tanning lamps.
Photochemistry is simply chemistry that takes place as the result of light's interaction with molecules. Virtually any wavelength of light can induce some sort of photochemistry. In biology, however, UVR wavelengths seem to cause the most significant changes in humans.
The body has many molecules that can absorb UVR and hence participate in photochemical reactions. For instance, nucleic acids (DNA, RNA), melanin, various proteins, hormones, and many drug metabolites absorb UVR. All of these molecules are potentially changed and damaged by sun exposure.
It is known that UVR consistently and specificially damages DNA and one gene in particular, known as p53, is a marker for such damage. Although the precise pathway is not yet mapped, this is certainly one avenue that can lead to skin cancer. It is also well documented that UVR induces the formation of free radicals in the skin. These free radicals go onto produce many deleterious reactions that can lead to damage, including skin cancer.
Skin cancer occures more frequently than all other cancers combined. In the U.S. alone, there will be about 1,000,000 new cases of skin cancer this year. Importantly, the rate of skin cancer is increasing rapidly for reasons that are not entirely clear but surely have something to do with sun exposure.
There are two types of skin cancer, Melanoma and Non-Melanoma. Melanoma is a cancer (maglignant neoplasm) that forms melanocytes. Melanocytes are cells that form melanin (the brown pigment we all make). Melanomas usually occure in the skin but can form anywhere there are melanocytes such as the eyes, nails, central nervous system and mucosal surfaces.
Of the types of skin cancer, melanoma is by far the most fatal, accounting for the vast majority of skin cancer related deaths. The sun's role in melanoma is not yet definitively established, although it seems likely that UVR exposure is critical.
Non-Melanoma Skin Cancer (NMSC) is a term that collectively refers to the two less lethal forms of sun related skin cancer. Basal Cell Carcinoma (BCC) and Squamous Cell Carcinoma (SCC). Together they occure far more often than Melanoma but are also, fortunately, less likely to cause death.
Other Sun-Related Skin Changes
Aside from causing skin cancer, the sun is responsible for many other changes in our skin. Sun exposure will cause wrinkling, yellowing, and thinning of the skin. In addition, sun exposed skin will lose its elasticity. It is estimated that 90% of the skin changes that we associate with aging are actually due to sun exposure. These changes are collectively referred to as photoaging. In other words, if we were constantly protected from the sun, our skin would barely change from the time we were about 20 years old. Most photoaging is the result of damage to collagen and elastin, two important structural components of the skin. Between them, they provide the skin with strength and elasticity.
The photochemical reactions described above are the result of specific wavelengths interacting with specific molecules. The collection of wavelengths that can cause a particular reaction is called Action Spectrum for that reaction. For example, the Action Spectrum for sunburn is predominately in the UVB range. In other words, UVB causes sunburn.
A long suspected, but only recently proven fact is that the Action Spectrum for photoaging is in the UVA region, specifically in the UVA I portion of the ultaviolet spectrum. This is important because zinc oxide effectively block this portion of the UVR spectrum.
Since the late 1920's, sunscreens have been used by people to protect themselves from the harmful effects of the sun. The original sunscreens were organic molecules that worked by absorbing the sun's radiation. Para-Amino Benzoic Acid (PABA), perhaps the best known sunscreen, was patented in 1943. and enjoyed a long period of common usage. It has since fallen out of favor, but other organic sunscreens still make up the vast majority of the chemicals used for this purpose.
In addition to the organic sunscreens, there are also several inorganic chemicals used to block the sun. Some common ones are zinc oxide, titanium dioxide, and iron oxide. The inorganic chemicals offer good protection and, unlike their organic counterparts, are not absorbed into the skin. Because of this, they do not cause adverse reactions such as allergies and are thought to be generally safer. Among these, zinc oxide has an unmatched history of safe and effective long-term use.
In the past, sun damage was synoymous with sunburn. It was assumed that the same rays that caused sunburn would also cause skin cancer and other sun related problems. Accordingly, sunscreen developement was aimed at the invention of sunscreens that stopped sunburn which is a UVB phenomemon.
We now know that the entire UV spectrum, and not just UVB, is important. Photoaging and perhaps even some forms of skin cancer can be caused by UVA. There is a lack of ingredients that effectively block UVA, especially UVA I. Fortunately, zinc oxide does.
Sun Protection Factor SPF
How much protection a sunscreen provides is described by the SPF number on the container. This system is used virtually worldwide.
The test consists of determining how much UVR (mostly UVB) it takes to cause a barely detectable sunbburn on a given person. As an example, if it takes 10 minutes to "burn" without the sunscreen and then 100 minutes with the sunscreen then that product has a SPF of 10 (100/10). An SPF 15 blocks about 94% of the UVR and is generally considered adequate for most people most of the time.
Since the SPF system is based on sunburn (a UVB phenomenon), it tells you very little about UVA protection. Again, this stems from the fact that, at the time the SPF system was created (the 1970's), UVB was considered the only important part of the spectrum. A meaningful test for UVA is currently being devised, but for the time being, consumers have to rely on the sunscreen manufacturers to use the right ingredients, like zinc oxide.
One should note that a claim of "Broad Spectrum" on the label is no quarantee of adequate UVA coverage. This is because a "Broad Spectrum" claim can be made simply by including one of several ingredients that block only a portion of the UVA spectrum. As stated above, this is based on an old assumption that UVA was not very important. It is expected that the various regulatory agencies around the world will correct this problem in the near furture.
Zinc Oxide is a True Broad-Spectrum Sunblock
Zinc oxide has been used for centuries to protect and heal the skin. When it was first intentionally used as a sunblock is not clear, but at the least during this century, it has been considered common knowledge that zinc oxide is the most effective sunblock available. Everyone can recall lifeguards and tennis players with zinc oxide on their lips and noses. The military even issued zinc oxide paste to pilots during both World Wars to use in case they were downed and excessively subjected to the elements.
UVB causes sunburn and UVA causes skin aging and skin darkening (tanning). Both UVA and UVB are involved in skin cancer. Given this knowledge, any consumer product claiming sun protection needs to block both UVA and UVB. To do anything else is doing a disservice to the public. Recall that a sunscreen that blocks only UVB will still prevent a sunburn.
Sunburn is the body's built in alarm system that tells us when we have had too much UVR. When this response is "bypassed" peiople stay out longer than they normally would, and if only using a UVB block, expose themselves to unnaturally high doses of UVA radiation. This is one of several proposed explanations for the rising incidence of skin cancer.
Fortunately, zinc oxide blocks virtually the entire UVA and UVB spectrum. This makes it the most complete block known. Why then, is it not used in all sunscreen products?
There are a couple of answers to this. First, the recognition of zinc oxide's potential as the ultimate sunblock is a relatively recent event. Second, the traditional organic sunscreens have a tremendous amount of commercial inertia that need to be overcome. Third, even in a microfine form, zinc oxide will appear white when used in large amounts. To deal with this, the cosmetic formulators must be educated on the proper use of particulates.
In the past couple of years, the market has seen the first elegant sunscreens based on zinc oxide become available. Sometimes zinc oxide is used alone but usually it is used in combination with one of the organic sunsreens. More and more products based on zinc oxide are expected to appear in the market in the future. This is especially true given the trend towards using sunscreens in daily wear products such as moisturizers. In this type of formulation, it is particularily important to use ingredients like zinc oxide that are not only effective, but also non-irritating.
Dr. Mark Mitchnick, Medical Director, sunSmart, Inc.