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Sunlight & Radiation

The Chemistry of Sunscreen

Physical Blockers

Chemical Filters

Cell Protectants

SPF rating does not adequately measure protection from all the damaging radiation effects of light. SPF is only a determination of protection from one specific wavelength of ultraviolet radiation, the UVB (290nm - 320nm).* Unfortunately, there is no currently approved standard to rate the quality of a sunscreen's UVA protective capabilities. UVA (320nm - 400nm) is the deeper penetrating wavelength more often associated with skin changes of wrinkling, pigmentation, and long term damage. The SPF rating system does not accurately or completely define a sunscreen's protective capabilities from any other harmful ultraviolet radiation, except the UVB wavelength.

UVB creates a red, painful irritation first experienced during early sun exposure, but UVB is not the only ultraviolet wavelength damaging to the skin. In fact, UVB has only a minimal effect upon the deeper depth of skin. UVB and UVA radiation are both recognized as causing skin cancer. SPF 30 is not enough.


SUNLIGHT'S DAMAGING RADIATION

Skin is our largest organ and is an integral part of our immune system, so it is critical to protect our skin from any injury, but in particular, damage from sunlight. In many cases, skin is our primary line of defense against external trauma and environmental insult. When our skin is damaged, our immune system is weakened. While the skin is an effective barrier against many environmental insults, its natural protective capacity against radiation is skin type dependent. Furthermore, the sun's UV rays have a tremendous immunosuppressive effect and are known to cause skin cancer.

Sunlight consists of five forms of radiation ranging from wavelengths of 100 nanometers (nm)* to beyond one million nm (infinity). These radiation wavelengths are what cause excessive pigment changes, pre-cancerous and cancerous skin lesions, wrinkles and skin aging, along with triggering other adverse lightsensitive responses. Furthermore, there are two forms of radiation emitted from artificial sources (mercury vapor lamps and welding arcs) that play a role in damaging skin.

* one billionth of a meter

Sunlight's Five Forms of Radiation

Ultraviolet C (UVC) - 100 - 290nm
UVC wavelengths are the shortest ultraviolet rays, extending from 100nm to 290nm, and are the most carcinogenic. While the sun generates ultraviolet C, the atmospheric ozone layer screens out virtually all UVC from reaching us. However, Ultraviolet C may become increasingly problematic for those living at high altitudes. If the depletion of the ozone layer through pollution continues, the consequences will be life-threatening on a large-scale. UVC is severely photo damaging to the skin, resulting in skin burn with exposure. Artificial sources, such as some mercury arc-welding units, and germicidal lamps emit ultraviolet C. These wavelengths can very efficiently kill germs, giving rise to their common name, 'germicidal waves'.

Ultraviolet B (UVB) - 290nm - 320nm
The current SPF rating system addresses only this specific wavelength. UVB is the intermediate wavelength of Ultraviolet rays, and causes the initial appearance of redness, commonly called 'sunburn.' UVB creates painful irritation, but is believed by many to be less damaging than UVA (320nm-400nm), which causes the pigmentation changes associated with tanning. UVB primarily damages the outer most layer of the skin, the epidermis. The result is skin redness and thickening of the the stratum corneum, (our body's attempt to reduce UVB impact on the epidermis). Excessive exposure to UVB is the foremost promoter of premature aging of the skin. This type of damage is cumulative, potentially resulting in basal cell and squamous cell cancers.

Ultraviolet A (UVA) - (320nanometers - 400nm)
The longer UVA wavelengths were once thought of as essentially harmless, contributing only to a 'healthy tan.' Scientific evidence now indicates that this is not true. UVA induces cutaneous photo damage, usually seen as dryness, uneven pigmentation, inflammation, skin darkening (tanning), and fine wrinkles along with skin cancer. Since even a low dose of UVA can penetrate to the underlying dermis, resulting photodamage will cause wrinkles and sagging skin. Furthermore, UVA adversely affects the deep dermis far more than the superficial 'sunburn' caused by UVB rays, resulting in a loss of the elastic quality of its supportive collagen, causing premature aging. Unlike the shorter UVB (290-320nm) wavelengths, UVA easily penetrates window glass.

Interestingly, the amount of UVA reaching the earth, unlike UVB, retains essentially the same energy level every day of the year, morning, noon, and afternoon. Deeply penetrating UVA radiation presents the same damaging effect to the skin in mid-December at 9 am as it does in mid-July at 4 pm. Therefore, those of us with sensitivities to light, whether the cause is genetic, disease related (such as Lupus and Rosacea), drug related (as with certain antibiotics and diuretics) or related to photodynamic therapy (PDT), need year-round, everyday, morning-to-night protection from all forms of light.

It is estimated that 10 to 12 times more UVA than UVB reaches the earth's surface at sea level. UVA protection is not numerically addressed although you may see UVA and broad-spectrum protection on a package. Remember that broad spectrum is not total spectrum protection. The SPF rating system does not predict the ability of sunscreens to block UVA wavelengths.

The most important aspect of UVA is the cumulative tissue damage that results from these deeply penetrating UV rays. Studies to date support the relationship of such UV exposure to the development of basal and squamous cell cancers, as well as pre-cancerous lesions. Recently, it has been reported that depletion of Vitamin A in the skin by UVA exposure may contribute to both photo aging and cancers of the skin.

Visible Light (400nm - 760nm)
Nearly 50% of the sun's radiation reaching us at sea level is within the visible range. As the name describes, these are the wavelengths that humans can see (violet, indigo, blue, green, yellow, orange, red.). Distributed from approximately 400nm to 760nm, the energy level of visible light is lower than that of the ultraviolet wavelength. Prestigious journals such as, the 'Journal of Investigative Dermatology', 'Cancer Research', and the 'British Journal of Dermatology', have published reports showing that visible light is capable of precipitating phototoxic reactions, promoting DNA cross-linking and enhancing tumor growth. This lower energy has the ability to penetrate the skin deeper than UVA, reaching down within the dermis. Adverse skin reactions can occur within this visible light wavelength. It is a misconception to think of visible light as being harmless to human skin. An indication of the significance of visible light as an active wavelength is its current use for multiple Photo Dynamic Therapies (PDTs), which are used for the treatment of esophageal cancer, certain lung cancers and premalignant skin cancer.

Infrared- 'IR' (greater than 760nm to 1,000,000nm)
Infrared goes from above 760nm to infinity, but most of the energy is from 760nm to about 1800nm, comprising more than 40% of the sun's rays reaching us at sea level. These wavelengths warm us when we stand in the sun (perceived as deeply penetrating heat), and are emitted by stoves, furnaces, light bulbs, heat lamps, ovens, and space heaters. A number of studies have implicated infrared waves as photodamaging. Infrared has been known to cause cancer, such as Kang Cancer in China, Kangri in Kashmir, Kairo in Japan, and Peat Fire Cancer in Ireland. Chronic exposure to infrared light leads to mottled pigmentation, loss of elastin, (elastosis) and the typical characteristics seen in photo-aged skin (wrinkling, sagging, leathery-feel).


THE CHEMISTRY OF SUNSCREEN

Sunscreen: (snskrn) n. Chemical or physical agents that protect the skin from sunburn and erythema by absorbing or blocking ultraviolet radiation. A preparation, often in the form of a cream or lotion, used to protect the skin from the ultraviolet rays of the sun.

Sunscreens use chemical absorbers and/or physical blockers formulated to protect the skin. This section provides brief technical descriptions of how sunscreens work.


I. PHYSICAL BLOCKERS

Physical blockers fall into three categories:

  • Direct physical blockers
  • Indirect blockers that assist by increasing distribution of direct blockers
  • Polymers, often starch derived, that substantially increase the effective length of the pathway that the sun's rays must travel to reach the skin.

Direct Physical Photoblockers

Most of the physical photoblockers are compounds of metals (iron, chromium, zinc, titanium, etc) that occur naturally, while some, such as bismuth are man-made. In addition to their photoprotective attributes, these substances also assist in preventing windburns and skin damage from wind driven micro particles of dirt and grime. An additional significant property of these physical blockers is their ability to offer a defense against infrared ('heat') rays by two distinct means.

First, particles large enough to be visible (i.e. reflect visible light) will also reflect and refract infrared waves most harmful to skin (760nm - 1,800nm). Second, regardless of their particle size, these metal-based materials act as a 'heat sink' and thereby reduce the heat effect on the skin.

Three important photoprotective blockers are discussed in this section.

Titanium Dioxide

This white pigment powder is widely used in cosmetics. The purpose of large particle titanium is to give opacity to the products containing it, and to lighten (or whiten) their color. Opaque titanium dioxide highly reflects and strongly scatters all UV and visible rays. It also reflects much of the skin-damaging infrared waves, keeping the skin cooler, reducing 'heat' damage and its subsequent photoaging.

To photo-stabilize titanium dioxide, it must be micro-coated with its own protectant such as silicone or aluminum oxide. An alternate procedure to inhibit breakdown is to incorporate other appropriate blockers together with titanium dioxide since titanium dioxide spreads poorly on the skin. To achieve cosmetic elegance and usefulness, microcoating the titanium dioxide is common; designing a vehicle to assure good, even application to the skin is essential. Large particle titanium dioxide products produce a very white, opaque appearance on the skin when applied. Therefore, submicronizing the titanium dioxide powder creates small particles to absorb visible light, enabling products to be offered as sun protectors that help protect the skin from most UVB and some UVA, but are invisible on the skin.

Transparent (sub-micronized) titanium dioxide works by absorbing, reflecting and scattering UVB and some UVA rays. However, protection against UV, visible and infrared is significantly limited when submicronized titanium dioxide is the primary protectant.

Zinc Oxide

Zinc Oxide has been known and used topically for centuries as a skin protectant and wound healing adjuvant and is a recognized mild antimicrobial agent. More than 50 years ago, zinc oxide was indicated as a block for ultraviolet light (UVB/UVA). It also reflects infrared from the skin, as does titanium dioxide. However, its ability to protect in the long UVA range, (300 - 400 nm) is much higher than titanium dioxide. Zinc oxide absorbs, rather than scatters, most UVA, while titanium dioxide primarily scatters these wavelengths. Thus, formulated in combination with titanium dioxide, ultrafine zinc oxide 'closes the window' in the UVA range. Zinc oxide works to both complement titanium dioxide's protection and extend photoprotection to the skin where titanium dioxide is insufficient. The optimal particle size range for ultraviolet blocking zinc oxide (without blocking visible wavelengths) is approximately 80 to 150 nanometers (1,000 nanometers = 1 micron)

Iron Oxides

We most commonly see iron oxide in two areas; as rust on exposed iron and in cosmetics to give the cover-up color desired. While not approved by the FDA as an active ingredient in sunscreens, many companies use them in their sunscreen products. Cosmetic iron oxides are man-made to very high purity, desired color and particle size.

Iron oxide pigments for cosmetic use are micronized powders. By controlling the purity, particle size, temperature and rate of drying during manufacture, they become available in a number of shades and tones of red, yellow, black and brown (and blends of these basic colors). These cosmetic pigments, if incorporated at adequate concentration and when properly dispersed in well-designed vehicles, not only add color to the lotion (or cream, powder, etc.), but contribute significant protection of the skin from multiple wavelengths of light.

Ultra-submicronized iron oxides protect against visible light waves, but add little color to the finished product. This allows for the addition of higher levels of infrared protecting iron oxide while retaining the cosmetic elegance and shade of the final preparation. Considerable blocking of ultraviolet rays is also reported with submicronized iron oxides, complementing further the primary UV blocking agents.

Indirect Physical Blocker Aids

Examples of these particles can be natural talc or mica, and are usually flat and oval in shape. They are very small particles, though they are much larger than direct physical blockers. A portion of very small physical blocker particles will coat the larger flat talc (mica, etc.). Being flat and smooth, the coated talc will easily slide over each other, overlapping themselves and effectively increasing protective coverage on the skin.

Polymers

Polymers can be natural substances from plants, modified semi-natural, animal derived (modified chitin, from the 'shells' of shrimp etc. is commonly employed) or synthetic substances such as micronized nylon. Certain polymers, when carefully formulated into a photoprotective preparation, create a maze-like 'cage' structure that forces the ultraviolet and visible rays (100nm - 760nm) to go through a 'maze' rather than directly reaching the skin. This longer route helps to increase protection by either preventing some rays from reaching the skin (or reaching the skin with greatly reduced energy) or by increasing the contact time between the rays and the organic filters/physical blockers.

By themselves, such polymers (which incidentally also improve the feel of the cosmetic finished product on the skin) provide little to no useful skin photoprotection, but they do help to defend the skin from wind and wind-blown dirt and grime pollution particles. However in the presence of active photoprotective agents, these polymers can increase the Sun Protection Factor (SPF) by 3 to 5 points.


II. CHEMICAL ABSORBERS/ORGANIC FILTERS

Chemical Sunscreens (or Organic Filters) are usually soluble in oils or water. These filter either/or UVB and UVA irradiation to varying efficiency. No organic filter completely blocks the UVB and/or UVA rays from the skin. Further, the actual protection offered by any and all sun-protective products relates directly to their level of concentration, the film thickness applied to the skin, as well as the careful, total coverage of the exposed skin sites.

The most common chemical absorbers used in sunscreens include:

Octyl Salicylate

Salicylates are the oldest class of sunscreens, with octyl salicylate the most widely used. While it is strictly a UVB absorber, and a weak one at that, it offers several positive qualities, including: Octyl salicylate is virtually nonirritating and nonsensitizing to skin. Cosmetically, it is an easy to handle emollient 'oil' that acts as a good solvent (solubilizer) for other, solid organic sunscreens, such as the benzophenones.

Octyl Dimethyl PABA (Padimate O)

This oil-like UVB absorber is the most efficient for this ultraviolet range, absorbing best at the maximum sunburn frequencies (310nm - 312nm). It was the most popular UVB sunscreen in the United States, but adverse reports (not necessarily proven) have reduced its use. Padimate - O is a PABA derivative, but quite distinct. Today's purified material is essentially free of PABA.

Octyl Methoxycinnamate

Currently, this oily liquid is the most widely utilized organic UVB absorber in the world. It is second in efficiency to Padimate-O, but offers broader protection (300nm -315nm) in the sunburn region of UVB. It has a very good safety record and is relatively easy to formulate with. Additionally, it is moisturizing and water insoluble, adhering tenaciously to the skin.

Menthyl Anthranilate

An old and safe absorber, but overall weak, menthyl anthranilate absorbs moderately in the UVB range from about 300nm and somewhat more strongly into the UVA (up to about 340nm). It can somewhat enhance the UVB and lower (320nm to 340nm) UVA absorption of more active absorbers.

Oxybenzone (Benzophenone-3) and Sulisobenzone (Benzophenone-4)

These are closely related solid (powder) absorbers. Oxybenzone is water-insoluble, while the acid form, sulisobenzone, can be made soluble in water when it is neutralized. While these compounds are classified as UVA absorbers they are also UVB absorbers. Overall, they offer only moderate protection through both the UVB range and part of the UVA (320nm - 360nm). They are quite stable and can enhance effectiveness of stronger UVB absorbers.

Avobenzone (Parsol™1789)

This solid (powder) absorber exhibits marginal UVB and lower (320nm - 330nm) UVA absorption. It gives good UVA absorption from about 330nm to 340nm and very good absorption in the UVA range up to about 370nm, where it loses effectiveness. In addition, avobenzone can convert to its inactive form in the presence of sunlight.

Octocrylene

An emollient, water resistant UVB/UVA absorber; while octocrylene is a relatively weak sunscreen, it gives some protection in the UVB and lower (320 - 350 nm) UVA range. Most important, octocrylene is a very stable absorber and both protects and augments other UV absorbers, while improving their uniform skin coating.

FDA APPROVED ACTIVE INGREDIENTS

Sec. 352.10 Sunscreen active Ingredients:

Aminobenzoic acid (PABA) up to 15 percent
Avobenzone up to 3 percent
Cinoxate up to 3 percent
Dioxbenzone up to 3 percent
Homosalate up to 15 percent
Menthyl anthranilate up to 5 percent
Octocrylene up to 10 percent
Octyl methoxycinnamate up to 7.5 percent
Octyl salicylate up to 5 percent
Oxybenzone up to 6 percent
Padimate O up to 8 percent
Phenylbenzimidazole sulfonic acid up to percent
Sulisobenzone up to 10 percent
Titanium dioxide up to 25 percent
Trolamine salicylate up to 12 percent
Zinc oxide up to 25 percent

III. CELL PROTECTANTS

Sunscreens also contain substances that help protect our skin from other unseen damage from the sun. This section discusses these substances, known as cell protectants.

Background

UVA radiation penetrates deeply into our skin and initiates oxidation processes at the cellular level. Exposure to UVA causes pigmentation changes such as tanning or burning. A variety of cell-damaging free-radical oxygen species, including superoxide (*O2), and hydroxy radicals (*OH) are released vis--vis UVA induction. Cellular damage then occurs, particularly by membrane lipids' peroxidation. Hydrogen peroxide may also form, adding to cellular damage.

The primary action of UVA is to add energy to molecules in our skin, including ubiquinone (Coenzyme Q10), that go on to interact with oxygen to produce the highly reactive oxygen forms mentioned above. These 'oxygen' moieties degrade DNA in our cells.

Evidence of UVA damage becomes visible first as sunburn (where it adds to UVB burning), then inflammation and skin darkening, and later as photoaging and skin cancers. The Skin Cancer Foundation has reported that depletion of Vitamin A in the skin by UVA exposure may contribute to both photoaging and cancers of the skin.

Active and Supportive Cellular Protection

Protecting the skin from the adverse effects of UVB and UVA is the first line of defense. For UVB (290nm - 320nm), adequate concentrations of approved 'sunscreens' will achieve a protection factor (SPF) of 30 - plus. Several chemical absorbers will give moderate (not adequate) protection against the lower-half of the UVA spectrum (i.e., 320nm - 350/360nm). The most recently approved UVA absorbing chemical, Parsol 1789 (avobenzone), gives good protection through a greater portion of the UVA region (up to approximately 370nm - 374nm), but is still incomplete and believed to be photo-unstable.

Additionally, avobenzone is reported to be photo-unstable, rapidly degrading on exposure to UV radiation. Of equal or greater concern are reports indicating that UVB sunscreens may be degraded by an avobenzone - photo-sensitized mechanism. In other words, avobenzone appears to not only rapidly lose its UVA photoprotecting ability, but may actually decrease the protection level of UVB sunscreens. These reports urge very careful formulating and thorough testing, as well as adding stabilizer molecules when/if incorporating avobenzone into a UV protecting product.

To increase protection against UVB and UVA cellular damage, physical blockers, including iron oxides, titanium dioxide and zinc oxide, as well as extenders (particles that extend the effectiveness of the smaller particles of iron oxide, titanium dioxide and zinc oxide) such as mica and talcum must be included. Of primary importance is that these physical protectants be incorporated at adequate concentration to afford complete protection for extended periods.

Supplementary cellular protectants are not intended to act as primary UV absorbers (though some may exhibit slight absorption within the UVB - UVA spectrum). Rather, they act to prevent damage to the cells directly and indirectly.

A partial list of examples of common cellular protectants used in sunscreen follows. This is not an all inclusive list; but rather, a review of cell protectants with different modes or sites of active protection.

Vitamin E

In its pure active 'natural ' state, as tocopherol, vitamin E protects products from oxidizing, but is too reactive to retain adequate activity within the skin when topically applied. Fortunately, our skin can metabolize more stable forms of vitamin E to release tocopherol where it is needed. Tocopheryl acetate and tocopheryl linoleate are among the more popular forms used in sunscreens. As an oil-soluble antioxidant, it gives considerable protection to our skins' cells. Vitamin E 'breaks' the chain-reaction of free- radicals before they can cause lipid peroxidation-induced destruction of the cellular membranes. However, it requires a regeneration agent, a substance that prevents it from being rapidly depleted. Vitamin C (see below) is one such regeneration agent.

Vitamin C

Vitamin C (ascorbic acid) is one of the most effective antioxidants available and is used in sunscreen to regenerate the lipid-soluble vitamin E (so that it retains its cellular membrane protective activity).Vitamin C is available in many forms, some of which are water-soluble (ascorbyl acid phosphate, for example), while others are lipid-soluble, such as ascorbyl palmitate. Ascorbyl palmitate, topically applied, has also been reported to exhibit some protection against UVB burns, and has anti-inflammatory activity. Combinations of vitamin C compounds with vitamin E appear to offer greater protection against cellular insult from UVB and/or UVA exposure than either antioxidant alone. Additionally, Vitamin C moderately protects against UVB photodamage as well as UVA-promoted phototoxic responses.

Beta-Carotene (B-Carotene)

This pre-cursor of vitamin A, a lipid-soluble (i.e. oil soluble) yellow-orange/orange-red pigment, is found in most vegetables. Beta-Carotene is an excellent quencher of singlet oxygen (free radical) as well as free radicals that participate in lipid peroxidation. B-Carotene has been reported to be of value in the treatment of erythropoietic protoporphyria (EPP), a disease that causes photosensitivity to upper UVA and sections of visible light (380nm - 560nm). Additionally, there is evidence that Beta-Carotene inhibits UV's promoted carcinogenesis.

Anthocyanins/Proanthocyanins

These bioflavanoid-like antioxidants are found in vegetation such as pine bark (The Maritime Pine yields a highly active proanthocyanidin, offered under the trademark Pycnogenol) and grapes. These compounds are among the most active free-radical quenchers known. Anthocyanins increase the action of ascorbates (Vitamin C) and supplement the protective qualities of tocopherol (Vitamin E). Published reports describe the ability of these highly specialized antioxidant bioflavanoids to not only potentiate vitamin C, protect cells and collagen tissue, but to strengthen blood vessels and maintain capillaries.

Selenium

Numerous medical, pharmaceutical and nutritional publications describe the ability of selenium, in very low doses, to help prevent cancer, including skin cancer, act as an anti inflammatory, and aid in cellular DNA repair. It has also been reported that selenium reduces the reactivity of skin cells to UV exposure. Complex selenium compounds, topically applied in concentrations of less than 0.05% (selenium), significantly reduce UV skin damage (manifested as less inflammation, less pigmentation, and retardation of and diminished levels of skin cancer).

Chelates

Chelates are compounds that bind metals, particularly iron, and remove them from interacting with other materials. Some chelates are formed naturally, others are synthesized. Iron chelators protect against cellular damage from free-radical(s) oxygen. A few chelating compounds are ortho-phenanthroline, edetic acid (and its salts/derivatives) and dipyridylamine. Topical chelate application prior to UV exposure is reported to reduce and/or delay visible skin wrinkling caused by UV exposure, as well as tumor formation.

Miscellaneous Photoprotective /Cell Aids

Some materials indirectly protect the skin cells from light wave damage by either maintaining the UV absorbers on the skin surface, such as octyldodecyl neopentanoate, or by forming a maze (matrix) - like film that tightly bonds to the skin surface. These materials, such as acrylates/octylpropenamide copolymer and aluminum starch octenylsuccinate, significantly lengthen the pathway of light trying to reach to skin, thereby reducing the light's ability to damage skin cells.