Sunscreen products protect against the harmful effects of the sun’s ultraviolet rays. All of us require some exposure to sunshine—about 15 minutes of exposure a day helps the body make an adequate supply of vitamin D, which is vital for healthy bones and teeth. However, longer exposure may cause adverse effects such as skin aging and damage to cells. Specifically, the sun’s rays penetrate into the skin’s inner layers, damaging collagen and elastic fibers and causing wrinkles. Chronic exposure can cause DNA damage leading to various forms of skin cancer. The correlation between tanning and the rise of malignant melanoma is well known. Furthermore, scientists have warned that the thin layer of ozone that protects life on earth from the sun’s ultraviolet (UV) radiation is being depleted. If this trend continues, the incidence of skin cancer is expected to rise dramatically.
Sunscreens help protect against the sun’s damaging effects. But just how much protection they provide is controversial. Many experts insist that sunscreens create a false sense of security, convincing consumers to stay in the sun beyond safe limits. Amazingly, a number of studies suggest that people who use sunscreens may actually increase their risk of melanoma because they spend too much time in the sun.1 According to the American Academy of Dermatology, sunscreens are just one source of protection; others include tightly-woven clothing and wide-brimmed hats.
Loosely-woven fabrics may transmit up to 30% of ultraviolet radiation with wet fabrics such as swimsuits. Interestingly, a white T-shirt has an SPF of 5 to 9.2 Ultraviolet protective fabrics raise the SPF to 30.3 A baseball cap with a 4-inch wide circumference brim is required to cover the entire face and neck.4
The U.S. Food and Drug Administration (FDA) regulates sunscreen products as over-the-counter drugs. The final over-the-counter Drug Products Monograph on Sunscreens (Federal Register 1999: 64: 27666-27963) establishes the conditions for safety, efficacy and labeling of these products.
The sun protection factor (SPF) is defined as the dose of ultraviolet radiation (UVR) that is required to produce one minimal erythema dose (MED) on protected skin after the application of 3mg/cm2 of product divided by the UVR required to produce one MED on unprotected skin. Or, to put it another way:
SPF Value = Exposure Time Interval (MED on protected skin)/Exposure Time Interval
(MED on unprotected skin)
Thus, a water-resistant product maintains its SPF level after 40 minutes of water immersion, whereas a very water-resistant (formerly waterproof) product maintains the SPF level after 80 minutes of water immersion. A broad-spectrum or full-spectrum sunscreen provides protection in both the UVB and UVA I and UVA II ranges.
Traditionally, sunscreens have been divided into chemical absorbers and physical blockers on the basis of their mechanism of action. Chemical sunscreens are generally aromatic compounds conjugated with a carbonyl group. These chemicals absorb high-density UV rays with subsequent excitation to a higher energy state. Upon return to the ground state, the energy absorbed in the photochemical excitation will result in the emission of longer wavelength radiation. The precise nature of this emitted lower energy and longer wavelength radiation depends on the type of sunscreen chemical. The more resonance delocalization occurs, the more efficient the chemical is in absorbing harmful ultraviolet radiation.
Recent research indicates that the newer micronized forms of physical blockers may also function in part by absorption. Sometimes referred to as non-chemical sunscreens, they may be more appropriately designated as inorganic particulate sunscreen ingredients. Permissible ingredients and maximum allowable concentrations, as listed in the FDA monograph, are shown in Table I. Sunscreen ingredients can also be classified by the portion of UVR that they effectively absorb.
A Closer Look at UVB Screens
Para-aminobenzoic acid (PABA). One of the earliest and widely-used UVB absorber; it requires alcohol solvent as the dispensing vehicle. It is notorious for staining clothes and is associated with a number of adverse reactions. Ester derivatives, primarily padimate O or octyl dimethyl PABA, became more popular due to greater compatibility in a variety of cosmetic vehicles and a lower potential for staining and adverse reactions. Because of problems with PABA formulations, irritation and staining, manufacturers adopted the PABA-free claim.
Octyl methoxycinnamate. It is the most frequently-used sunscreen ingredient today. It is less potent in an order of magnitude than padimate O. Diethanolamine methoxycinnamate is a water-soluble cinnamate derivative.
Octyl salicylate. When used in combination with other sunscreens, it boosts UVB protection. Salicylates are weak UVB absorbers with a good safety record.
Octocrylene. Used in combination with other UV absorbers to achieve higher SPF numbers. Octocrylene used in combination with other sunscreen ingredients, such as avobenzone, may add to the overall stability of these ingredients in a specific formula.
Phenylbenzimidazole sulphonic acid. Most chemical sunscreens are oils and are soluble only in the oil phase of emulsion systems, accounting, in part, for the heavy and greasy oily aesthetics of many of these products. It is water-soluble and is included in sunscreen products to impart lighter and less oily feel in daily use moisturizers. It is a selective UVB filter, allowing nearly all UVA transmission. It is a known sunscreen because of its broad spectrum action.
|Table I: FDA Sunscreen Final Monograph Ingredients|
|Drug Name||Concentration %||Absorbance|
|Aminobenzoic acid||up to 15||UVB|
|Cinoxate||up to 3||UVB|
|Dioxybenzone II||up to 3||UVB, UVA I & II|
|Homosalate||up to 15||UVB|
|Menthyl anthranilate||up to 5||UVA II|
|Octyl methoxycinnamate||up to 7.5||UVB|
|Octisalate||up to 5||UVB|
|Oxybenzone||up to 5||UVB|
|Padimate O||up to 8||UVB|
|Phenylbenzimidazole sulfonic acid||up to 4||UVB|
|Sulisobenzone||up to 10||UVB, UVA II|
|Trolamine salicylate||up to 12||UVB|
Oxybenzone. Although benzophenones are primarily UVB absorbers, oxybenzone absorbs well through UVA II. Oxybenzone can be considered a broad-spectrum absorber. It significantly augments UVB protection when used in a given formula.
Menthyl anthranilate. An- thranilates are weak UVB filters, and they absorb mainly in the near UVA portion of the spectrum. Anthranilates are less effective in this range than benzophenones, and they are not used frequently.
Avobenzone. Butyl methoxydibenzoylmethane (Parsol 1789) provides superior protection through a larger portion of the UVA range, including UVA-I. It is a significant addition to the sunscreen products for true broad-spectrum UV protection, however concerns have been raised regarding its photostability and its potential to degrade other sunscreen ingredients.
Ultrafine titanium dioxide and zinc oxide are two of the best known examples of physical blockers. They are chemically inert, and absorb or reflect the full UV spectrum.
Despite advances in the technology, formulating with these ingredients can be tricky. Zinc oxide is less whitening than titanium dioxide, and it may provide better UVA-I protection. Hybrid products containing a combination of chemical UV absorbers with inorganic particulate sunscreens represent a better compromise.
A Regulatory Minefield
In 1978, Congress ordered FDA to issue regulations on the prevention and treatment of sunburn. The agency completed the sunscreen monograph in May 1999 to meet the congressional mandate, and it initially gave the industry until May 2001 to implement it. The new regulation established a list of active ingredients that companies can use in sunscreen products and simplified labels so consumers will know how to use these products properly.
The monograph also pertains to sunscreen containing cosmetics. But questions about the harmfulness of a certain type of ultraviolet light and how to best label sunscreens led The Food and Drug Administration to delay these regulations until further studies could be conducted. The FDA has delayed the implementation of the so-called final monograph on sunscreens. Although the FDA delayed implementation of its new rules on sunscreens, one new requirement in the monograph for over-the-counter sunscreen products went into effect on May 22, 2000, as originally scheduled. This regulation requires all tanning products that do not contain sunscreen to bear the following warning:
“WARNING—This product does not contain a sunscreen and does not protect against sunburn. Repeated exposure of unprotected skin while tanning may increase the risk of skin aging, skin cancer, and other harmful effects to the skin even if you do not burn.”
Tanning products that do not contain sunscreens and do not protect against the harmful effects of UV light are regulated as cosmetics. FDA requires this warning statement so that consumers are fully informed that such products do not provide protection from the sun.
There is much to be learned from the natural world around us. For example, the topmost leaves of plants do not get sunburned despite hours of exposure. Even more extreme are those desert plants that grow in white reflective sand and “catch rays” for years without showing signs of damage. What does the plant possess that is so effective against solar ultraviolet rays? UV light causes DNA damage so rapidly that it sterilizes bacterial cultures within minutes, and yet the chromosomes, chloroplasts and mitochondria of the leaves go unscathed for years. How can this be?
Two possible adaptations have been discovered by scientific research. One is that plants possess a marvelously efficient DNA-repair system, and the plant surface is coated with superlative sunscreen.
A second alternative has been substantiated by certain experiments.5 There are two theories that either the leaves reflect UV light, absorb it or perform a combination of both. Until now the plant behavior regarding protection from UV rays has not yet been revealed.
Phytoplankton protect themselves against the damaging effects of UV with enzymes or unknown compounds. Phytoplankton are tiny microscopic algae that grow in oceans and lakes. These require the sun for photosynthesis. Phytoplankton contain enzymes that can withstand bombardment from UV radiation. These plants form the basis of the marine food chain. Scientists at the Plymouth Marine Laboratory in the UK have worked with The Boots Company to develop skin care products based on these natural sunscreens. Sunscreens from algae is one of many science achievements featured in the Annual Report of Natural Environment Research, published July 19, 2001.
Six frequently used UVA and UVB sunscreens were tested by the Institute of Pharmacology and Toxicology at the University of Zurch, Switzerland for estrogenicity, and reported in Environmental Health Perspective February 28, 2001.
Chemicals that are estrogenic can be considered endocrine disrupters, fooling the body into believing that they are natural hormones. Chemicals belonging to this group include:
• homosalate (HMS);
• 4-Methyl-benzylidene camphor (4-MBC);
• octyl-methoxycinnamate (OMC);
• octyl-dimethyl-PABA (OD-PABA) and
• butyl-methoxydibenzoylmethane (B-MDM).
The first five of these chemicals, found in sunscreens of every type, are estrogenic, whereas the sixth, B-MDM, is inactive. Researchers concluded that there was enough evidence of estrogenicity to warrant further studies. Abiding by the precautionary principle, it makes sense to choose products free of these chemicals until more research has been completed into possible long-term effects in humans and wildlife.
Chemical-free and natural sunscreens use titanium dioxide, zinc oxide, talc, red petrolatum and PABA. They are physical barrier-type sunscreens and generally less irritating. Zinc oxide and titanium dioxide protect against UVA and UVB rays. Physical sunscreens containing zinc oxide or titanium dioxide are less allergenic. 6,7
Zinc oxide and titanium dioxide tend to reflect sunlight across the whole spectrum from visible light to infra-red radiation. They tend to be opaque particulate products and thus can be seen when applied to the skin.
Sun care products contain the sunscreen active ingredients as well as a vast number of chemical additives necessary to ensure stability. The vehicle type is of paramount importance for determining sunscreen efficacy and aesthetics. Ingredients such as film formers, emollients and surfactants have a profound effect on the strength of UV absorbance by the active ingredient and which wavelengths they absorb.
The Proper Vehicle
All the additives add to the cost of the product and it does not mean an expensive product will be an efficient sunscreen product. In the laboratory, UV radiation from a fixed source is applied to the back of human volunteers. The dose necessary to cause minimal erythema is calculated. Because the UV lamps used in the testing procedure have a fixed output, the dose delivered becomes a function of the time for which the skin is exposed to the radiation source. In practice, the dose is changed by changing the time of exposure. The efficacy of sunscreen within the UVB range is assessed according to a sun protection factor (SPF) rating.
The SPF rating of a sunscreen product is derived from the proportional increase in the dose required to cause minimal erythema. Thus a product which requires four times the dose of UVB to produce minimal erythema is labeled an SPF 4. For a product to be labeled SPF 4, it must reduce the erythemally active incident UVB dose by 75%.
This is a laboratory measure produced under carefully controlled conditions. These conditions are difficult to reproduce in everyday life and therefore the number is not produced with the aim of direct application for daily use, rather, it is a laboratory grading.
In order to achieve a broad spectrum of protection, there should also be an accompanying filtration of the product in the UVA range. The assessment techniques of UVA filter are in question because of the very long time necessary to deliver a dose of UVA to volunteers sufficient to cause minimal erythema.
|Fair-skinned consumers can suffer the most from UV damage.|
It is not possible to create a UVA rating similar to that which measures UVB. It would be necessary to keep the volunteer in front of the radiation source for several days! For this reason in vitro or physical filtration methods are currently used in Australia and the U.S.8 Similar procedures have been adapted in many other countries. However, a number of other methods are also available.
This involves measuring the proportion of incident UVB and UVA which penetrates through a thin film layer of the sunscreen under examination. Thus the measurement is a reading of the total amount of a radiation penetrating, rather than the biologically active or erythemal radiation dose as is the case in the UVB testing.
There has been considerable discussion about which particular SPF number is adequate and concerns about the accuracy of the testing method in the high SPF range. It is unlikely that there is substantial improvement in benefits from a product with higher SPF number. Nevertheless, in the U.S., there is no ceiling on SPF levels. Products have been developed and promoted publicly with SPF ratings of 50, 60 or higher. The Food and Drug Administration in the U.S. has expressed concern about the misleading nature of these high SPF numbers. Following considerable discussion, the FDA released a statement recommending that there be a sealing on the labeling and promotion of SPF 30 numbers in the U.S.9 This SPF ceiling is similar to Australia’s standards.
Several theories propose that skin cancer in humans develop as a result of exposure to the whole spectrum of sunlight. A choice of a sunscreen containing both UVB and UVA blockers likely provides maximum protection. Most experts agree that SPF (minimum 15) and the frequency of application are of paramount importance. Products are frequently applied in insufficient amounts; in a manner that provides inadequate coverage in real use. Even if they are applied correctly, sunscreen products are frequently removed by constant touching, rubbing, sweating, swimming or other similar activities leading to a reduction in the amount of sunscreen applied. For this reason, recommendations regarding sunscreen applications include product application at least 15 minutes before going outdoors. It is also recommended that they are to be reapplied every one to two hours as necessary to remain outdoors in the sun for that time.10,11
Sunscreens with an SPF rating are not complete blockouts. They allow the penetration of a proportion of UVR. In doing so, the radiation gradually increases their cumulative dose over a given time period. It has been established that once erythema occurs, reapplying a sunscreen will not prevent further sunburn. Leaving the sun is the recommendation once erythema is developed, rather than reapplication of a strong sunscreen. Some experts recommend a very high SPF number to compensate for inadequate application.12
Due to an increase in melanoma cases worldwide, manufacturers should educate customers about the proper way to apply sunscreens, rather than rely on costly products with high concentrations or SPF.
Some Potential Problems
The most common skin problem associated with sunscreen is irritation, which may include an itchy rash that may lead to eczema and pruritus. This is a non-allergic inflammatory response that is seen most frequently around the eye.13 True allergic contact dermatitis is very rare.14 However, allergic reactions in the form of contact dermatitis have been observed significantly and more frequently with perfumes, preservatives and other ingredients in the formula.
Less common skin reactions include contact urticaria- and acne-forming reactions. Concern has been expressed that vitamin D deficiency could be produced by regular application of sunscreens, particular in people whose diets are low in vitamin D-containing foods. Although this is a theoretical possibility, no study has yet demonstrated a decrease in vitamin D. In fact, a prospective study in Maryborough (Australia) revealed that vitamin D levels in users of an SPF 17 sunscreen were exactly the same as those in users of the base cream from this sunscreen product. The effect of regular sunscreen use on vitamin D levels was studied in Australia.15
The skin of infants has the potential to absorb a greater proportion of chemicals than adult skin. This led to a concern about the use of sunscreens at a very young age. For this reason, it has been suggested that sunscreen use be limited to children over the age of six months. In practice, there has never been any report of an adverse reaction occurring in infants as a result of absorption of sunscreen chemicals. When used as recommended for the small areas of skin that cannot be protected by clothing or other natural methods, such as the face, back of the hands or back of the feet, it is unlikely that there will be enough organic sunscreen absorption from these areas to constitute an excess load for hepatic metabolism.
As stated previously, the most common misunderstanding regarding sunscreen use is the false sense of security regarding an adequate amount of a sunscreen application compared with sun exposure. It is a common experience for people to develop areas of sunburn on skin to which sunscreen has not been adequately applied. In fact, areas are frequently missed and it is not apparent because of the transparent nature of the sunscreen products once applied to the skin. Although there have been reports of some of the sunscreen chemicals being carcinogenic, a recent review has not confirmed these fears.16
Many of the organic chemicals commonly used in sunscreen products have not been established safe for long-term human use. For example, titanium dioxide- and zinc oxide-based sunscreens are being promoted on the basis that they may be less harmful than organic sunscreen absorbers. The use of microfine titanium dioxide as a sunscreen product also has no long-term safety data.
High-quality, natural suntan and after-sun products are found in abundance at natural foods stores. From avocado oil to botanicals such as rosemary and comfrey, these ingredients soothe and protect the skin.
Some plant oils contain natural sunscreens. Sesame oil resists 30% of UV rays, while coconut, peanut, olive, and cottonseed oils block out about 20%. Mineral oil doesn’t resist any UV rays and dissolves the sebum secreted from oil glands that helps inhibit water evaporation from the skin.
Aloe vera is a well-known sun care product and is included in many natural sun care products for its soothing and healing properties. It is also known as the anti-burn plant. The gel from aloe is believed to stimulate skin and assist in new cell growth. It is used to relieve sunburn, blisters or heat rash, or is mixed with sunscreens to moisturize skin. Aloe in a highly concentrated and pure form helps alleviate immune suppression by ultraviolet radiation in mice.18
Tea tree oil is an ancient aboriginal remedy. It is an effective antiseptic, fungicide and germicide. It is a popular component of many sunscreen formulations that relieve sunburn by increasing blood flow in capillaries, bringing nutrients to damaging skin.
Walnut extract is obtained from the leaves and shells of Juglans regia L. It is said to contain a naphthoquinone that reacts with keratin, forming a “sclerojuglonic” compound. It is this reaction product that provides UV protection.19
Rhatania root extract is a light yellow, oil-soluble liquid is extracted from rhatania root, a South American shrub officially known as Krameria triandra. Spectrophotometer testing indicates that as a concentrated extract it absorbs 25-30% of the amount of UV radiation typically absorbed by octyl methoxycinnamate. Based on these readings, the extract appears to display a somewhat broader, albeit lower, absorption range, as it absorbs slightly more UV light in the 340-380nm range than the organic sunscreen does.20
Vitamin E has been commonly used as a free radical scavenger and an emollient too.
Some Formulation Tips
The vehicle used is critical for determining sunscreen efficacy and aesthetics. Ingredients such as solvents, emulsifiers/surfactants, film-formers, emollients, oils and esters can have a profound effect on the strength of UV absorbance by the active ingredients and at which wavelengths they absorb. It is also essential to improve the effectiveness of sunscreens by increasing skin accumulation of UV absorbers with minimal permeation to the systemic circulation, thereby increasing the product’s substantivity. Previous research shows that Transcutol CG (ethoxy diglycol) increases both accumulation and photoprotection of oxybenzone.
Film-formers and emulsifiers determine the nature of the film on the skin’s surface. Higher SPF products require a formula that provides a uniform and thick sunscreen film with minimum interaction of inert ingredients with the actives. Durability and water-resistance is obviously vehicle-dependent. Lastly, product aesthetics play a large role in consumer acceptance.
The most popular sunscreen vehicles are creams and lotions. Two phase oil-in-water or water-in-oil emulsion systems allow for the widest variety in formulation. Most sunscreen ingredients are lipid-soluble and are incorporated into the oil phase of the emulsion. Higher SPF products may contain 20-40% sunscreen oils, accounting for the occlusive greasy feel of many of these products. Dry lotions, often presented as sport lotions, represent the formulator’s attempt to provide a less oily product.
Gels, sticks, and aerosols are other vehicles to deliver organic sunscreens. Water- or alcohol-based gels provide less greasy aesthetics, but they rely on the more limited number of water-soluble sunscreen ingredients and are less substantive with a greater potential for irritation. Sticks readily incorporate lipid-soluble sunscreens thickened with waxes and petrolatum and are heavier on application, but they are useful for protecting limited areas, such as the lips, the nose or around the eyes. Aerosols provide some convenience on application, but they may be difficult to apply evenly, which may result in a discontinuous film.
During the past decade, sunscreens have been incorporated into a broader range of consumer products, including daily use moisturizers. The FDA monograph now distinguishes between beach and non-beach products. The availability of sunscreens in this fashion offers many advantages. Daily protection is facilitated for a large segment of the population. UV protection is encouraged by the glamour image associated with cosmetic use. Sunscreens containing moisturizers are available year-round, as opposed to seasonal beach products. Moisturizers that incorporate sunscreens are generally oil-in-water emulsions. Water-soluble sunscreen ingredients are often used to decrease the oil phase and to increase cosmetic elegance. Thanks to pigment content, foundation makeup without sunscreen generally provides a SPF of 3 or 4. By raising the level of pigments, including inorganic sunscreen (particularly titanium dioxide and zinc oxide), higher SPF can be achieved with or without the inclusion of organic sunscreen. Makeup with sunscreen has intrinsic full-spectrum UVA protection based on opacity. Chemical sunscreens are generally added to lipsticks to provide enhanced SPF protection. The inclusion of pre-dispersions of titanium dioxide or zinc oxide in emollient esters are highly recommended to be included in sunscreen products in order to provide uniform particle distribution and less whitening effects.
The growing consumer awareness of the dangers of the sun has influenced the cosmetics industry and the sun care segment in particular. The consumer wants products that enable them to stay in the sun for a longer period of time. Most products are targeted at a specific market and nearly every manufacturer has a complete range of products—sunless tanners, high SPF sunblocks, tanning lotions. The most recent introductions have focused on products for children, athletes or for those who want UVA protection. These niche markets have enhanced sales for several years. In the past two years, most cosmetic companies have launched products containing sunscreens, moisturizers, antioxidants or a combination of all three.
The newest trend will be to stay continuously cool, while remaining in the sun. Cooling is achieved through evaporating water, alcohol or any other low-density vapor producing solvents or materials that leave a cooling effect on the skin. Thanks to the encapsulating technology that can deliver water or other cooling agents with intermittent release via a rub mechanism. This will provide some relief to the body from very high temperature in the sun without compromising SPF numbers. Another form is an intermittent spray/water-based thin emulsion, containing sunscreens, menthol and cooling esters with continuous intact SPF number.
In addition, another form can be hydro-alcohol-based systems, with silicone and cooling esters in a thin light gel vehicle and a fresh fragrance note. This will reinforce the SPF and at the same time keep the body temperature cool and refreshed.
The identification of naturally-derived sunscreens from the herbs is not impossible, and it requires more concerted efforts. We have plants available that have given indications of protecting themselves from intense heat and UVR from the sun. We know at this time no material is available from plants or marine life that can be called a true sunscreen. However, we cannot rule out this option for future product development.
Some of the current technology partially replaces the chemical sunscreens or reduces the whitening effect of titanium dioxide or zinc oxide. Encapsulated melanin has been a good example. Nevertheless, more research trials are needed.
Another area that requires more attention is to improve the effectiveness of sunscreens by increasing skin accumulation of UV absorbers with minimal permeation to the systemic circulation. This can be achieved in increasing the product’s substantivity. A good example, Transcutol CG, increases both accumulation and photoprotection of oxybenzone (unpublished research data). In this regard many of the newest polymers and sunscreens compatible esters can be tested. An ideal product with cosmetic appeal can be constructed in the form of a thin sprayable gel with a film forming polymer combined with emollients and cooling ingredients.
Looking for a new supplier of UV absorbers and blockers? A list of them begins on p. 54 in the print version of Happi.