Richard Thomas, MD, FRCPC
Department of Dermatology and Skin Science, University of British Columbia, Vancouver, BC, Canada
There are benefits and risks to sunlight exposure of the skin. Our skin utilizes ultraviolet light to produce vitamin D and provides protection against the harmful effects of the sun’s ultraviolet light (UVL). Fair skin more easily synthesizes vitamin D but is more vulnerable to sun-induced skin cancer, compared with darker skin which is better suited for accommodating intense sunlight.1 However, there is no such thing as a “safe” suntan. This review discusses ultraviolet A (UVA) and ultraviolet B (UVB) light induced skin reactions and various aspects of sun protection with sunscreen.
UVA and UVB Light-Induced Skin Reactions
- Sunburn is produced largely by UVB exposure. Most other photosensitive reactions, such as idiopathic, phototoxic and photoallergic reactions, as well as sun rashes and the aggravation of existing conditions, are predominantly produced by UVA exposure.
- The types of UVL reactions can be categorized as short- and long-term effects (Table 1).
- Short-term effects of UVL include sunburn and idiopathic reactions to UVL (polymorphous light eruption – PLE).
- PLE is an itchy rash seen within hours of sun exposure. It may last for days, in contrast to solar urticaria which develops within minutes of exposure and lasts for hours only.1
- Further short-term effects are phototoxic and photo-allergic skin reactions, caused by a number of drugs and chemicals (e.g. tetracyclines and thiazides) which can produce either a phototoxic or photo-allergic reaction.1
- Many pre-existing conditions are triggered by UV radiation. Some of the significant ones include: rosacea; seborrheic dermatitis; melasma; discoid and systemic lupus; dermatomyositis; herpes simplex; viral exanthema; pemphigus; Darier’s disease; and porphyria.
- Long-term effects of UVL include photodamage and actinic keratosis, photo-aging and skin cancer.
- The primary environmental cause of skin cancers and aging of the skin is UVA and UVB radiation from the sun.2-5 Its damaging effects are cumulative, so daily protection throughout life is important.5-6
- Given the aging “baby boomer” population, an increase in the incidence of skin cancer is likely to occur but will not be limited to this cohort.
- The rate of melanoma in young Caucasian women (<44 years) has increased 6.1% annually, which may reflect recent trends in indoor tanning, such as by tanning beds that use mainly UVA light.7
|Comparisons||UVA (320-400nm)||UVB (290-320nm)|
|Levels||Levels are constant throughout the year||Amounts vary and increase in the summer, at noon and on the equator|
|Penetration||Penetrates into the lower dermis||Most only penetrates the epidermis|
|Levels through glass||Penetrates glass||Does not go through glass|
|Other||95% of UVL is UVA||Sun protection factor (SPF) measures UVB blockage|
|Effects on the Skin||UVA (cumulative damage)||UVB|
|Carcinogenic level||May be important in causing melanoma||More carcinogenic than UVA|
|Changes to the skin||85% of sunburn effect|
|Systemic effects||Immunosuppressive||Vitamin D production|
|Table 1. Comparison of UVA and UVB light16|
Sunscreen Application is One of the Necessary Measures for Photo-Protection
- Photoprotection should be encouraged to prevent both immediate and long-term adverse effects of excessive sun exposure.
- Minimizing skin exposure to the sun, including wearing protective clothing and seeking shade, remain the most effective means of avoiding sun-induced skin cancer.
- Because being in the sun provides a great sense of well-being, it is important to mitigate its harmful effects through the regular use of sunscreens.
- Sunscreens are either:
- physical (absorb and reflect sunlight) and are made of inorganic particles such as zinc oxide or titanium dioxide;
- chemical organic substances (absorb light energy) such as cinnamates (octinoxate), benzophenone (oxybenzone) salicylates (octisalate), octocrylene, ensulizone (phenylbenzimidazole sulfonic acid) and avobenzone (butyl methoxydibenzoylmethane).
- Sunscreen is available as a lotion, spray, gel or other topical product that absorbs or scatters and reflects part of the sun’s UV radiation on the skin to help prevent photodamage.2-4
- Originally developed to block UVB light to prevent sunburn, sunscreens inadvertently left the user unprotected against the negative effects of increased UVA exposure.
- Recent technological advances have produced a new generation of full spectrum UVA stable sunscreens, which, when used regularly, are likely to help reduce the risk of photo-aging and skin cancer.8
- Avobenzone is likely the most effective broad spectrum UVA sunscreen.9 However, by itself it is unstable when exposed to sunlight. Consequently, several different companies developed systems to stabilize it.10 A combination of diethyl 2,6-naphthalate, avobenzone and oxybenzone has been formulated under the name Helioplex™ (Neutrogena®/Johnson & Johnson) (Figure 1).11
- Another example is Ecamsule (Mexoryl™ SX, L’Oreal), a benzylidene derivative, which has a peak absorption in the UVA spectrum at 345nm.
- Many of the chemicals effective in UVA protection automatically raise the sun protection factor (SPF) value of the vehicle, as they also protect against the UVB part of the spectrum.
- An ideal sunscreen should block the whole spectrum of UVL.
Figure 1: Avobenzone spectrum of protection compared with physical screens.
Sun Protection Factor
- The SPF is a measure of the protectiveness of a sunscreen against the sun’s UV rays.
- Approximately 80% of sunburn comes from the UVB rays in sunlight and 20% is attributable to the UVA range.
- SPF is based on determining the minimum erythema dose (MED), defined as the smallest amount of energy required for triggering the erythema, in areas of sunscreen protected and unprotected skin.12,13
- Studies have shown that protection against sunburn with sunscreen is directly related to SPF levels.14 The protection provided is related to the amount of sunscreen applied to the skin.12 The SPF is calculated using 2 mg/cm2 (an equivalent of two tablespoons) of sunscreen applied to the area of interest. If the unprotected skin burns in 10 minutes, properly applied sunscreen at SPF 15 will provide protection against sunburn for 150 minutes (2 ½ hours).
- The protection provided by sunscreen is related to the amount of product applied as well as the correct selection of the sunscreen.12 Most people apply only 25-50% of the correct amount of sunscreen required, which means they do not have the level of SPF protection indicated on the label. Application of an adequate amount of sunscreen is by far the most important factor influencing efficacy.12
Adherence to Sunscreen Use
- Sunscreen compliance is, in part, related to the degree of individual awareness of the cumulative nature of sun damage, balanced against the desire to have a sun tan.
- The initial consideration when choosing a sunscreen should be sunburn prevention, using a sunscreen that offers broad spectrum UV protection.
- Good UVA protectors also tend to increase the SPF because they are broad spectrum. Choosing a cream, lotion, stick, gel or spray that feels good on the skin is critical for good adherence: it is unreasonable to expect that sunscreens will be used if they do not feel comfortable.
- Generally speaking, the lighter feeling sunscreens tend to be more popular. Physical blocks tend to give a white appearance to the skin and are not as protective with respect to UVA as organic screens. However, developments including micronization of the particles and nanotechnology have improved these types of sunscreens.15
- Daily use of sunscreen is important in people at high risk of skin cancer. At sufficient SPF levels, sunscreens are effective in protecting the skin from actinic keratoses and squamous cell carcinomas.8
- Daily use of sunscreens is also important for people who want to maintain youthful looking skin.
Potential Issues When Using Sunscreen
- A burning or stinging sensation upon application is the most frequent complaint with sunscreen application.
- Contact urticaria may develop on application of sunscreen in some individuals.
- Irritant dermatitis and allergic contact dermatitis to fragrances and preservatives may occur.
- While it has long been thought that sunscreens are the most common source of photo-allergy, this has been disputed of late.16
- Acne may be induced or exacerbated by sunscreens.16
- Individuals with oily skin need oil-free, alcohol-based gels or lighter lotions.
- The US Food & Drug Administration recently issued a warning regarding flammability of alcohol-based sunscreens, based on reports of burn incidents. The agency recommends that while applying and wearing sunscreen products labeled as flammable the wearer does not smoke and should avoid open flames from lighting cigarettes, lit cigarettes, grilling, candles or sparking materials.
Some Controversies Around Sunscreens
- There are claims that avobenzone and oxybenzone may be linked to breast cancer because of their estrogenic effects. Despite the fact that benzene rings are proficient at neutralizing free radicals they are also mutagenic.17 It should be noted that these estrogenic effects are hundreds of times weaker than those associated with soy. 17
- Controversy also exists regarding retinyl palmitate (also benzene ring) and an increased risk of skin cancer. This increase was seen in albino mice when retinyl palmitate was used at significantly high doses and combined with UV light. Experience over several decades has shown a skin cancer protective effect with the use of vitamin A products.18
- There is some concern around the use of nanotechnology in physical sunscreens related to their systemic absorption. However, it appears that the particles stick together in what is called “agglomeration,” preventing systemic absorbption.16
Vitamin D and Sunscreens
- UVB is needed for vitamin D production in the skin. While there are conflicting reports, it is thought that sunscreens, applied for intended use (according to the manufacturer’s instructions) will not be sufficient to prevent vitamin D production in most people.
- People with ethnically dark skin and the elderly are more at risk for reduced vitamin D production with the use of sunscreens.
- Additionally, obese individuals who experience fat malabsorption may also be at greater risk of reduced vitamin D absorption, as they are less able to absorb oral vitamin D. 20
The 5 Sun Protection Commandments
- Choose sunscreen with SPF 30 or more that is cosmetically acceptable to you.
- Apply 15 minutes prior to heading outdoors and re-apply every 2 hours when exposed.
- Use daily if you have a risk of skin cancer or have a photosensitive condition.
- Choose a water resistant product during recreational activity.
- Wear broad-brimmed hats plus proper sunglasses.
While sun exposure does have negative effects on the skin, modern sunscreens are safe and effective in preventing sunburn, photodamage and skin cancer. However, it must be noted that the SPF is essentially a laboratory number: in real world experience people do not typically apply the same quantity of sunscreen on the skin as is used in laboratory tests. This causes confusion and means that the protection offered is in fact less than appears on the product label. To some extent, this shortfall may be compensated for by using a higher SPF sunscreen. With respect to safety concerns around sunscreens, they have yet to be scientifically proven. Therefore, regular sunscreen use should be mandatory for those with a personal or family history of skin cancer light-induced dermatoses, or connective tissue disease. Further, individuals with fair skin should also be advised on the risk of cumulative sun exposure and the benefits of sunscreen use.
- Lim WL, Jlm Hawk. Photodermatologic disorders. Chapter 87. Dermatology textbook Bolognia et al 3rd edition.
- Schalka S, et al. Photodermatol Photoimmunol Photomed. 2009;25(4): 175-180.
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- Nicolaidou E, et al. et al. J.Cosmet Dermatol. 2006;5(4):322-327.
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- Moyal D, et al. J. Am Acad Dermatol. 2008;58(5 Suppl 2): S149–S154.
- Little EG, et al. Dermatol Clin. 2012;30(3):355-361
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- Cole C. Photodermatol Photoimmunol Photomed. 2001;17(1): 2-10
- Cole C, et al. Comparison of photoprotection efficacy and photostability of broad spectrum sunscreens. Presented at: The 64th Annual Meeting of the American Academy of Dermatology; San Francisco, CA; March 3-7, 2006. Poster #P2617.
- Faurschou A, et al. Br. J. Dermatol. 2007;156(4):716–719
- Bissonnette R, et al. J. Am Acad Dermatol. 2000;43(6):1036-1038.
- Ou-Yang H, et al. J Am Acad Dermatol. 2012; 67(6):1220-1227.
- Lodén M, et al. Br. J. Dermatol. 2011; 165(2):255-262.
- Shaw T, et al. Dermatitis. 2010; 21(4):185-198.
- Wang SQ, et al. Arch Dermatol. 2011;147(7):865-866.
- Shapiro SS, et al. J Drugs Dermatol. 2013;12(4):458-463.
- Diehl JW, et al. Dermatol Ther. 2010;23(1):48-60
- AAD recommendations (Dermatology Daily–AAD July 8, 2013). Personal communication to author July 8, 2013.