Summer fun ... and trouble

May 2004

With the summer months approaching, this special issue’s Pharmacology Consult will review protection against two common summer-related dangers — the sun and biting insects. These subjects were last reviewed several years ago in Pharmacology Consult, and changes in treatment recommendations and product selection will be highlighted.

Sun protection

Although no data from clinical trials exist that sunscreens prevent skin cancer, professional health organizations continue to recommend their use, including the American Cancer Society, the American Academy of Dermatology and the AAP. Sunscreen use is one part of a complete sun protection program, which should also include avoidance of peak sun activity and use of hats and protective clothing. Ultraviolet radiation (UVR) most damaging to human skin includes UVA and UVB, with energy wavelengths of 320-400 nm and 290-320 nm, respectively. UVB is most active at producing erythema and sunburn; UVA is less likely to produce sunburn, although it can still promote skin damage and is more responsible for aggravating photosensitivity disorders. UVR may cause not only skin cancers but also premature skin wrinkling and discoloration. Approximately 80% of lifetime sun exposure occurs prior to 18 years of age. Periods of acute high exposure to UVR (ie, sunburns) in childhood and adolescence increase the risk of melanoma development in adult life.

Geller and colleagues published an interesting study in 2002 in which adolescents (12 to 18 years of age) were surveyed (1999) by a self-report questionnaire on their use of sunscreen, tanning beds and sunburning. More than 10,000 males and females were included from all 50 states. The prevalence of sunscreen use was 34.4%, while 9.5% had used a tanning bed in the previous year. Eighty-three percent of patients reported at least one sunburn during the previous summer, and 36% reported three or more sunburns. The study investigators concluded that many children are at significant risk of skin cancer because of these practices.

Sunscreen products

Numerous over-the-counter (OTC) sunscreens are available to consumers for use in children and adults. These products contain at least one of 16 sunscreen active ingredients. Most of these ingredients function to absorb UVR, thus blocking its transmission to the epidermis, while two ingredients (titanium dioxide and zinc oxide) function as physical sunscreens, by reflecting UVR. Consumers are most likely to be familiar with a sunscreen’s SPF rating – sun protection factor — and are likely to choose a specific product based largely upon this characteristic. SPF is defined as the ratio of the minimal erythema dose (UVR required to produce perceptible redness) with use of a sunscreen to the minimal erythema dose without sunscreen use. Thus, use of sunscreen provides protection for a defined period, as determined in part by the SPF. Use of a sunscreen with SPF 15 would allow one to be exposed to the sun’s rays for 150 minutes before development of erythema if the same degree of erythema would develop in 10 minutes without use of sunscreen. While the product’s SPF is important, consumers and clinicians should understand the implications of relying solely upon SPF ratings for product efficacy and sun protection.

Consumers may believe a higher SPF rating equates with greater sun protection, a belief that is partly true, yet with significant limitations. Products with higher SPF primarily provide a longer protection and may not inherently provide a higher quality of protection as compared with products with lower SPF ratings.

The FDA recently limited the SPF rating to a maximum of “30+”, with the intention that a SPF 15-30 product should provide protection for most sunscreen users, including those who sunburn easily. Additional reasoning for this limitation included the relatively small increase in UVR protection with higher SPF ratings – SPF 15 absorbs 93% UVR, SPF 30 absorbs 96.7% UVR, SPF 40 absorbs 97.5% UVR.

It is important to consider other characteristics of sunscreen products when choosing a product and promoting its effective use. SPF ratings correspond to protection against UVB, the principal cause of sunburn. Protection against UVA is also important, as UVA additionally contributes to skin damage and may produce photosensitivity reactions. Not all of the active ingredients in sunscreen products provide broad protection against UVB and UVA. Many products include more than one ingredient to provide broad UVB and UVA protection and thus are more protective (such products will often state this on the product label). Titanium dioxide and zinc oxide provide the broadest protection (290-770 nm), and products containing these ingredients in more cosmetically acceptable formulations are becoming more widely available.

Perhaps the most important consideration for the effective use of a sunscreen relates not to product characteristics, such as SPF rating, but to proper application and use. The most common means whereby consumers misuse sunscreens are application of inadequate amounts and infrequent application. The average adult should apply nine portions of approximately ½ teaspoonful each (about 1 oz in total) to body parts likely to be exposed to the sun. While younger children may require less, this adult dose estimation is still useful. Application of an insufficient amount of sunscreen can decrease a product’s SPF rating by 50% (eg, application of too little SPF 15 may result in SPF 7-8 protection). Sunscreens should also be applied 30 minutes prior to sun exposure, as this time is necessary for the active ingredients to bind to the skin. Sunscreen should then be reapplied every two hours, regardless of the product SPF rating, as activity, sweating and other factors may diminish the protective effects of applied sunscreen. By FDA definition, “water-resistant” or “sweat-resistant” products retain their SPF rating for 40 minutes, and thus frequent reapplication is still necessary. “Very water-resistant” products retain activity for 80 minutes. It is also helpful to use sunscreens on cloudy days, as UVR may be decreased by only 20% to 40%.

The application of sunscreens to infants younger than 6 months retains some degree of controversy, as the effects of topical application of a chemical substance upon the young infant are not well known. Because direct evidence linking sunscreen use and toxicity does not exist for young infants, the Australian Cancer Society recommends that infants younger than 6 months be treated with sunscreen. (Australia has the highest rate of melanoma.) The AAP’s recommendations are similar – sunscreen can be applied to infants younger than 6 months as part of a complete sun-protection plan (avoid direct sun, use protective clothing, among other sun-protective means).

Insect repellents

DEET (N,N-diethyl-meta-tolumide) remains the most effective insect repellent available OTC. Products differ by the concentration of DEET, ranging from 7% to 100%. The superior efficacy of DEET has been shown in several published studies. Products containing other non-DEET active ingredients (eg, citronella, eucalyptus oil, soy bean oil) are available and may be advertised as “natural.” While these agents afford some protection against mosquitoes, their duration of protection is not likely to be as great as compared with DEET-containing products. DEET provides activity against many species of mosquitoes, ticks, fleas and biting flies.

Recent changes affecting the use of DEET in children include current recommendations by the AAP that DEET concentrations up to 30% may be used in children, as 30% DEET appears equally safe as 10% DEET (the prior recommended maximum concentration for pediatric use). As an increase in DEET concentration affects primarily duration of action, products with 30% DEET should provide protection for five to six hours. Duration of action does not appear to increase significantly more with DEET concentrations above 30% to 50%.

While some published data exist describing the potential toxicity of DEET, much of these data relate to prolonged or excessive use of DEET, in addition to toxicity from accidental oral ingestion. Toxicity data from more appropriate use is limited and not clear as to causation. The Environmental Protection Agency (EPA), a regulating body of DEET use, has concluded that DEET is safe when applied in appropriate concentrations, and that lower concentrations (<30%) are not necessarily safer for children.

An additional measure that may be useful to repel insects is application of permethrin to clothing, tents and sleeping bags. Permethrin (an insecticide, not a repellent) is available as a spray (Repel Permanone, WPC Brands) for this use.

For more information:

• Committee on Environmental Health, AAP. Ultraviolet light: a hazard to children. Pediatrics. 1999;104:328-333.
• Geller AC. Use of sunscreen, sunburning rates and tanning bed use among more than 10,000 U.S. children and adolescents. Pediatrics. 2002;109:1009-1014.
• Bug off! How to repel biting insects. Consumer Reports. 1993;451-454.
• Committee on Environmental Health, AAP. Follow safety precautions when using DEET on children. www.aap.org/family/wnv-jun03.htm; 2003.
• Insect repellents. Med Lett Drugs Ther. 2003;45:41-42.
• Oransky S. Seizures temporally associated with use of DEET insect repellent – New York and Connecticut. MMWR. 1989;38:678-690.
• Fradin MS. Comparative efficacy of insect repellents against mosquito bites. N Engl J Med. 2002:347:13-18.