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When should sunscreen be reapplied?
When should sunscreen be reapplied? Brian L. Diffey, DSc Newcastle upon Tyne, United Kingdom Background: A common recommendation by many public health agencies is to reapply sunscreen every 2 to 3 hours. Is this recommendation effective in minimizing ultraviolet exposure of the skin during time in the sun? Objective: The purpose of this article is to determine how the time of sunscreen reapplication affects the solar ultraviolet exposure of the skin. Methods: A mathematical model was derived that took into account typical amounts of sunscreen application and sunscreen substantivity to determine how these factors, when combined with the time of sunscreen reapplication, influence the photoprotection provided by sunscreen during exposure for several hours around mid day in strong sunshine. Results: Using a sunscreen that is readily removed from the skin achieves little in the way of sun protection, no matter when it is reapplied. For sunscreens that bind moderately or well to skin, typical of modern waterproof or water-resistant products, the lowest skin exposure results from early reapplication into the sun exposure period, and not at 2 to 3 hours, after initial application. Typically reapplication of sunscreen at 20 minutes results in 60% to 85% of the ultraviolet exposure that would be received if sunscreen were reapplied at 2 hours. Conclusions: Advice given to sunscreen users should be to apply sunscreen liberally to exposed sites 15 to 30 minutes before going out into the sun, followed by reapplication of sunscreen to exposed sites 15 to 30 minutes after sun exposure begins. Further reapplication is necessary after vigorous activity that could remove sunscreen, such as swimming, toweling, or excessive sweating and rubbing. (J Am Acad Dermatol 2001;45:882-5.)
T
he protection offered by a sunscreen— defined by its sun protection factor (SPF)—is assessed after phototesting in vivo at an internationally agreed application thickness of 2 mg/cm2. However, several studies have shown that consumers apply much less than this,1-5 typically between 0.5 and 1.5 mg/cm2. Application thickness has a significant effect on protection, with most users probably achieving a mean value of between 20% and 50% of that expected from the product label as a result of common application thicknesses.6 Compounded with this is the likely variability of protection over the skin surface because of uneven
From the Regional Medical Physics Department, Newcastle General Hospital. Funding: None. Conflict of interest: None. Accepted for publication May 9, 2001. Reprints not available from author. Copyright © 2001 by the American Academy of Dermatology, Inc. 0190-9622/2001/$35.00 + 0 16/1/117385 doi:10.1067/mjd.2001.117385
882
application technique.7 Once a sunscreen has been applied to skin, its substantivity can become compromised because of factors such as immersion in water8 and abrasion with beach sand.9 Therefore the reasons for reapplying sunscreen during sun exposure are to compensate for initial underapplication so as to achieve an SPF more in line with the rated value and to replace sunscreen that may have been removed by water, vigorous toweling, or friction with clothing or sand. Inadequate application is the primary purpose for reapplication since modern waterproof or water-resistant sunscreens exhibit good sunstantivity,10,11 even after immersion in water.8,12 Guidance on sunscreen packs about reapplication is generally to “reapply frequently” or “reapply regularly.” Given considerations of cost, convenience, and human nature, it is unlikely many users will reapply sunscreens frequently or regularly. In addition, general guidance to reapply sunscreens frequently or regularly is vague and nonspecific, and there are no published data on how well consumers comply with this advice. A common recommendation by many public health agencies13 is to reapply sun-
Diffey 883
J AM ACAD DERMATOL VOLUME 45, NUMBER 6
screen every 2 to 3 hours. The analysis given herein will examine whether this more specific recommendation is effective in minimizing ultraviolet exposure of the skin for a given duration in the sun.
METHOD
plicity it is taken to be constant at 6 SED/h in these calculations because this would be a typical average skin irradiance for someone in tropical or subtropical summer sunshine around mid day. Calculations were carried out for a sunscreen that is readily removed from the skin (t ⁄ = 0.5 hour), one that is moderately removed (t ⁄ = 2 hours), and one that binds well to the skin (t ⁄ = 8 hours). In keeping with typical usage,1-5 it is assumed that sunscreen is applied at an average density (x0) of 1 mg/cm2 on each occasion. 1
Let us assume someone is intending to stay in the sun for a 4-hour period around mid day and they apply an SPF15 sunscreen at a mean thickness of x0 mg/cm2 before going outside. Sunscreen is removed from the skin by clothing or sweating such that the thickness remaining on the skin can be shown to decrease with time in an exponential manner12 with a substantivity “half-life” of t ⁄ hours. After time T hours, sunscreen is reapplied, again at a thickness of x0 mg/cm2. The thickness [x(t)] of sunscreen expressed as milligrams per square centimeter remaining on the skin at time t hours after commencing sun exposure is: 1
2
x(t) = [x0 + x(nT)].exp[–λ(t – nT)] where n is the number of times sunscreens have been reapplied by time t, and λ is the rate constant for sunscreen removal and is equal to loge2/t ⁄ . Before the first reapplication (ie, at times t < T), n is equal to zero and x(0) is also zero. The cumulative skin exposure expressed in units of standard erythema dose (SED)14* throughout the 4-hour period in the sun is expressed as follows: 1
2
4h
E(t)/SPF[x(t)]dt
SED
0
where E(t) is the erythemal irradiance incident on the skin in SED/h at time t hours after commencing sun exposure and SPF[x(t)] is the equivalent SPF when the product thickness on the skin at time t is x(t) mg/cm2. Calculations are presented herein for a sunscreen of rated SPF15; the equivalent SPF when such a sunscreen is applied at a thickness x mg/cm2 on the skin can be estimated as6: SPF(x) = 1 + 2x + 2.6x2 Although the erythemal irradiance E(t) varies with climatic, temporal, and geographic factors, for sim-
1
1
2
2
2
RESULTS An example of how sunscreen thickness and associated SPF vary during the sun exposure period is illustrated in Fig 1. In this example a product of nominal SPF15 that binds moderately to the skin (t ⁄ = 2 hours) is applied initially at 1 mg/cm2 with a second application 1 hour later. The corresponding equivalent SPF is only about 5 initially, rises to around 12 immediately after reapplication, and falls to less than 4 by the end of the 4-hour exposure period. Fig 2 shows the effect of reapplying sunscreen once during the 4-hour exposure period at times ranging from 20 to 240 minutes after the initial application. It can be seen that using a sunscreen that is readily removed from the skin (t ⁄ = 0.5 hour) achieves little in the way of sun protection, no matter when it is reapplied. The skin would receive an exposure of between 14 and 18 SED regardless when reapplication took place, an exposure sufficient to result in severe, painful sunburn. For sunscreens that bind moderately (t ⁄ = 2 hours) or well (t ⁄ = 8 hours) to skin, typical of modern waterproof or water-resistant products, the lowest skin exposure results from reapplication at 20 minutes into the 4hour period, and not at 2 to 3 hours, after initial application. Typically reapplication of sunscreen at 20 minutes results in 60% to 85% of the ultraviolet exposure that would be received if sunscreen is reapplied at 2 hours. The reason for this is that an adequate thickness of sunscreen is applied to the skin early into the sun exposure period with consequently good sun protection. For a product that binds well to skin, sunburn is unlikely to occur for reapplication at 20 minutes, but waiting until 2 hours before reapplication could result in mild erythema. 1
1
1
2
2
2
1
2
DISCUSSION *The SED is a measure of erythemal ultraviolet radiation14; it requires an exposure of about 1.5 SED to produce just perceptible reddening of skin (erythema) in the unacclimatized skin of sun-sensitive persons who burn easily and never tan (Fitzpatrick skin type I), about 2 SED in subjects who burn easily but tan minimally (skin type II), and 3 SED in subjects who will burn but tan readily (skin type III).
Application of sunscreen is just one element of a strategy aimed at controlling sun exposure, which includes also seeking shade around mid day and wearing clothing and wide-brimmed hats. Sunscreen application can be problematic. Insufficient sunscreen is normally applied to achieve the rated SPF,
884 Diffey
J AM ACAD DERMATOL DECEMBER 2001
Fig 3. Cumulative skin exposure after an application of a highly substantive (t ⁄ = 8 hours) SPF15 sunscreen at 1 mg/cm2 on each occasion during a 6-hour exposure period of mean skin irradiance 6 SED/h. ▲, One sunscreen application, time of application (0), skin exposure (SED/h) after 6 hours (8.9); , 2 sunscreen applications, time of application (0 and 20 minutes), skin exposure (SED/h) after 6 hours (3.7); ■, 3 sunscreen applications, time of application (0, 2 hours, and 4 hours), skin exposure (SED/h) after 6 hours (4.0). 1
Fig 1. Sunscreen thickness on skin (solid line; left-hand axis) and equivalent SPF (broken line; right-hand axis) for an initial application of 1 mg/cm2 SPF15 sunscreen that binds moderately to the skin (t ⁄ = 2 hours), followed by a second application of the same thickness 60 minutes later during a 4-hour exposure. 1
2
Fig 2. Skin exposure after initial application of SPF15 sunscreen at 1 mg/cm2 followed by a single reapplication at the same thickness during a 4-hour exposure period of mean skin irradiance 6 SED/h. Sunscreen substantivity “half-life”: 30 minutes (); 2 hours (■); 8 hours (▲).
and that which is applied is often spread nonuniformly, thereby resulting in some exposed skin sites achieving little or no protection. In addition, sunscreen is removed to a greater or lesser extent because of activities such as swimming and toweling. For these reasons, both manufacturers and health professionals recommend reapplication at one or more times during the period of sun exposure. In a field study11 comparing the efficacy of sunscreen reapplication regimens in children exposed to ambient sunlight, it was shown that 1 or 4 applications of a highly substantive SPF25 sunscreen were equally effective in protecting against sunburn during a 6-hour sun exposure period. However, when the sun exposure period was extended to 8 hours,
2
reapplication of sunscreen resulted in less erythema than in those body sites that had received only a single application. Calculations using the above model showed that a single reapplication of a substantive SPF15 sunscreen at 20 minutes into a 6-hour exposure period resulted in a lower cumulative skin ultraviolet dose at the end of this period than either an initial application only or reapplication at 2 and 4 hours (Fig 3). More importantly, Fig 3 shows that the benefit of early reapplication of sunscreen is greater if the exposure period is less than initially intended. This finding reinforces the need to apply an adequate quantity of sunscreen early into the sun exposure period, a conclusion that is independent of the sunscreen rated SPF or the solar ultraviolet intensity. Clearly, the magnitude of skin exposure (expressed as SED) decreases with increasing SPF and decreasing ultraviolet intensity. The data in Fig 3 also indicate that reapplication of sunscreen results in a skin exposure that is 2 to 3 times less than that achieved with a single application of sunscreen. The same increase in protection from sunburn with sunscreen reapplication was found in an experimental study using 104 adult Caucasian volunteers.15 Consumers are advised to apply sunscreen liberally or generously. However, many studies1-5 have shown that users feel uncomfortable with application at 2 mg/cm2 and prefer to apply quantities averaging approximately half this thickness. Therefore it is likely that campaigns to encourage people to apply a greater quantity of sunscreen at a single application
J AM ACAD DERMATOL VOLUME 45, NUMBER 6
will fail. An important factor in sunscreen performance is the uneven nature of the skin surface. If an opaque sunscreen is applied to the skin, particularly at low surface densities typical of practical usage, the epidermal markings become visible as the surface grooves, or sulci, are filled.16 With further application, the intervening ridges are also covered and the surface becomes more featureless. The situation is analogous to painting a wall with a textured surface when two coats of paint are almost always required for satisfactory coverage. In the same way two “coats of sunscreen” may be required for adequate protection. Advising people to reapply sunscreen early into their sun exposure period may well be more successful than at 2 to 3 hours when the reasons they have for being in the sun are more likely to make them forget. Furthermore, the analysis presented herein indicates that the common public health recommendation13 to reapply every 2 to 3 hours needs to be revised. In conclusion, it is proposed that the advice given to sunscreen users should be as follows: • Apply sunscreen liberally to exposed sites 15 to 30 minutes before going out into the sun. • Reapply sunscreen to exposed sites 15 to 30 minutes after sun exposure begins. • Further reapplication of sunscreen is necessary after vigorous activity that could remove sunscreen, such as swimming, toweling, or excessive sweating and rubbing. REFERENCES 1. Azurdia RM, Pagliaro JA, Diffey BL, Rhodes LE. Sunscreen application by photosensitive patients is inadequate for protection. Br J Dermatol 1999;140:255-8.
Diffey 885
2. Bech-Thomsen N, Wulf HC. Sunbathers’ application of sunscreen is probably inadequate to obtain the Sun Protection Factor assigned to the preparation. Photodermatol Photoimmunol Photomed 1993;9:242-4. 3. Diffey BL, Grice J.The influence of sunscreen type on photoprotection. Br J Dermatol 1997;137:103-5. 4. Gottlieb A, Bourget TD, Lowe NJ. Sunscreens: effects of amounts of application of Sun Protection Factors. In: Lowe NJ, Shaath NA, Pathak MA, editors. Sunscreens: development, evaluation, and regulatory aspects. New York: Marcel Dekker; 1997. p. 583-8. 5. Stenberg C, Larkö O. Sunscreen application and its importance for the Sun Protection Factor. Arch Dermatol 1985;121:1400-2. 6. Stokes RP, Diffey BL. How well are sunscreen users protected? Photodermatol Photoimmunol Photomed 1997;13:186-8. 7. Rhodes LE, Diffey BL. Quantitative assessment of sunscreen application technique by in vivo fluorescence spectroscopy. J Soc Cosmet Chem 1996;47:109-15. 8. Stokes RP, Diffey BL.Water resistance of sunscreen and day-care products. Br J Dermatol 1999;140:259-63. 9. Stokes RP, Diffey BL. A novel ex vivo technique to assess the sand/rub resistance of sunscreen products. Int J Cosmet Sci 2000;22:329-34. 10. Agin PP, Levine DJ. Sunscreens retain their efficacy on human skin for up to 8h after application. J Photochem Photobiol B 1992;15:371-4. 11. Odio MR, Veres DA, Goodman JJ, Irwin C, Rob LR, Martinez J, et al. Comparative efficacy of sunscreen reapplication regimens in children exposed to ambient sunlight. Photodermatol Photoimmunol Photomed 1994;10:118-25. 12. Rhodes LE, Diffey BL. Fluorescence spectroscopy: a rapid, noninvasive method for measurement of skin surface thickness of topical agents. Br J Dermatol 1997;136:12-7. 13. MacKie RM. Skin cancer. 2nd ed. London: Martin Dunitz; 1996. p. 331. 14. CIE Standard. Erythema reference action spectrum and standard erythema dose. CIE S 007/E-1998. Vienna: Commission Internationale de l’Éclairage; 1998. 15. Pruim B, Green A. Photobiological aspects of sunscreen reapplication. Australas J Dermatol 1999;40:14-8. 16. Farr PM, Diffey BL. How reliable are sunscreen protection factors? Br J Dermatol 1985;112:113-8.
T
he protection offered by a sunscreen— defined by its sun protection factor (SPF)—is assessed after phototesting in vivo at an internationally agreed application thickness of 2 mg/cm2. However, several studies have shown that consumers apply much less than this,1-5 typically between 0.5 and 1.5 mg/cm2. Application thickness has a significant effect on protection, with most users probably achieving a mean value of between 20% and 50% of that expected from the product label as a result of common application thicknesses.6 Compounded with this is the likely variability of protection over the skin surface because of uneven
From the Regional Medical Physics Department, Newcastle General Hospital. Funding: None. Conflict of interest: None. Accepted for publication May 9, 2001. Reprints not available from author. Copyright © 2001 by the American Academy of Dermatology, Inc. 0190-9622/2001/$35.00 + 0 16/1/117385 doi:10.1067/mjd.2001.117385
882
application technique.7 Once a sunscreen has been applied to skin, its substantivity can become compromised because of factors such as immersion in water8 and abrasion with beach sand.9 Therefore the reasons for reapplying sunscreen during sun exposure are to compensate for initial underapplication so as to achieve an SPF more in line with the rated value and to replace sunscreen that may have been removed by water, vigorous toweling, or friction with clothing or sand. Inadequate application is the primary purpose for reapplication since modern waterproof or water-resistant sunscreens exhibit good sunstantivity,10,11 even after immersion in water.8,12 Guidance on sunscreen packs about reapplication is generally to “reapply frequently” or “reapply regularly.” Given considerations of cost, convenience, and human nature, it is unlikely many users will reapply sunscreens frequently or regularly. In addition, general guidance to reapply sunscreens frequently or regularly is vague and nonspecific, and there are no published data on how well consumers comply with this advice. A common recommendation by many public health agencies13 is to reapply sun-
Diffey 883
J AM ACAD DERMATOL VOLUME 45, NUMBER 6
screen every 2 to 3 hours. The analysis given herein will examine whether this more specific recommendation is effective in minimizing ultraviolet exposure of the skin for a given duration in the sun.
METHOD
plicity it is taken to be constant at 6 SED/h in these calculations because this would be a typical average skin irradiance for someone in tropical or subtropical summer sunshine around mid day. Calculations were carried out for a sunscreen that is readily removed from the skin (t ⁄ = 0.5 hour), one that is moderately removed (t ⁄ = 2 hours), and one that binds well to the skin (t ⁄ = 8 hours). In keeping with typical usage,1-5 it is assumed that sunscreen is applied at an average density (x0) of 1 mg/cm2 on each occasion. 1
Let us assume someone is intending to stay in the sun for a 4-hour period around mid day and they apply an SPF15 sunscreen at a mean thickness of x0 mg/cm2 before going outside. Sunscreen is removed from the skin by clothing or sweating such that the thickness remaining on the skin can be shown to decrease with time in an exponential manner12 with a substantivity “half-life” of t ⁄ hours. After time T hours, sunscreen is reapplied, again at a thickness of x0 mg/cm2. The thickness [x(t)] of sunscreen expressed as milligrams per square centimeter remaining on the skin at time t hours after commencing sun exposure is: 1
2
x(t) = [x0 + x(nT)].exp[–λ(t – nT)] where n is the number of times sunscreens have been reapplied by time t, and λ is the rate constant for sunscreen removal and is equal to loge2/t ⁄ . Before the first reapplication (ie, at times t < T), n is equal to zero and x(0) is also zero. The cumulative skin exposure expressed in units of standard erythema dose (SED)14* throughout the 4-hour period in the sun is expressed as follows: 1
2
4h
E(t)/SPF[x(t)]dt
SED
0
where E(t) is the erythemal irradiance incident on the skin in SED/h at time t hours after commencing sun exposure and SPF[x(t)] is the equivalent SPF when the product thickness on the skin at time t is x(t) mg/cm2. Calculations are presented herein for a sunscreen of rated SPF15; the equivalent SPF when such a sunscreen is applied at a thickness x mg/cm2 on the skin can be estimated as6: SPF(x) = 1 + 2x + 2.6x2 Although the erythemal irradiance E(t) varies with climatic, temporal, and geographic factors, for sim-
1
1
2
2
2
RESULTS An example of how sunscreen thickness and associated SPF vary during the sun exposure period is illustrated in Fig 1. In this example a product of nominal SPF15 that binds moderately to the skin (t ⁄ = 2 hours) is applied initially at 1 mg/cm2 with a second application 1 hour later. The corresponding equivalent SPF is only about 5 initially, rises to around 12 immediately after reapplication, and falls to less than 4 by the end of the 4-hour exposure period. Fig 2 shows the effect of reapplying sunscreen once during the 4-hour exposure period at times ranging from 20 to 240 minutes after the initial application. It can be seen that using a sunscreen that is readily removed from the skin (t ⁄ = 0.5 hour) achieves little in the way of sun protection, no matter when it is reapplied. The skin would receive an exposure of between 14 and 18 SED regardless when reapplication took place, an exposure sufficient to result in severe, painful sunburn. For sunscreens that bind moderately (t ⁄ = 2 hours) or well (t ⁄ = 8 hours) to skin, typical of modern waterproof or water-resistant products, the lowest skin exposure results from reapplication at 20 minutes into the 4hour period, and not at 2 to 3 hours, after initial application. Typically reapplication of sunscreen at 20 minutes results in 60% to 85% of the ultraviolet exposure that would be received if sunscreen is reapplied at 2 hours. The reason for this is that an adequate thickness of sunscreen is applied to the skin early into the sun exposure period with consequently good sun protection. For a product that binds well to skin, sunburn is unlikely to occur for reapplication at 20 minutes, but waiting until 2 hours before reapplication could result in mild erythema. 1
1
1
2
2
2
1
2
DISCUSSION *The SED is a measure of erythemal ultraviolet radiation14; it requires an exposure of about 1.5 SED to produce just perceptible reddening of skin (erythema) in the unacclimatized skin of sun-sensitive persons who burn easily and never tan (Fitzpatrick skin type I), about 2 SED in subjects who burn easily but tan minimally (skin type II), and 3 SED in subjects who will burn but tan readily (skin type III).
Application of sunscreen is just one element of a strategy aimed at controlling sun exposure, which includes also seeking shade around mid day and wearing clothing and wide-brimmed hats. Sunscreen application can be problematic. Insufficient sunscreen is normally applied to achieve the rated SPF,
884 Diffey
J AM ACAD DERMATOL DECEMBER 2001
Fig 3. Cumulative skin exposure after an application of a highly substantive (t ⁄ = 8 hours) SPF15 sunscreen at 1 mg/cm2 on each occasion during a 6-hour exposure period of mean skin irradiance 6 SED/h. ▲, One sunscreen application, time of application (0), skin exposure (SED/h) after 6 hours (8.9); , 2 sunscreen applications, time of application (0 and 20 minutes), skin exposure (SED/h) after 6 hours (3.7); ■, 3 sunscreen applications, time of application (0, 2 hours, and 4 hours), skin exposure (SED/h) after 6 hours (4.0). 1
Fig 1. Sunscreen thickness on skin (solid line; left-hand axis) and equivalent SPF (broken line; right-hand axis) for an initial application of 1 mg/cm2 SPF15 sunscreen that binds moderately to the skin (t ⁄ = 2 hours), followed by a second application of the same thickness 60 minutes later during a 4-hour exposure. 1
2
Fig 2. Skin exposure after initial application of SPF15 sunscreen at 1 mg/cm2 followed by a single reapplication at the same thickness during a 4-hour exposure period of mean skin irradiance 6 SED/h. Sunscreen substantivity “half-life”: 30 minutes (); 2 hours (■); 8 hours (▲).
and that which is applied is often spread nonuniformly, thereby resulting in some exposed skin sites achieving little or no protection. In addition, sunscreen is removed to a greater or lesser extent because of activities such as swimming and toweling. For these reasons, both manufacturers and health professionals recommend reapplication at one or more times during the period of sun exposure. In a field study11 comparing the efficacy of sunscreen reapplication regimens in children exposed to ambient sunlight, it was shown that 1 or 4 applications of a highly substantive SPF25 sunscreen were equally effective in protecting against sunburn during a 6-hour sun exposure period. However, when the sun exposure period was extended to 8 hours,
2
reapplication of sunscreen resulted in less erythema than in those body sites that had received only a single application. Calculations using the above model showed that a single reapplication of a substantive SPF15 sunscreen at 20 minutes into a 6-hour exposure period resulted in a lower cumulative skin ultraviolet dose at the end of this period than either an initial application only or reapplication at 2 and 4 hours (Fig 3). More importantly, Fig 3 shows that the benefit of early reapplication of sunscreen is greater if the exposure period is less than initially intended. This finding reinforces the need to apply an adequate quantity of sunscreen early into the sun exposure period, a conclusion that is independent of the sunscreen rated SPF or the solar ultraviolet intensity. Clearly, the magnitude of skin exposure (expressed as SED) decreases with increasing SPF and decreasing ultraviolet intensity. The data in Fig 3 also indicate that reapplication of sunscreen results in a skin exposure that is 2 to 3 times less than that achieved with a single application of sunscreen. The same increase in protection from sunburn with sunscreen reapplication was found in an experimental study using 104 adult Caucasian volunteers.15 Consumers are advised to apply sunscreen liberally or generously. However, many studies1-5 have shown that users feel uncomfortable with application at 2 mg/cm2 and prefer to apply quantities averaging approximately half this thickness. Therefore it is likely that campaigns to encourage people to apply a greater quantity of sunscreen at a single application
J AM ACAD DERMATOL VOLUME 45, NUMBER 6
will fail. An important factor in sunscreen performance is the uneven nature of the skin surface. If an opaque sunscreen is applied to the skin, particularly at low surface densities typical of practical usage, the epidermal markings become visible as the surface grooves, or sulci, are filled.16 With further application, the intervening ridges are also covered and the surface becomes more featureless. The situation is analogous to painting a wall with a textured surface when two coats of paint are almost always required for satisfactory coverage. In the same way two “coats of sunscreen” may be required for adequate protection. Advising people to reapply sunscreen early into their sun exposure period may well be more successful than at 2 to 3 hours when the reasons they have for being in the sun are more likely to make them forget. Furthermore, the analysis presented herein indicates that the common public health recommendation13 to reapply every 2 to 3 hours needs to be revised. In conclusion, it is proposed that the advice given to sunscreen users should be as follows: • Apply sunscreen liberally to exposed sites 15 to 30 minutes before going out into the sun. • Reapply sunscreen to exposed sites 15 to 30 minutes after sun exposure begins. • Further reapplication of sunscreen is necessary after vigorous activity that could remove sunscreen, such as swimming, toweling, or excessive sweating and rubbing. REFERENCES 1. Azurdia RM, Pagliaro JA, Diffey BL, Rhodes LE. Sunscreen application by photosensitive patients is inadequate for protection. Br J Dermatol 1999;140:255-8.
Diffey 885
2. Bech-Thomsen N, Wulf HC. Sunbathers’ application of sunscreen is probably inadequate to obtain the Sun Protection Factor assigned to the preparation. Photodermatol Photoimmunol Photomed 1993;9:242-4. 3. Diffey BL, Grice J.The influence of sunscreen type on photoprotection. Br J Dermatol 1997;137:103-5. 4. Gottlieb A, Bourget TD, Lowe NJ. Sunscreens: effects of amounts of application of Sun Protection Factors. In: Lowe NJ, Shaath NA, Pathak MA, editors. Sunscreens: development, evaluation, and regulatory aspects. New York: Marcel Dekker; 1997. p. 583-8. 5. Stenberg C, Larkö O. Sunscreen application and its importance for the Sun Protection Factor. Arch Dermatol 1985;121:1400-2. 6. Stokes RP, Diffey BL. How well are sunscreen users protected? Photodermatol Photoimmunol Photomed 1997;13:186-8. 7. Rhodes LE, Diffey BL. Quantitative assessment of sunscreen application technique by in vivo fluorescence spectroscopy. J Soc Cosmet Chem 1996;47:109-15. 8. Stokes RP, Diffey BL.Water resistance of sunscreen and day-care products. Br J Dermatol 1999;140:259-63. 9. Stokes RP, Diffey BL. A novel ex vivo technique to assess the sand/rub resistance of sunscreen products. Int J Cosmet Sci 2000;22:329-34. 10. Agin PP, Levine DJ. Sunscreens retain their efficacy on human skin for up to 8h after application. J Photochem Photobiol B 1992;15:371-4. 11. Odio MR, Veres DA, Goodman JJ, Irwin C, Rob LR, Martinez J, et al. Comparative efficacy of sunscreen reapplication regimens in children exposed to ambient sunlight. Photodermatol Photoimmunol Photomed 1994;10:118-25. 12. Rhodes LE, Diffey BL. Fluorescence spectroscopy: a rapid, noninvasive method for measurement of skin surface thickness of topical agents. Br J Dermatol 1997;136:12-7. 13. MacKie RM. Skin cancer. 2nd ed. London: Martin Dunitz; 1996. p. 331. 14. CIE Standard. Erythema reference action spectrum and standard erythema dose. CIE S 007/E-1998. Vienna: Commission Internationale de l’Éclairage; 1998. 15. Pruim B, Green A. Photobiological aspects of sunscreen reapplication. Australas J Dermatol 1999;40:14-8. 16. Farr PM, Diffey BL. How reliable are sunscreen protection factors? Br J Dermatol 1985;112:113-8.