Photodermatology, Photoimmunology & Photomedicine
REVIEW ARTICLE
Application of sunscreen − theory and reality Bibi Petersen & Hans Christian Wulf
Department of Dermatological Research, Bispebjerg Hospital, University of Copenhagen, Copenhagen, Denmark.
Key words: sun protection factor; sunscreen application; sunscreen use
Correspondence: Miss Bibi Petersen, M.D., PhD, Department of Dermatological Research, Bispebjerg Hospital, University of Copenhagen, Bispebjerg Bakke 23, Copenhagen 2400, Denmark. Tel: +45 35316007 Fax: +45 35316010 e-mail: [email protected]
SUMMARY We present research on sunscreen use with possible pitfalls and discuss theory vs. reality. A literature review in PubMed was conducted using the terms ‘sunscreen application’, ‘sunscreen use’ and ‘sun protection factor’. The sun protection factor (SPF) of sunscreens are tested using a thickness of 2 mg/cm2, but investigations show that sunscreen under natural conditions is applied insufficiently with amounts about 0.39 to 1.0 mg/cm2, which decreases the protection factor considerably. It has been shown that early reapplication or use of very high SPF (70–100) may partly compensate for the discrepancy between the amounts of sunscreen applied during testing and in reality, and that sunscreen application can be improved by education of consumers. Missing areas and ultraviolet radiation exposure before sunscreen application are other pitfalls that reduce the protective effect of sunscreens considerably. Current sunscreen labelling overrates the protective effect of a given sunscreen when the reality of sunscreen use is taken into account. This may possibly mislead consumers to feel it is safe to extend sun exposure. Alternatively to educating people to use large amounts of sunscreen, we suggest a simple teaching strategy: (1) Apply before sun exposure and (2) Reapply once within 1 h.
Accepted for publication: 3 December 2013
Conflicts of interest: None declared.
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Photodermatol Photoimmunol Photomed 2014; 30: 96–101 There are several ways to protect the skin from solar ultraviolet radiation (UVR). The main recommendations are to seek the shade, to avoid sun exposure around noon and to apply sunscreen. Sunscreen application is usually the preferred way of sun protection. The World Health Organization (WHO) recommends that sunscreen should be applied 20 min before sun exposure with reapplication every second hour, and repeated after swimming or bathing. The protection afforded by sunscreen is expressed as the sun protection factor (SPF), which is the factor by which the sun exposure dose can be increased before skin erythema develops. The minimal erythema dose (MED) is the UVR dose needed to provoke just perceptible MED with sunscreen . erythema. The SPF is calculated from the formula: SPF = MED without susncreen According to the Guidelines from the American Food and Drug Administration (1) and the International Organization for Standardization (2) the amount of sunscreen applied when testing SPF should be 2 mg/cm2. How much of the labelled SPF is obtained when using a sunscreen is therefore dependent on the amount of sunscreen applied and whether it is spread uniformly. In reality, however, consumers apply much less sunscreen than 2 mg/cm2. External factors such as swimming, physical activity, sweating, and contact with beach sand and clothing can reduce the © 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd doi:10.1111/phpp.12099
Application of sunscreen − theory and reality
amount and thereby the effect. However, there are also studies on how to compensate for the insufficient sunscreen application layer obtaining increased protection. In this article we summarize present research on sunscreen use in theory and in reality with a discussion of future perspectives.
MAIN TEXT Application thickness The recommended sunscreen application thickness of 2 mg/cm2 for SPF testing is based on studies showing that this amount generates the best reproducible results with the lowest variation of test results (1, 3), which are needed to ensure confidence in the efficacy classification of sunscreens. The actual amount of sunscreen applied has been shown to be about 20–50% of the amount needed to achieve the labelled SPF (4–12). A simple method used to determine application thickness has been to divide the amount (grams) of sunscreen used by the sunscreen treated skin area (cm2). In two studies sunscreens were handed over to the participants and the bottles returned and weighed every third month. Diary registrations of the skin area with sunscreen application made it possible to roughly calculate the median amount of sunscreen applied, which was 0.39 and 0.79 mg/cm2, respectively (4, 9). In once-weekly evaluations children’s sunscreen use was found to be 0.48 mg/cm2 (7), and in a 1-day study on a nude beach, in which 42 women, men and children applied their own sunscreen, the application thickness was found to be 0.5 mg/cm2 by weighing sunscreen bottles before and after an unobserved full-body sunscreen application (6). A holiday study with intensive sunbathing, diary registrations and daily weighing of sunscreen bottles showed that on average volunteers applied sunscreen with 0.79 mg/cm2 (men with 0.93 mg/cm2 and women with 0.66 mg/cm2) (10). In a laboratory volunteers were instructed to apply sunscreen as usual, and the sunscreen application thickness was found to be 1 mg/cm2 (12). This was also found in another laboratory study in which the exact area with sunscreen application was determined by adding a fluorescent to the sunscreen (11). In 1997 Rhodes and Diffey reported fluorescence spectroscopy as a rapid, non-invasive method for measurement of skin surface thickness of topical agents (13). The method was used in a laboratory study showing that even photosensitive patients, who could be expected to have a stronger incitement to obtain sun protection, applied Photodermatol Photoimmunol Photomed 2014; 30: 96–101 © 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
sunscreen with a median of merely 0.5 mg/cm2 (5). However, this was only applicable in laboratory studies. In 2003 Whiteman et al. introduced a method for detection of sunscreen application on the skin with swabs, by which spectrophotometric analysis of the swabs can detect if sunscreen has been applied or not (14). Later the concentration–absorption relationship for sunscreen in the swab was used to determine sunscreen application thickness in the field, by swabbing of the skin and subsequent spectrophotometric analysis in the laboratory (15). The group made a pilot field study of 14 volunteers, in which the average sunscreen application thickness proved to be 1.4 mg/cm2 (15). Tape stripping was also used to determine sunscreen application thickness in another study conducted on a beach with recruitment of 60 volunteers who were unaware of the aim. The application thickness on all body sites was found to be 0.2 mg/cm2 or less (8).
Relationship between application thickness and SPF There are different reports on the relationship between sunscreen application thickness and SPF. A linear relationship between application thickness and SPF 30–100 was suggested by Ou-Yang et al. (16), and in a multicentre study Bimczok et al. (3) studied three different sunscreens with SPF25 and found a close to linear relationship between application thickness and SPF. However, the range of application thickness in this study was small, which unavoidably gives a close to linear relationship. In addition, if there was a true linear relationship, the intercept with the SPF axis should have been 1, but for two of the three products this was not the case. Conversely, three studies conducted on Danish, Brazilian and Korean volunteers (17–19), respectively, found the relationship to be exponential. In the Danish study (17) the relationship was tested with SPF4 on 20 volunteers with an application thickness in the range of 0.5–4 mg/cm2. In the Korean study (18) SPF30 and SPF35 were tested on 15 volunteers with an application thickness in the range of 0.5–2.0 mg/ cm2. Finally, in the Brazilian study (19) SPF15 and SPF30 were tested on 20 volunteers each with an application thickness in the range of 0.5–2.0 mg/cm2. The consequence of an exponential relationship is that if a sunscreen with SPF16 is applied insufficiently with 1 mg/cm2 the SPF is dramatically reduced by the square root to SPF4, and if applied with 0.5 mg/cm2 it is reduced by the fourth root to SPF2. Recently, a study testing SPF50 on 13 volunteers with an application thickness of 0.5–2 mg/cm2 proposed that the 97
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relationship between sunscreen application thickness and SPF followed a logarithmic curve (20). The relationship is therefore not fully determined. A study proposed that this might be due to the relationship not being uniform, as the authors tested sunscreens with both high and low SPF and found a linear dose–response relationship for the sunscreens with low SPF (4 + 15) and an exponential dose–response relationship for the sunscreens with high SPF (30 + 55) (21). However, this does not correspond to previous results, as the studies that found the relationship to be linear used sunscreen with SPF25–100 (3, 16), while those studies that found the relationship to be exponential used sunscreens with either high or low SPF (17–19).
Technical factors affecting SPF Studies adding fluorescence to sunscreens have documented ‘missing areas’ after self-application of sunscreen (5, 8, 22, 23), which is an important factor when sunscreen fails to protect against sunburn. Techniques of sunscreen application were tested, and it was found that rubbing instead of gentle application decreased the SPF up to 20%, which has to be considered in real life as well as during research in laboratories (24). Conversely, when using a spray sunscreen, rubbing in the sunscreen immediately after spraying is necessary to ensure proper coverage (25). The homogeneity of UV filters in a sunscreen and the formulation have a significant impact on the final SPF (26). Products that are easy to apply are more uniformly applied resulting in a smooth surface with low variation in SPF and are associated with a greater application thickness (27). Some sunscreen formulations are preferred to others by consumers (28), and it has been shown that the formulation of an inorganic sunscreen can make the consumer apply about only 2/3 of the amount they apply using an organic sunscreen with the same SPF (29). Type of dispenser has also been shown to influence the amount of sunscreen applied, with a pump system inducing the greatest amount compared to squeeze bottle and roll-on (7). After application of a sunscreen several factors can decrease the SPF. It is recommended to wait at least 20 min before dressing after sunscreen application, but it has been shown that the negative effect of clothing is dependent on application thickness. If the sunscreen is applied with 1–2 mg/cm2 the time-interval to clothing can be reduced to 8 min without a significant effect on SPF, while with a shorter interval the SPF will be significantly reduced (30). Therefore it can be difficult to apply sunscreen correctly on children on a busy morning. When the application thickness is less than 1 mg/cm2, the time interval has no impact 98
on the SPF, probably because there is little or no sunscreen to be removed by the clothing (30). The effect of clothing, physical activity and bathing on SPF was tested on 19 subjects, and about 60% and 44% of the initial SPF was still left on the skin after 4 and 8 h, respectively (31). In a laboratory study isolated epidermis was used to test the effect of sand on 10 different sunscreens, and it was found that 15 to 59% of the sun protective effect was lost after contact with and subsequent removal of sand (32). The same method with isolated epidermis was used to test sunscreen products labelled waterproof or water resistant, and during four 20-min. immersions in water the two types of products lost 20–40% of their protective effect (33). In a recent study (34) comprising 18 subjects the water resistance of two sunscreens was found to be significantly reduced by pre-treatment of the skin with a cosmetic moisture-containing lotion or cream. The reduction was registered by use of tape stripping and spectroscopic measurements before and after swimming. Another pitfall concerning sunscreen use is the potential UVR exposure before application. Unpublished data from a holiday study (10) showed that on 48% of days with sunscreen use skin sites had received more than 0.1 SED (standard erythema dose) before sunscreen was applied, showing that it was applied outdoors. The limit of 0.1 SED was chosen to eliminate insignificant, accidental exposure. The skin sites were exposed for 100 min [range 56–392] and received in average 1.3 SED [range 0.11–9.3 SED] prior to the first application, which constituted approximately 30% of the constitutive MED [range 3%– 234%] and 22% of the total dose that day [range 1.5%–100%].
Sunscreen use related to sun exposure and sunburn Data from questionnaires and diaries have shown that sunscreen use is positively associated with duration of sun exposure (35, 36) and personal UVR doses (36, 37). It has also been shown that the duration of sun exposure is positively associated with SPF and the number of sunscreen applications (38). Furthermore, sunburns have been found to be more frequent on days with sunscreen use (36, 37), paradoxically, as sunscreen is supposed to protect against sunburn. It has been suggested that sunscreen use encourages users to extend time with intentional sun exposure, because they believe they can tolerate higher doses without sunburning (38, 39). Others have suggested that people apply sunscreen, attempting to protect themselves when they know they will be exposed to high UVR doses (40), as well as it has been shown that sun-sensitive persons are more likely to use sunscreen (41). These findings have Photodermatol Photoimmunol Photomed 2014; 30: 96–101 © 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
Application of sunscreen − theory and reality
been proposed as an explanation for the association between sunscreen use and sunburning and for the positive association found between sunscreen use and malignant melanoma (42–44). A meta-analysis based on 18 studies could not confirm this positive association between sunscreen use and malignant melanoma (45), and other epidemiological studies have shown a protective effect of sunscreens against malignant melanoma (46–48).
Is it possible to compensate for insufficient sunscreen use? In 6 photosensitive patients the effect of education was investigated. By use of fluorescence spectroscopy the patients’ sunscreen application thickness was measured before education, after 2 weeks and after 6 months. The patients were informed of deficiencies in their sunscreen application technique found by use of fluorescence spectroscopy. This was followed by a consultation with a demonstration in sunscreen application. The education had a significant effect on sunscreen application thickness, which increased from 0.11 mg/cm2 before education to 0.82 mg/cm2 after 2 weeks and 1.13 mg/cm2 after 6 months (49). In another study 105 volunteers were asked to apply sunscreen as they would do before sunbathing. Half of the volunteers were given instructions on methods for efficient photoprotection and informed that an application thickness of 2 mg/cm2 is needed for maximum protection, whereas the other half received no instructions. Subjects who were instructed in sunscreen use applied 0.9 mg/cm2 on average, which was significantly more than the uninstructed group, who on average applied 0.7 mg/cm2 (50). WHO recommends to reapply sunscreen every second hour. A mathematical model was used to calculate the consequences of early sunscreen reapplication and showed that reapplication after 20 min resulted in a lower cumulative dose than that after either a single application or reapplication after 2 h (51). Sunscreen reapplication after 20 min resulted in 60–85% of the UVR exposure received with reapplication after 2 h (51). A study of 104 volunteers using biophysical measurements and determination of MED found that reapplication of sunscreen can induce a two-to-three-fold increase in protection from sunburn (52). A Japanese group found that under laboratory conditions no significant differences were observed in SPF values achieved with either single or double applications resulting in the same total quantity, which means that double application of 1 mg/cm2, is as effective as a single application of 2 mg/cm2 (20). In addition 23 Japanese subjects were asked to apply sunscreen twice. Application thickness was Photodermatol Photoimmunol Photomed 2014; 30: 96–101 © 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
estimated separately for the first and second applications to on average 1.27 mg/cm2 and 0.74 mg/cm2, respectively (20). This amount used when reapplying sunscreen was not confirmed on volunteers in another study, where the sunscreen layer on the skin was 0.95 mg/cm2 after reapplication (53). Double application or early reapplication is therefore a possible solution to compensate for inadequate sunscreen application, or at least to improve it. In an Australian survey of 807 subjects 76% reported using sunscreen and 61% reported re-applying sunscreen during the 3 preceding months (54). At the moment there are no recommendations for using sunscreens with SPF higher than 50. However, results from a randomized study (16) performed in a laboratory indicate that very high SPF of 70–100 may partly compensate for the insufficient amount applied by sunscreen users. The results suggested a linear relationship between application thickness and SPF, and the authors found that sunscreens labelled SPF100 or SPF70 applied with 0.5 mg/cm2 resulted in average SPFs of 27 and 19, respectively. In another study, under investigator observation, 58 skiers applied SPF50 on one side of the face and SPF85 on the other side of the face. An average of 5.0 h’ UVR exposure resulted in 8 cases of sunburn on the side with SPF50, which was significantly more than the one sunburn case detected on the side protected with SPF85, indicating an increased benefit from very high protection factors (55). Finally, results from a real life study (36) showed that the median UVR dose received on a day causing sunburn could be reduced to the median UVR dose received on a day without sunburn by applying an effective SPF2, what might be obtained by applying SPF16 with 0.5 mg/cm2.
CONCLUSION It is clear from the studies presented in this review that there is a discrepancy between the amount of sunscreen applied during testing and in reality. The amount of 2 mg/ cm2 is used for SPF testing because it gives more reproducible results (1) than a smaller amount of 0.5–1.0 mg/cm2 as is applied in real life (4–12). The relationship between the sunscreen application thickness and the SPF is probably product dependant and may not always be fully exponential (3, 16–21). When the reality of sunscreen use is taken into consideration, current sunscreen labelling is seen to overrate the protective effect of sunscreens, which may possibly mislead consumers to feel safe to extend sun exposure. The effect of education on sunscreen application thickness was studied, but it only resulted in sunscreen layers of 0.8–1.13 mg/cm2 (49, 50). In vivo studies showed 99
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that early reapplication can increase the sunscreen layer to 0.95–2.01 mg/cm2 (20, 53). Other studies showed that very high SPF (70–100) may compensate for insufficient application thickness (16, 55). Other major pitfalls in sunscreen use are ‘missing areas’ (5, 8, 22, 23) and UVR exposure before sunscreen application. In a holiday study sunscreen was not applied until outdoors in 48% of the cases and these skin sites received approximately 30% of the constitutive MED [range 3%– 234%] prior to sunscreen application (10).
Educating the entire population on how to apply sunscreen or increasing the amount of sunscreen by a factor of 2 to 4 may not be realistic, whereas re-application seems to be a more realistic way to obtain a thicker sunscreen layer with increased protection. Using very high SPF may be recommended, but the products are typically more difficult to apply uniformly. We therefore suggest a simple teaching strategy: (1) Apply before sun exposure and (2) Reapply once within 1 h.
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REVIEW ARTICLE
Application of sunscreen − theory and reality Bibi Petersen & Hans Christian Wulf
Department of Dermatological Research, Bispebjerg Hospital, University of Copenhagen, Copenhagen, Denmark.
Key words: sun protection factor; sunscreen application; sunscreen use
Correspondence: Miss Bibi Petersen, M.D., PhD, Department of Dermatological Research, Bispebjerg Hospital, University of Copenhagen, Bispebjerg Bakke 23, Copenhagen 2400, Denmark. Tel: +45 35316007 Fax: +45 35316010 e-mail: [email protected]
SUMMARY We present research on sunscreen use with possible pitfalls and discuss theory vs. reality. A literature review in PubMed was conducted using the terms ‘sunscreen application’, ‘sunscreen use’ and ‘sun protection factor’. The sun protection factor (SPF) of sunscreens are tested using a thickness of 2 mg/cm2, but investigations show that sunscreen under natural conditions is applied insufficiently with amounts about 0.39 to 1.0 mg/cm2, which decreases the protection factor considerably. It has been shown that early reapplication or use of very high SPF (70–100) may partly compensate for the discrepancy between the amounts of sunscreen applied during testing and in reality, and that sunscreen application can be improved by education of consumers. Missing areas and ultraviolet radiation exposure before sunscreen application are other pitfalls that reduce the protective effect of sunscreens considerably. Current sunscreen labelling overrates the protective effect of a given sunscreen when the reality of sunscreen use is taken into account. This may possibly mislead consumers to feel it is safe to extend sun exposure. Alternatively to educating people to use large amounts of sunscreen, we suggest a simple teaching strategy: (1) Apply before sun exposure and (2) Reapply once within 1 h.
Accepted for publication: 3 December 2013
Conflicts of interest: None declared.
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Photodermatol Photoimmunol Photomed 2014; 30: 96–101 There are several ways to protect the skin from solar ultraviolet radiation (UVR). The main recommendations are to seek the shade, to avoid sun exposure around noon and to apply sunscreen. Sunscreen application is usually the preferred way of sun protection. The World Health Organization (WHO) recommends that sunscreen should be applied 20 min before sun exposure with reapplication every second hour, and repeated after swimming or bathing. The protection afforded by sunscreen is expressed as the sun protection factor (SPF), which is the factor by which the sun exposure dose can be increased before skin erythema develops. The minimal erythema dose (MED) is the UVR dose needed to provoke just perceptible MED with sunscreen . erythema. The SPF is calculated from the formula: SPF = MED without susncreen According to the Guidelines from the American Food and Drug Administration (1) and the International Organization for Standardization (2) the amount of sunscreen applied when testing SPF should be 2 mg/cm2. How much of the labelled SPF is obtained when using a sunscreen is therefore dependent on the amount of sunscreen applied and whether it is spread uniformly. In reality, however, consumers apply much less sunscreen than 2 mg/cm2. External factors such as swimming, physical activity, sweating, and contact with beach sand and clothing can reduce the © 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd doi:10.1111/phpp.12099
Application of sunscreen − theory and reality
amount and thereby the effect. However, there are also studies on how to compensate for the insufficient sunscreen application layer obtaining increased protection. In this article we summarize present research on sunscreen use in theory and in reality with a discussion of future perspectives.
MAIN TEXT Application thickness The recommended sunscreen application thickness of 2 mg/cm2 for SPF testing is based on studies showing that this amount generates the best reproducible results with the lowest variation of test results (1, 3), which are needed to ensure confidence in the efficacy classification of sunscreens. The actual amount of sunscreen applied has been shown to be about 20–50% of the amount needed to achieve the labelled SPF (4–12). A simple method used to determine application thickness has been to divide the amount (grams) of sunscreen used by the sunscreen treated skin area (cm2). In two studies sunscreens were handed over to the participants and the bottles returned and weighed every third month. Diary registrations of the skin area with sunscreen application made it possible to roughly calculate the median amount of sunscreen applied, which was 0.39 and 0.79 mg/cm2, respectively (4, 9). In once-weekly evaluations children’s sunscreen use was found to be 0.48 mg/cm2 (7), and in a 1-day study on a nude beach, in which 42 women, men and children applied their own sunscreen, the application thickness was found to be 0.5 mg/cm2 by weighing sunscreen bottles before and after an unobserved full-body sunscreen application (6). A holiday study with intensive sunbathing, diary registrations and daily weighing of sunscreen bottles showed that on average volunteers applied sunscreen with 0.79 mg/cm2 (men with 0.93 mg/cm2 and women with 0.66 mg/cm2) (10). In a laboratory volunteers were instructed to apply sunscreen as usual, and the sunscreen application thickness was found to be 1 mg/cm2 (12). This was also found in another laboratory study in which the exact area with sunscreen application was determined by adding a fluorescent to the sunscreen (11). In 1997 Rhodes and Diffey reported fluorescence spectroscopy as a rapid, non-invasive method for measurement of skin surface thickness of topical agents (13). The method was used in a laboratory study showing that even photosensitive patients, who could be expected to have a stronger incitement to obtain sun protection, applied Photodermatol Photoimmunol Photomed 2014; 30: 96–101 © 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
sunscreen with a median of merely 0.5 mg/cm2 (5). However, this was only applicable in laboratory studies. In 2003 Whiteman et al. introduced a method for detection of sunscreen application on the skin with swabs, by which spectrophotometric analysis of the swabs can detect if sunscreen has been applied or not (14). Later the concentration–absorption relationship for sunscreen in the swab was used to determine sunscreen application thickness in the field, by swabbing of the skin and subsequent spectrophotometric analysis in the laboratory (15). The group made a pilot field study of 14 volunteers, in which the average sunscreen application thickness proved to be 1.4 mg/cm2 (15). Tape stripping was also used to determine sunscreen application thickness in another study conducted on a beach with recruitment of 60 volunteers who were unaware of the aim. The application thickness on all body sites was found to be 0.2 mg/cm2 or less (8).
Relationship between application thickness and SPF There are different reports on the relationship between sunscreen application thickness and SPF. A linear relationship between application thickness and SPF 30–100 was suggested by Ou-Yang et al. (16), and in a multicentre study Bimczok et al. (3) studied three different sunscreens with SPF25 and found a close to linear relationship between application thickness and SPF. However, the range of application thickness in this study was small, which unavoidably gives a close to linear relationship. In addition, if there was a true linear relationship, the intercept with the SPF axis should have been 1, but for two of the three products this was not the case. Conversely, three studies conducted on Danish, Brazilian and Korean volunteers (17–19), respectively, found the relationship to be exponential. In the Danish study (17) the relationship was tested with SPF4 on 20 volunteers with an application thickness in the range of 0.5–4 mg/cm2. In the Korean study (18) SPF30 and SPF35 were tested on 15 volunteers with an application thickness in the range of 0.5–2.0 mg/ cm2. Finally, in the Brazilian study (19) SPF15 and SPF30 were tested on 20 volunteers each with an application thickness in the range of 0.5–2.0 mg/cm2. The consequence of an exponential relationship is that if a sunscreen with SPF16 is applied insufficiently with 1 mg/cm2 the SPF is dramatically reduced by the square root to SPF4, and if applied with 0.5 mg/cm2 it is reduced by the fourth root to SPF2. Recently, a study testing SPF50 on 13 volunteers with an application thickness of 0.5–2 mg/cm2 proposed that the 97
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relationship between sunscreen application thickness and SPF followed a logarithmic curve (20). The relationship is therefore not fully determined. A study proposed that this might be due to the relationship not being uniform, as the authors tested sunscreens with both high and low SPF and found a linear dose–response relationship for the sunscreens with low SPF (4 + 15) and an exponential dose–response relationship for the sunscreens with high SPF (30 + 55) (21). However, this does not correspond to previous results, as the studies that found the relationship to be linear used sunscreen with SPF25–100 (3, 16), while those studies that found the relationship to be exponential used sunscreens with either high or low SPF (17–19).
Technical factors affecting SPF Studies adding fluorescence to sunscreens have documented ‘missing areas’ after self-application of sunscreen (5, 8, 22, 23), which is an important factor when sunscreen fails to protect against sunburn. Techniques of sunscreen application were tested, and it was found that rubbing instead of gentle application decreased the SPF up to 20%, which has to be considered in real life as well as during research in laboratories (24). Conversely, when using a spray sunscreen, rubbing in the sunscreen immediately after spraying is necessary to ensure proper coverage (25). The homogeneity of UV filters in a sunscreen and the formulation have a significant impact on the final SPF (26). Products that are easy to apply are more uniformly applied resulting in a smooth surface with low variation in SPF and are associated with a greater application thickness (27). Some sunscreen formulations are preferred to others by consumers (28), and it has been shown that the formulation of an inorganic sunscreen can make the consumer apply about only 2/3 of the amount they apply using an organic sunscreen with the same SPF (29). Type of dispenser has also been shown to influence the amount of sunscreen applied, with a pump system inducing the greatest amount compared to squeeze bottle and roll-on (7). After application of a sunscreen several factors can decrease the SPF. It is recommended to wait at least 20 min before dressing after sunscreen application, but it has been shown that the negative effect of clothing is dependent on application thickness. If the sunscreen is applied with 1–2 mg/cm2 the time-interval to clothing can be reduced to 8 min without a significant effect on SPF, while with a shorter interval the SPF will be significantly reduced (30). Therefore it can be difficult to apply sunscreen correctly on children on a busy morning. When the application thickness is less than 1 mg/cm2, the time interval has no impact 98
on the SPF, probably because there is little or no sunscreen to be removed by the clothing (30). The effect of clothing, physical activity and bathing on SPF was tested on 19 subjects, and about 60% and 44% of the initial SPF was still left on the skin after 4 and 8 h, respectively (31). In a laboratory study isolated epidermis was used to test the effect of sand on 10 different sunscreens, and it was found that 15 to 59% of the sun protective effect was lost after contact with and subsequent removal of sand (32). The same method with isolated epidermis was used to test sunscreen products labelled waterproof or water resistant, and during four 20-min. immersions in water the two types of products lost 20–40% of their protective effect (33). In a recent study (34) comprising 18 subjects the water resistance of two sunscreens was found to be significantly reduced by pre-treatment of the skin with a cosmetic moisture-containing lotion or cream. The reduction was registered by use of tape stripping and spectroscopic measurements before and after swimming. Another pitfall concerning sunscreen use is the potential UVR exposure before application. Unpublished data from a holiday study (10) showed that on 48% of days with sunscreen use skin sites had received more than 0.1 SED (standard erythema dose) before sunscreen was applied, showing that it was applied outdoors. The limit of 0.1 SED was chosen to eliminate insignificant, accidental exposure. The skin sites were exposed for 100 min [range 56–392] and received in average 1.3 SED [range 0.11–9.3 SED] prior to the first application, which constituted approximately 30% of the constitutive MED [range 3%– 234%] and 22% of the total dose that day [range 1.5%–100%].
Sunscreen use related to sun exposure and sunburn Data from questionnaires and diaries have shown that sunscreen use is positively associated with duration of sun exposure (35, 36) and personal UVR doses (36, 37). It has also been shown that the duration of sun exposure is positively associated with SPF and the number of sunscreen applications (38). Furthermore, sunburns have been found to be more frequent on days with sunscreen use (36, 37), paradoxically, as sunscreen is supposed to protect against sunburn. It has been suggested that sunscreen use encourages users to extend time with intentional sun exposure, because they believe they can tolerate higher doses without sunburning (38, 39). Others have suggested that people apply sunscreen, attempting to protect themselves when they know they will be exposed to high UVR doses (40), as well as it has been shown that sun-sensitive persons are more likely to use sunscreen (41). These findings have Photodermatol Photoimmunol Photomed 2014; 30: 96–101 © 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
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been proposed as an explanation for the association between sunscreen use and sunburning and for the positive association found between sunscreen use and malignant melanoma (42–44). A meta-analysis based on 18 studies could not confirm this positive association between sunscreen use and malignant melanoma (45), and other epidemiological studies have shown a protective effect of sunscreens against malignant melanoma (46–48).
Is it possible to compensate for insufficient sunscreen use? In 6 photosensitive patients the effect of education was investigated. By use of fluorescence spectroscopy the patients’ sunscreen application thickness was measured before education, after 2 weeks and after 6 months. The patients were informed of deficiencies in their sunscreen application technique found by use of fluorescence spectroscopy. This was followed by a consultation with a demonstration in sunscreen application. The education had a significant effect on sunscreen application thickness, which increased from 0.11 mg/cm2 before education to 0.82 mg/cm2 after 2 weeks and 1.13 mg/cm2 after 6 months (49). In another study 105 volunteers were asked to apply sunscreen as they would do before sunbathing. Half of the volunteers were given instructions on methods for efficient photoprotection and informed that an application thickness of 2 mg/cm2 is needed for maximum protection, whereas the other half received no instructions. Subjects who were instructed in sunscreen use applied 0.9 mg/cm2 on average, which was significantly more than the uninstructed group, who on average applied 0.7 mg/cm2 (50). WHO recommends to reapply sunscreen every second hour. A mathematical model was used to calculate the consequences of early sunscreen reapplication and showed that reapplication after 20 min resulted in a lower cumulative dose than that after either a single application or reapplication after 2 h (51). Sunscreen reapplication after 20 min resulted in 60–85% of the UVR exposure received with reapplication after 2 h (51). A study of 104 volunteers using biophysical measurements and determination of MED found that reapplication of sunscreen can induce a two-to-three-fold increase in protection from sunburn (52). A Japanese group found that under laboratory conditions no significant differences were observed in SPF values achieved with either single or double applications resulting in the same total quantity, which means that double application of 1 mg/cm2, is as effective as a single application of 2 mg/cm2 (20). In addition 23 Japanese subjects were asked to apply sunscreen twice. Application thickness was Photodermatol Photoimmunol Photomed 2014; 30: 96–101 © 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
estimated separately for the first and second applications to on average 1.27 mg/cm2 and 0.74 mg/cm2, respectively (20). This amount used when reapplying sunscreen was not confirmed on volunteers in another study, where the sunscreen layer on the skin was 0.95 mg/cm2 after reapplication (53). Double application or early reapplication is therefore a possible solution to compensate for inadequate sunscreen application, or at least to improve it. In an Australian survey of 807 subjects 76% reported using sunscreen and 61% reported re-applying sunscreen during the 3 preceding months (54). At the moment there are no recommendations for using sunscreens with SPF higher than 50. However, results from a randomized study (16) performed in a laboratory indicate that very high SPF of 70–100 may partly compensate for the insufficient amount applied by sunscreen users. The results suggested a linear relationship between application thickness and SPF, and the authors found that sunscreens labelled SPF100 or SPF70 applied with 0.5 mg/cm2 resulted in average SPFs of 27 and 19, respectively. In another study, under investigator observation, 58 skiers applied SPF50 on one side of the face and SPF85 on the other side of the face. An average of 5.0 h’ UVR exposure resulted in 8 cases of sunburn on the side with SPF50, which was significantly more than the one sunburn case detected on the side protected with SPF85, indicating an increased benefit from very high protection factors (55). Finally, results from a real life study (36) showed that the median UVR dose received on a day causing sunburn could be reduced to the median UVR dose received on a day without sunburn by applying an effective SPF2, what might be obtained by applying SPF16 with 0.5 mg/cm2.
CONCLUSION It is clear from the studies presented in this review that there is a discrepancy between the amount of sunscreen applied during testing and in reality. The amount of 2 mg/ cm2 is used for SPF testing because it gives more reproducible results (1) than a smaller amount of 0.5–1.0 mg/cm2 as is applied in real life (4–12). The relationship between the sunscreen application thickness and the SPF is probably product dependant and may not always be fully exponential (3, 16–21). When the reality of sunscreen use is taken into consideration, current sunscreen labelling is seen to overrate the protective effect of sunscreens, which may possibly mislead consumers to feel safe to extend sun exposure. The effect of education on sunscreen application thickness was studied, but it only resulted in sunscreen layers of 0.8–1.13 mg/cm2 (49, 50). In vivo studies showed 99
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that early reapplication can increase the sunscreen layer to 0.95–2.01 mg/cm2 (20, 53). Other studies showed that very high SPF (70–100) may compensate for insufficient application thickness (16, 55). Other major pitfalls in sunscreen use are ‘missing areas’ (5, 8, 22, 23) and UVR exposure before sunscreen application. In a holiday study sunscreen was not applied until outdoors in 48% of the cases and these skin sites received approximately 30% of the constitutive MED [range 3%– 234%] prior to sunscreen application (10).
Educating the entire population on how to apply sunscreen or increasing the amount of sunscreen by a factor of 2 to 4 may not be realistic, whereas re-application seems to be a more realistic way to obtain a thicker sunscreen layer with increased protection. Using very high SPF may be recommended, but the products are typically more difficult to apply uniformly. We therefore suggest a simple teaching strategy: (1) Apply before sun exposure and (2) Reapply once within 1 h.
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