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Background Sunscreens are an important component of healthy sun-protection behavior. To achieve satisfactory protection, sunscreens must be applied consistently, evenly and correctly. Consumers do not apply sunscreen properly and, therefore, do not achieve the protection indicated by the label 'sun protection factor' ( SPF). The objective of the present study was to determine the actual sun(burn) protection given by a range of sunscreen application thickness levels for both low and high SPF formulas. Subjects and Methods Forty study subjects were recruited from each of three geographical regions in China. Sunscreens with label SPFs of 4, 15, 30, and 55 were tested at application levels of 0.5, 1.0, 1.5, and 2.0 mg/cm2 in three laboratories using a standard SPF protocol. Results Sunscreens with lower SPFs (4 and 15) showed a linear dose-response relationship with application level, but higher SPF (30 and 55) product protection was exponentially related to application thickness. Conclusion Sunscreen protection is not related in one uniform way to the amount of product applied to human skin. Consumers may achieve an even lower than expected sunburn protection from high SPF products than from low SPF sunscreens. [ABSTRACT FROM AUTHOR]