Sunscreen skin penetration and safety evaluation should be conducted together to ensure that in vitro cytotoxicity studies investigate appropriate doses of these organic chemical ultraviolet (UV) filters to which living epidermal cells are exposed in real life. 

Hayden et al, in the study, determined if sufficient topically applied sunscreens penetrate the viable human epidermis to position the local keratinocyte cell populations at risk of toxicity. They evaluated the penetration and retention of the commonly employed sunscreen agents (avobenzone, octinoxate, octocrylene, oxybenzone and padimate O) in human skin to isolated human epidermal membranes.

After 24-hour of human epidermal exposure to sunscreens, they discovered detectable amounts of all sunscreens in the stratum corneum and viable epidermis. Interestingly, epidermal penetration was most obvious with oxybenzone. The concentrations of each sunscreen in the viable human epidermis after topical application, adjusting for skin partitioning and binding effects, were revealed to be at least five times lesser than those appearing to cause toxicity in cultured human keratinocytes. 

Hence, it is evident that the human viable epidermal sunscreens levels are too low to cause any substantial toxicity to the underlying human keratinocytes. Hence, sunscreen use should be continued to be advocated.

Source: Hayden CG, Cross SE, Anderson C, et al. sunscreen penetration of human skin and related keratinocyte toxicity after topical application. Skin Pharmacol Physiol. 2005;18(4):170-4.