A critical review of the existing evidence for sun-screens has been published.
According to it, the concerns about cancer are not well-evidenced.
They looked at:
- Whether sunscreens stop skin-cancers.
- Whether sunscreens hinder the production of Vitamin D.
- Whether the controversial ingredients were shown to cause harm, specifically:
- oxybenzone and hormonal homeostasis,
- nanoparticles of zinc oxide (ZnO) and titanium dioxide (TiO2) and toxicity through the skin.
- retinyl palmitate and cancer
For this question, only the last two issues are relevant.
The nanoparticle debate extends past just sunscreens, into other personal care and cosmetic products. They are widely used and have had a good safety record.
Nonetheless, some studies in rats suggested they may be photogenotoxic.
Considerable data assessing the potential toxicity of these materials in sunscreens has been published to date, and the studies referenced above were performed in controlled environments on healthy, undamaged skin. It has been established that the stratum corneum is an effective barrier preventing the entry of nano-ZnO and -TiO2 into deeper layers of the skin. Nonetheless, it remains to be determined whether a greater degree of penetration occurs through skin that is damaged, diseased or otherwise compromised. At the present time, however, the available data do not provide conclusive evidence demonstrating that damaged skin leads to an increased penetration of nanoparticles.
In summary, if your skin is undamaged, the nanoparticles can't get through. If your skin is damaged, there is no good evidence they get through, but more research is required.
In conclusion, the available evidence from in vitro and animal studies fails to demonstrate convincing evidence indicating that retinyl palmitate imparts an increased risk of skin cancer. Furthermore, while no human data examining this relationship are available, decades of clinical observations support the notion that retinyl palmitate is safe for use in topical applications such as sunscreens.
Given sunscreens prevent Squamous cell carcinoma (SCC) and sunburn (although this study claims it is inconclusive in preventing melanomas and and basal cell carcinoma (BCC)), their use seems justified despite the fears.