Exposure to ultraviolet light causes a striking and prolonged reduction in vitamin-A sensitive, cancer-reducing retinoic receptors and proteins, increasing the risk of skin cancer developing.

The study found that, eight hours after exposure to ultraviolet light, the levels of retinoic receptors and proteins had fallen by 65% and took 48 hours to recover. When the research subjects were exposed every 24 hours for three days, no recovery was seen until 48 hours after the final exposure. This suggested that daily sunbathing was more likely to lead to skin cancer than, say, sunbathing twice a week.

The researchers reasoned that applying retinoic acid to the skin before exposure might help, and found that this was indeed so. Where the skin had been treated with retinoic acid prior to exposure, full recovery only took 16 hours. But even treated skin took 48 hours to recover when exposure was daily.

The retinoic receptors and proteins appeared to protect against the tumour-promoting and skin-ageing properties of the transcription-factor activator-protein 1, which is itself promoted by exposure to ultraviolet light.