What does "photoaddition" refer to in tanning?

Photoaddition refers to the phenomenon that occurs when the skin is exposed to UV radiation from both indoor tanning devices and natural sunlight within a relatively short period, such as 24 hours. This process can amplify the tanning effect because the skin receives cumulative exposure to UV rays.

When the skin is already undergoing tanning from indoor exposure, additional outdoor UV exposure can increase the production of melanin, leading to a deeper tan. However, it is important to approach this practice with caution, as excessive exposure to UV radiation, whether from tanning beds or the sun, can increase the risk of skin damage and potential health concerns, such as skin cancer.

In contrast, tanning without sunscreen, tanning for too long, or tanning during high UV index times each pertain to risks associated with UV exposure but do not specifically describe the cumulative tanning effect that photoaddition entails.