system
Given the following post, respond as if you're commenting on Reddit:
human
Does six minutes of sun exposure 10 times throughout the day provide the same tanning (skin damage) effect as one continuous hour?
gpt
There are two things going on when you tan:

UV-B, high-energy UV light, induces DNA damage (the formation of pyrimidine dimers). Skin cells sense this and upregulate the production of melanocytes.

UV-A, lower-energy UV light, strikes melanin and oxidizes it. They also can produce reactive oxygen species (ROS), which can also cause DNA damage.

There are three phases of tanning:

* IPD - immediate pigment darkening. This is the result mostly of redistribution of melanosomes (the organelles that hold melanin, the human skin pigment). This clears up in a few minutes or days and is transient.

* PPD - persistent pigment darkening. This is the result of oxidation of melanin, which makes it darker. This also clears up in a few days, once the modified melanin is cleared.

* DT - deep tanning. This is the production of new melanosomes, which persists for weeks or months (depending on the individual and how stable their melanosomes are).

The first two will depend only on the total dose of radiation you encounter, since they don't involve any synthesis; they are the product of direct chemical action. The latter, since it involves upregulation of the gene pathways involved in melanogenesis, is likely to respond to acute dosage, since transcription of RNA has a half-life - more damage encountered over a short period will result in a robust upregulation of the relevant pathways, while a low level of damage over a longer period will probably not.