Conditions

Sun & Ultraviolet

Sunlight is two things at once: the only natural source of vitamin D, and a source of cumulative DNA damage. Both depend on the same narrow ultraviolet band, and the difference between them is dose.

Instrument

Vitamin D from sunlight

Synthesis estimate and burn threshold from UV index, skin type, exposed area and age.

Scope

What this section covers

UV index, vitamin D synthesis, skin type and the erythemal dose. The sun as a measurable input rather than a weather condition.

Field notes

Reference

SunHow Your Skin Actually Makes Vitamin DThe photochemical chain from UVB photon to circulating vitamin D, why the process self-limits, and what that means for how long you should stand in the sun.
SunReading the UV Index Like an Instrument, Not a WarningThe UV index is a measured physical quantity with a precise definition. Understanding what it counts, and what it deliberately ignores, changes how you plan a day outdoors.
SunWhy the Same Ten Minutes Is a Different Dose for Different PeopleThe Fitzpatrick scale, minimal erythemal dose, and how melanin changes both the vitamin D output and the damage cost of identical sun exposure.
SunVitamin D Winter: The Months Sunlight Stops WorkingAbove roughly 37 degrees of latitude, the winter sun cannot produce vitamin D at any exposure duration. The geometry behind that, and what to do about it.
SunDoes Sunscreen Block Vitamin D? The Answer Is Uncomfortably SpecificLaboratory studies say sunscreen should nearly eliminate vitamin D synthesis. Field studies find almost no effect. Both are correct, and the gap between them is where the useful answer lives.