Instrument 01
Estimates cutaneous vitamin D synthesis from UV index, skin type, exposed surface area, exposure duration and age — and reports where that exposure crosses your personal burn threshold.
From your weather app for the time you will be outside. Below 3, synthesis is minimal.
Classify by how your skin responds to first exposure after winter, not by appearance in August.
Skin loses 7-dehydrocholesterol with age, so the same exposure yields less over time.
Cutaneous vitamin D3 produced by this exposure.
Your burn threshold arrives at — of continuous exposure at this UV index.
1,000 IU — —
2,000 IU — —
4,000 IU — —
The UV index converts to erythemally weighted irradiance at 0.025 W/m² per index unit. Accumulated dose is expressed in standard erythema doses, where 1 SED equals 100 J/m². Minimal erythemal dose varies by Fitzpatrick type from roughly 2 SED at type I to 8 SED at type VI.
Vitamin D synthesis is modelled as proportional to accumulated dose, exposed body surface fraction, a melanin factor reducing photon availability to 7-dehydrocholesterol, and an age factor reflecting declining epidermal precursor concentration. Above roughly 0.75 MED the model applies sharply diminishing returns, reflecting photodegradation of previtamin D3 into lumisterol and tachysterol — the mechanism that makes solar vitamin D self-limiting.
Below a UV index of 3, output is scaled down steeply. At low solar elevation the atmospheric slant path removes nearly all UVB, which is the physical basis of vitamin D winter.
Treat the output as an order-of-magnitude estimate for comparing exposures, not as a clinical measurement. Serum 25-hydroxyvitamin D is the only way to know your actual status.
General education, not medical advice. Consult a clinician before beginning vitamin D supplementation or making changes to sun exposure if you have a photosensitivity condition, a history of skin cancer, or take photosensitising medication.