The Risk of Too Little Sun
A landmark Swedish study followed 30,000 women over 20 years and found that women who avoided the sun had a lifespan comparable to smokers. Complete sun avoidance increased all-cause mortality — not just from vitamin D deficiency, but from cardiovascular disease, diabetes, and other conditions.
80% of Americans are vitamin D deficient. The "stay out of the sun entirely" message has overcorrected.
Lindqvist et al., Journal of Internal Medicine, 2016
The Risk of Too Much Sun
Sunburn — caused by exceeding your skin's Minimal Erythemal Dose (MED) — is the primary risk factor for melanoma. UV damage is cumulative, and even one blistering burn significantly increases lifetime risk.
The science is clear: burning is harmful. But the solution isn't to eliminate sun exposure — it's to get the right amount.
dminder exists to navigate this balance.
Like all things in health, the answer isn't all-or-nothing. It's finding the dose that maximizes benefit while minimizing harm. dminder uses real satellite data and erythemal dose science to find your personal sweet spot — every day, at your location, for your skin type.
Safety First: The Erythemal Dose
Every person has a Minimal Erythemal Dose (MED) — the amount of UV energy that causes the first visible reddening of their skin. It's your personal UV budget for the day.
What determines your MED?
- ● Skin type — Fair skin (Type I) burns at ~200 J/m². Dark skin (Type VI) tolerates 1000+ J/m².
- ● Medications — Some antibiotics, retinoids, and blood pressure drugs increase UV sensitivity.
- ● Recent exposure — Tanning builds melanin, gradually raising your MED over days.
How dminder keeps you safe
- ✓ Integrates UV dose over your entire session in real-time
- ✓ Applies an 80% safety margin — you're warned well before reaching your MED
- ✓ Escalating alerts: gentle nudge at 80%, firm warning at 95%, alarm at 100%
- ✓ Accounts for your skin type, exposed area, and current solar conditions
The goal is simple: capture your vitamin D in the safe zone between "not enough UVB to matter" and "too much UV for your skin." dminder finds that window for you, every day, everywhere on Earth.
Real Data, Not Guesswork
Every vitamin D calculation in dminder is driven by real-world measurements, not rough estimates or lookup tables.
NOAA Satellite Ozone
The ozone layer filters UVB before it reaches you. We download real ozone measurements from NOAA's OMPS satellite instrument — a 126 KB global grid updated continuously.
Solar Position
We calculate the sun's exact elevation angle at your GPS location using astronomical algorithms. UVB only reaches you above ~30° — this determines your vitamin D window.
Weather Conditions
Cloud cover reduces UVB reaching your skin. We factor in real-time weather data so your vitamin D estimate accounts for overcast skies, not just clear-sky conditions.
Validated with a Solarmeter
Our base rate of 50 IU/min with 10% skin exposed (face and hands) was validated against a Solarmeter Model 6.4 — the clinical instrument used by dermatologists and photobiologists. When the Solarmeter reads 50, dminder predicts 1,000 IU in 20 minutes. They match.
Personalized to You
Two people standing in the same sunlight produce wildly different amounts of vitamin D. dminder accounts for every variable.
Fitzpatrick Skin Type
Your skin type determines both your vitamin D production rate and your burn threshold. Type I (very fair) burns in minutes but converts UVB to vitamin D efficiently. Type VI (very dark) has 5–10x more melanin, which competes with vitamin D precursors for UVB photons.
Exposed Skin Area
Vitamin D production scales linearly with exposed skin. Face and hands (10%) produce about 50 IU/min at peak conditions. Arms and legs (60%) produce about 300 IU/min. The slider in dminder adjusts your rate in real-time.
Age, Weight & Body Composition
Vitamin D is fat-soluble. Higher body mass means more volume for distribution, affecting blood levels. Aging skin is less efficient at converting UVB to vitamin D3. dminder factors both into its blood level estimates.
One Engine, Both Platforms
Version 10 is a ground-up rewrite. The core calculation engine is written in Rust — a single library that powers both the iOS and Android apps.
Why this matters
When the vitamin D calculation, MED safety model, and blood level estimator are in one shared library, there's zero chance of platform divergence. A Type III user in Denver gets the exact same burn time on iPhone and Android — down to the second.
Previous versions maintained separate calculation code for each platform. Subtle differences crept in over years. That era is over.
What's in the engine
- ● Solar position & elevation calculation
- ● Erythemal dose integration (burn time)
- ● Vitamin D rate calculation
- ● Blood level estimation & pharmacokinetic decay
- ● Coaching notifications & risk assessment
- ● Optimal session finder
Beyond Sun Sessions
dminder tracks your complete vitamin D picture — not just sun exposure.
Sun Sessions
Timed sessions with real-time vitamin D tracking and burn protection
Supplements
Log single doses or set up daily recurring supplements with auto-backfill
Food Sources
Track vitamin D from salmon, fortified milk, mushrooms, and other foods
Lab Results
Enter blood test results to calibrate estimates and track progress over time
Your Vitamin D Ledger
All sources feed into a pharmacokinetic model that estimates your current blood level (25-hydroxyvitamin D) with exponential decay and bioavailability factors. It's like a running balance sheet for your vitamin D — deposits from sun and supplements, withdrawals from metabolism.
Why We Model Incidental Sun Exposure
Dr. Kevin Liu, an internal medicine physician, tells the story of his father — an elderly man under routine physician care for typical aging concerns. One day they ran his vitamin D level. It came back as zero. His skeletal frame had already begun to collapse. Nobody had thought to check.
This is not rare. Millions of people — especially those with darker skin, indoor lifestyles, or at higher latitudes — are profoundly deficient and don't know it. The earlier version of D-Minder only counted vitamin D from tracked sessions and supplements. If you didn't press "Start," the app assumed you got nothing. That's wrong. Everyone gets some incidental sun — walking to the car, eating lunch outside, running errands.
Here's what most people don't know: when the US government established the 800 IU daily recommendation, the model they used to arrive at that number already includes incidental sun exposure. The 800 IU is literally the minimum daily intake to get you out of deficiency — not even into the range of sufficiency — and it assumes you're getting some sun on the side. But how much incidental exposure you get depends on where you live, what time of year it is, your skin type, and how much time you spend outdoors. The government's model treats that as a constant. Ours doesn't.
D-Minder now estimates your daily incidental vitamin D based on your actual location, the season, your skin type, and how much time you typically spend outdoors (a single question during setup). On a summer day in Miami, a few minutes of errands with face and hands exposed might contribute 150+ IU. On a winter day in London, the same errands contribute essentially zero — the sun never climbs high enough for UVB to reach you.
This matters because the blood level estimator uses every source of vitamin D to project where you are. Without incidental exposure, the estimator systematically underestimates your level, coaching nudges are too aggressive, and the app feels out of touch with reality. With it, D-Minder's estimate tracks much closer to what a blood test would actually show.
Incidental exposure estimates are generated daily in the background and feed into the pharmacokinetic model alongside your tracked sessions and supplements. They don't appear in your session history — they're invisible infrastructure that makes the estimator more accurate.
Want the Full Technical Details?
Our science page covers the solar physics, atmospheric absorption, MED tables by skin type, the pharmacokinetic decay model, and how we validated our rates with clinical instruments.
Read the ScienceStart tracking your Vitamin D today
Free on iOS and Android.
