Methodology
How UV Balance estimates burn time, vitamin D production, and UV dose — the formulas, reference values, and their limitations. This is an educational tool, not medical advice.
Units & Conversions
UV exposure is measured in two standard units defined by the WHO and ICNIRP.
Sources: WHO Global Solar UV Index: A Practical Guide (2002); ICNIRP UV Radiation Guidelines (2010).
Sunburn Model
A MED (Minimal Erythemal Dose) is the minimum UV dose required to produce the first detectable redness on untanned skin, measured 24 hours after exposure. It is the threshold UV Balance uses to estimate burn risk.
Dose warnings
UV Balance signals increasing risk as your accumulated dose rises:
| Dose reached | What UV Balance shows |
|---|---|
| 50% of your MED | Info alert — monitor your time in the sun |
| 75% of your MED | Caution — consider moving to shade |
| 90% of your MED | Warning — approaching your burn threshold |
| 100% of your MED | Danger — seek shade immediately |
Fitzpatrick Skin Type & MED Values
The Fitzpatrick scale classifies skin into six types by its response to UV. Each type has a reference MED — the dose for untanned, unprotected skin under standard conditions. Tanning, sunscreen, or prior UV exposure all raise the effective MED considerably. Individual values within each type vary widely; these are population-level midpoints.
| Type | Typical description | Typical sun reaction | MED (SED) | MED (J/m²) |
|---|---|---|---|---|
| 1 | Very fair; often red/blonde hair, blue/green eyes, freckles | Always burns, never tans | 2 | 200 |
| 2 | Fair, white skin; light hair; blue or hazel eyes | Usually burns, tans minimally | 3 | 300 |
| 3 | Medium or olive skin; brown hair and eyes | Sometimes burns, tans evenly | 4.5 | 450 |
| 4 | Olive to light brown skin; dark hair and eyes | Rarely burns, tans easily | 6 | 600 |
| 5 | Brown skin; dark hair and eyes | Very rarely burns, tans very easily | 8 | 800 |
| 6 | Dark brown or black skin; black hair; dark eyes | Never burns, deeply pigmented | 10 | 1000 |
Sources: Fitzpatrick TB (1988) Arch Dermatol; WHO UV Index Practical Guide (2002); IARC Monographs Vol 100D (2012).
Vitamin D Estimation
The vitamin D estimate is based on Holick’s rule, a well-known order-of-magnitude approximation from dermatology research:
“Approximately 1,000 IU of vitamin D3 is produced when one quarter of the body surface area is exposed to one quarter of an MED of UVB radiation.”
— Holick MF, Am J Clin Nutr (2004)
Orientation modifier
Not all exposed skin receives direct UV equally — it depends on how you are positioned relative to the sun. UV Balance applies an orientation modifier to the exposed BSA before calculating vitamin D. Burn time is not affected.
| Orientation | How effective BSA is calculated | Multiplier |
|---|---|---|
| Mixed / Walking | Moving around; different surfaces face the sun at different times | Total BSA × 0.60 |
| Lying Face Up | Front = full UV credit; back = 30% (scattered & reflected UV only) | (BSA × 0.5 × 1.0) + (BSA × 0.5 × 0.3) |
| Lying Face Down | Back = full UV credit; front = 30% (scattered & reflected UV only) | (BSA × 0.5 × 1.0) + (BSA × 0.5 × 0.3) |
| Sitting | Roughly half your exposed skin faces the sun at any given time | Total BSA × 0.50 |
| Rotating | Turning periodically; most exposed skin receives direct sun over the session | Total BSA × 0.85 |
The face-up / face-down logic treats front and back as equal halves of your total exposed BSA (~50% each). The shaded side receives approximately 30% of direct irradiance from scattering and ground reflection. These are population-level approximations.
When the estimate is reliable
| Dose accumulated | Reliability | Why |
|---|---|---|
| Below 0.5 MED | Good range | Linear scaling is a reasonable approximation at low doses |
| 0.5 – 1.0 MED | Less reliable | Tanning begins; photoproduction efficiency falls; estimate shown with caveat |
| Above 1.0 MED | Unreliable | Burn threshold exceeded; further UV degrades pre-vitamin D&sub3;; estimate is not meaningful |
| UV Index below 3 | Minimal production | Low solar angle means mostly UVA, which does not produce vitamin D — only UVB (280–315 nm) does |
Body Surface Area (BSA)
BSA fraction is the proportion of your skin directly exposed to sunlight — not shaded, not covered by clothing. It scales the vitamin D estimate and UV dose linearly. 100% means fully bare; 0% means completely covered.
UV Balance offers three ways to set your BSA, all producing the same result in the calculations:
| Mode | How it works | Best for |
|---|---|---|
| Clothing Estimate | Choose an upper-body garment, a lower-body garment, and optionally a hat. Each item has a known coverage fraction; the app subtracts covered area from 100%. | Quick everyday use |
| Exposed Areas | Tick individual body regions that are exposed to the sun. Each region contributes its percentage of total body surface (based on the Rule of Nines — see below). | When you know specifically which parts of you are in the sun |
| Custom % | Type any number from 0 to 100. | Overriding with your own estimate |
All three modes are educational approximations. Actual exposed area varies with posture, garment fit, and individual body proportions.
Clothing Coverage Values
The coverage fractions below are anatomical estimates for an average adult. Upper and lower garments combine additively; a hat adds 5%. One-piece swimwear replaces both upper and lower selections.
Upper body
| Garment | Body area covered | What it covers |
|---|---|---|
| Nothing (bare chest/torso) | 0% | Nothing — full torso and arms exposed |
| T-shirt | 38% | Torso and partial upper arms |
| Long-sleeve shirt | 48% | Full torso and complete arms |
| Tank top | 28% | Torso core; shoulders and arms remain exposed |
| Bikini top | 9% | Chest only |
Lower body
| Garment | Body area covered | What it covers |
|---|---|---|
| Nothing (bare legs/hips) | 0% | Nothing — full legs and hips exposed |
| Shorts | 22% | Pelvis and upper thighs |
| Pants / trousers | 42% | Entire legs and pelvis |
| Speedo / swim brief | 6% | Groin and pelvis |
| Bikini bottom | 3% | Groin only |
Swimwear & hat
| Item | Body area covered | Notes |
|---|---|---|
| Hat | 5% | Added on top of any outfit; covers the top of the head (~5%). |
| Swimsuit (one-piece) | 28% | Replaces upper and lower selections; covers 28% of total body surface. |
Example outfits
| Outfit | Estimated body area exposed |
|---|---|
| Naked | 100% |
| Naked + hat | 95% |
| Speedo (bare torso) | 94% |
| Bikini (top + bottom) | 88% |
| One-piece swimsuit | 72% |
| Tank top + shorts | 50% |
| T-shirt + shorts | 40% |
| T-shirt + shorts + hat | 35% |
| Long-sleeve shirt + shorts | 30% |
| T-shirt + pants | 20% |
| Long-sleeve shirt + pants | 10% |
| Long-sleeve shirt + pants + hat | 5% |
Body Region Weights (Exposed Areas mode)
When using the Exposed Areas mode, each body region you tick contributes a fixed percentage of total skin surface area. The percentages are based on the Rule of Nines (Wallace, 1951) — a standard anatomical reference used clinically for burn assessment, adapted here for UV exposure. Selecting all twelve regions adds up to exactly 100%.
| Body region | % of total skin surface |
|---|---|
| Head & neck | 9% |
| Chest (front) | 9% |
| Abdomen (front) | 9% |
| Upper back | 9% |
| Lower back | 9% |
| Right arm | 9% |
| Left arm | 9% |
| Right thigh | 9% |
| Left thigh | 9% |
| Right lower leg | 9% |
| Left lower leg | 9% |
| Genitalia | 1% |
| All regions selected | 100% |
Genitalia is included as a selectable region at 1%, consistent with the clinical Rule of Nines standard. The trunk and legs are each split into two regions (front/back and thigh/lower leg) to allow more granular selection — the underlying percentages follow the standard Rule of Nines distribution.
Assumptions & Limitations
- Unprotected, untanned skin: All MED values assume no prior tan and no sunscreen. SPF 30 reduces effective UV dose by ~97%; SPF 50 by ~98%. Sunscreen reduces vitamin D production by a similar proportion.
- UV Index accuracy: Cloud cover, smoke, and air pollution can reduce actual UVI below the reported value by 10–90%. UV Balance uses real-time UV data from Open-Meteo where available, which already accounts for cloud cover.
- Altitude: UV increases approximately 10–12% per 1,000 m of elevation. People at altitude will burn faster than calculated.
- Reflective surfaces: Snow reflects up to 85% of UV; water 10–30%; sand 10–15%. Near these surfaces, effective UV exposure is significantly higher than the ambient UV Index suggests.
- Vitamin D requires UVB, not just any sunlight: Only UVB (280–315 nm) produces vitamin D. In the early morning, late afternoon, or in winter at higher latitudes, the sun's angle filters out most UVB even when it is bright outside. A simple guide: if your shadow is longer than your height, UVB is too low for meaningful vitamin D synthesis.
- Winter at higher latitudes: Above roughly 35° north or south, UVB is insufficient for vitamin D synthesis from approximately November through March.
- Age affects vitamin D production: An 80-year-old's skin produces around four times less vitamin D than a 20-year-old's at the same UV dose. This model does not adjust for age.
- Skin type is a range, not a precise value: Actual MED within a Fitzpatrick type can vary by a factor of 2–3. The values used are population-level reference midpoints.
- Clothing and body region estimates are averages: Coverage values assume standard adult body proportions, upright posture, and typical garment fit. Actual exposed area will vary.
- Windows block vitamin D production: Standard glass filters essentially all UVB. Sitting or driving by a window does not produce vitamin D.
- Some medications increase burn risk: Certain antibiotics, diuretics, retinoids, and anti-inflammatory drugs can make skin more sensitive to UV. Check with a pharmacist or doctor if you take regular medication.
- Temperature has no effect on burn risk: UV Index — not heat — determines how quickly you burn. A cool, overcast day can still carry significant UV. UV Balance shows temperature for context only.
- Vitamin D formula is a rough approximation: Real vitamin D photosynthesis is a complex process that does not scale linearly at high doses. The estimate is useful for order-of-magnitude guidance only and becomes less reliable above half your burn threshold.
References
- World Health Organization (2002). Global Solar UV Index: A Practical Guide. WHO/SDE/OEH/02.2. Geneva: WHO. who.int
- International Commission on Non-Ionizing Radiation Protection (2010). Limits of Exposure to Ultraviolet Radiation of Wavelengths 180–400 nm. Health Physics, 87(2), 171–186. icnirp.org
- Holick MF (2004). Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. Am J Clin Nutr, 80(6 Suppl), 1678S–1688S. PubMed 15585788
- Webb AR, Engelsen O (2006). Calculated ultraviolet exposure levels for a healthy vitamin D status. Photochem Photobiol, 82(6), 1697–1703. PubMed 16999790
- Fitzpatrick TB (1988). The validity and practicality of sun-reactive skin types I–VI. Arch Dermatol, 124(6), 869–871. PubMed 3377516
- Lucas R et al. (2006). Solar ultraviolet radiation — Global burden of disease from solar ultraviolet radiation. Environmental Burden of Disease Series No. 13. WHO, Geneva. who.int
- IARC (2012). Solar and Ultraviolet Radiation. IARC Monographs, Vol 100D. IARC
- Bouillon R et al. (2008). Vitamin D and human health: lessons from vitamin D receptor null mice. Endocr Rev, 29(6), 726–776. PubMed 19196837
- Wallace AB (1951). The exposure treatment of burns. Lancet, 261(6653), 501–504. (Original description of the Rule of Nines.)
- Open-Meteo (2024). Open-Meteo Weather API Documentation. open-meteo.com