Evidence review12 July 202614 min

Healthy sun exposure: scientific foundations

A review of the evidence on phototypes, vitamin D synthesis, the benefits and risks of sunlight, and the daily scoring model that follows from it.

Abstract

A review of the available evidence on ultraviolet exposure: UV index and safe times by phototype, cutaneous vitamin D synthesis and its saturation, documented physiological benefits, the risks of overexposure, and a proposed daily scoring model based on dose rather than minutes. Official bodies (WHO, European Commission) and clinical studies are cited for each point.

This document reviews the scientific evidence on sun exposure, skin phototypes, vitamin D synthesis, and the benefits and risks of sunlight, and proposes from it a daily scoring model — "recommended sun taken today" — for the product.

Phototypes, UV index and safe time in the sun

UV index

The UV index measures the intensity of ultraviolet radiation. The World Health Organization recommends taking precautions when it reaches 3 or above. In the hours around solar noon — roughly ±2 h — rays are most intense, so exposure in that window should be limited.

Fitzpatrick phototypes (I–VI)

These classify skin by its response to sun. Low phototypes (I–II, very fair) burn far sooner than high ones (V–VI, dark skin). Indicative times to burn without protection:

Phototype	At UVI 5	At UVI 9
I — very fair	~20–30 min	~11–15 min
II — fair	~30–40 min	~17–23 min
III — intermediate	~45–60 min	~25–35 min
VI — dark	—	> 65 min

These figures are initial references only. Latitude, season, cloud cover, altitude and reflective surfaces all change the dose actually received.

Radiation increases by roughly 10–12% per 1,000 m of elevation, and surfaces such as snow or sand raise the net dose through reflection.

A practical rule

To synthesise vitamin D efficiently it helps to be out when the UV index is moderately high. A common field rule: the index above 3 and your own shadow shorter than you are, which typically happens near midday in summer.

Vitamin D synthesis in the skin

The sun is the main source: more than 90% of our vitamin D comes from solar UVB. Ten to thirty minutes a day on uncovered skin, depending on phototype and hour, provides a significant contribution. Various bodies note that around 15 minutes a day with face, arms and hands exposed, in summer or at a good UV index, is usually enough to maintain adequate levels.

Darker phototypes need longer to produce the same amount as fair ones.
In winter or at high latitudes more exposure is needed; northern latitudes usually call for supplementation in winter.
There is a saturation point: past a certain threshold the skin stops producing more vitamin D even under continued radiation.

That saturation is the key property of the whole model: it defines a point beyond which staying out adds no benefit and only adds risk.

Physiological benefits of sunlight

Many studies support positive effects beyond vitamin D. Those with the strongest evidence:

Vitamin D production. Strengthens bones and teeth, regulates the immune system and muscle function. Adequate levels are associated with lower risk of bone disease and better immune function.
Improved mood. Sunlight increases serotonin production, lifting mood. Several studies show reduced symptoms of seasonal affective disorder.
Sleep–wake regulation. Morning exposure synchronises the circadian rhythm: daylight signals the brain to reduce melatonin, promoting daytime alertness and better sleep at night.
General wellbeing. Associated with more energy, better concentration and less stress. It also contributes to nitric oxide production in the skin, with a vasodilatory effect that temporarily lowers blood pressure.

Risks of excess sun

Unprotected overexposure has demonstrated adverse consequences:

Sunburn (erythema). Even a mild burn damages skin DNA. Each burn raises future skin cancer risk, especially melanoma if it occurs in childhood. The WHO stresses that a tan is not "healthy" but a sign of damage.
Photoaging. Radiation degrades collagen and elastin, accelerating wrinkles, spots and sagging. The effects are cumulative.
Skin cancer. Chronic exposure is a principal factor in all skin cancers; repeated burns raise melanoma risk.
Eye damage. Accumulated radiation can cause cataracts and premature macular degeneration. Eye protection is recommended alongside sunscreen.
Immunosuppression. Ultraviolet radiation has a local and systemic immunosuppressive effect, which can reduce the skin's defences against tumours.

Accumulating minutes is not enough: the quality of the exposure — UV intensity, phototype, protection applied — is what determines the outcome.

The daily scoring model

It follows from the review above that the product's main bar should not count minutes, but the percentage of healthy sun exposure reached (0–100%). The idea is to compute how much of the daily target the user has achieved, weighted by the real intensity of the radiation.

01Daily effective UV dose. Integrate the radiation actually received minute by minute, weighted by phototype and exposed body surface. Each interval dt contributes a dose proportional to UVI(t) × exposed_area × SPF_transmission.
02Target dose. Define the dose equivalent to that needed for the recommended benefit without risk, calibrated against what is reached in 10–30 minutes of direct sun depending on hour and latitude.
03Saturation function. Apply a curve reflecting that, past a certain point, benefit stops increasing but risk does not.
04Percentage. Truncate the result at 100%, so exceeding the target does not encourage excess.

VitD(D) = min(V_max, k · D)

Where D is accumulated UV dose and V_max the daily production ceiling. The exact shape matters less than the property: it saturates.

Adjustment factors

Sunscreen. Multiply effective exposure by the transmission actually achieved, not the nominal figure on the label.
Exposed area. The portion of the body in the sun, estimated from clothing.
UV variation during the session. Use real readings to integrate correctly: a minute at UVI 9 is worth more than one at UVI 3.
Multiple sessions. The percentage accumulates across the day.
Benefit versus risk. The main bar should focus on benefit; risk warnings belong in separate indicators.

What the history should record

Each session and daily summary should store considerably more than the UV index:

Timing of the exposure: date, start and end time.
Effective UV index: average and peak during the session.
Total duration.
Exposed body area and the user's phototype.
Sunscreen used: factor, and time of application and reapplication.
Computed accumulated dose, both erythemal and for vitamin D.
Score obtained in the session and the daily running total.
Computed risk: percentage of the MED reached.
Environmental conditions: location, cloud cover, ground reflection.

This allows the history to show the real evolution of the sun received each day — with intensity varying across the session — instead of a single static UV index number.

Communicating the scientific backing

The application should convey that its calculations rest on evidence, without saturating the interface. The practical conclusion was to use discreet signals: a note reachable from the main screen, an information icon beside the indicator, and a consultable sources section. The point is not to display the bibliography, but to make clear that it exists and can be checked.