Broncea
A consumer application built on a dosimetric model of ultraviolet exposure. It answers a question people usually settle with a guess: given where you are, right now, how long is it sensible to stay in the sun — and why that number and not another.
- Platform
- Android
- Languages
- 6
- Accounts required
- None
- Personal data on our servers
- Zero
01The model
A dose, not a duration.
A minute under a UV index of 9 delivers nine times the erythemally weighted energy of a minute under an index of 1. Any recommendation expressed in minutes is therefore wrong by up to an order of magnitude, while sounding precise.
D(t) = ∫₀ᵗ k · UVI(τ) · TSPF(τ) · s dτ ⩽ MED(tone)
Accumulated erythemal dose is integrated over the real hourly forecast, attenuated by the transmission actually achieved by the sunscreen in use, and compared against a personal minimal erythemal dose interpolated from published thresholds by skin tone. The recommendation is the time at which a limit is first reached — and the app reports which limit it was.
A dose engine, not a stopwatch
One minute under a UV index of 9 is not one minute under a UV index of 1. The model integrates the erythemally weighted irradiance actually received across a session, adjusted for skin tone, sensitivity and sunscreen, against a personal minimal erythemal dose.
Hyperlocal forecasts, hour by hour
UV index and daylight are read from Open-Meteo for the exact coordinates the user is at — or for any beach or city they search — for today and the days ahead. Not a regional average.
Benefit and risk as separate channels
Vitamin D synthesis and erythema accumulate on different curves, so they are modelled and displayed separately. Progress towards a healthy dose never decreases; burn risk has its own independent indicator.
The science is in the app
A dedicated screen lists every mechanism the model relies on, its strength of evidence, and the source — CIE, WHO/INTERSUN, IARC, and the peer-reviewed literature. Users can audit the reasoning, not just trust it.
Local-first by construction
No account, no sign-up, no advertising. Profile and history live only in the device's own storage. Location is used to query the forecast and is never stored on a server, because there is no server.
02Evidence
Every mechanism is listed inside the app.
Broncea contains a dedicated screen naming each mechanism the model relies on, the strength of the evidence behind it, and the source. A user can audit the reasoning rather than take it on faith — which is also the only honest way to ship a model that affects behaviour.
- Erythemal action spectrum & UV indexCIE 209:2014 · McKinlay–Diffey · WMO / WHO
- Sunscreen use and vitamin D statusPasseron et al., British Journal of Dermatology, 2019
- Photoprotection under real-world applicationYoung et al., PNAS, 2021
- Solar UV as a carcinogenIARC Monograph 100D · WHO
- Public-health exposure guidanceWHO INTERSUN programme
- Skin tone colorimetryChardon et al. · CIE-Lab
Source names are shown untranslated in all six of the app's languages, because they are proper nouns and translating them would make them harder to look up.
03Data
There is no server to leak.
Broncea has no backend and no database. Profile, calibration and session history live in the device's own storage and are never transmitted. There is no account to create, no advertising identifier, and nothing to sell.
Location leaves the device for exactly two purposes: retrieving the UV forecast from Open-Meteo, and converting coordinates into a readable place name through BigDataCloud. Both requests are encrypted, carry nothing but coordinates, and are not retained by us — because there is nowhere for us to retain them.
Deleting everything is a single action in the app's settings, and it is immediate and permanent. That is not a policy commitment; it is a consequence of the architecture.
Next
This is the first product, not the company.
Broncea demonstrates the method: a real model, validated honestly, shipped inside something ordinary people can use. The same process is now being applied to problems that have nothing to do with sunlight.