Estimate safe sun exposure time by UV index.
One UV index unit corresponds to 25 milliwatts per square metre of erythemally weighted irradiance, so time to a minimal erythemal dose falls in inverse proportion to the index. Skin type sets how large that dose is — type I reaches it several times faster than type IV — and reflective surfaces raise the effective index substantially, with fresh snow nearly doubling it. Protection is advised from index 3 upward.
Unprotected exposure time
Effective UV = UV index x surface reflectance factor; unprotected minutes = 200 x skin type factor / (effective UV x 10)
These are population averages. Individual sensitivity varies with medication, medical conditions and history of skin cancer. Cumulative UV exposure causes skin damage at any level — consult a dermatologist for personal advice.
For fair skin, often under twenty minutes before erythema begins. Darker skin types tolerate several times longer, but DNA damage accumulates in all skin types.
Fresh snow reflects up to 80% of UV, which nearly doubles the dose reaching your face. That is why sunburn at altitude in winter is so common.
From index 3 upward, official guidance recommends shade, clothing, a hat and sunscreen during peak hours.