Understanding the UV Index (UVI): How UV-A, UV-B, and Solar Angle Drive Vitamin D

Content last reviewed: October 2026

The Global Solar UV Index (UVI) is an international standard developed by the World Health Organization (WHO), the World Meteorological Organization (WMO), and the International Commission on Illumination (CIE). It measures the strength of sunburn-producing ultraviolet radiation reaching the Earth's surface at a given place and time. On a physical scale, 1 UV Index point equals 25 milliwatts per square meter (25 mW/m²) of erythemally weighted UV irradiance (ISO/CIE 17166) on a horizontal surface.

1. UV-A vs. UV-B: Why Only UV-B Produces Vitamin D

Solar ultraviolet radiation that reaches the ground is divided into two main bands: UV-A (315–400 nm) and UV-B (280–315 nm). UV-C (100–280 nm) is completely absorbed by stratospheric oxygen and ozone. Although UV-A accounts for more than 95% of total solar UV energy at the surface, its photons do not carry enough energy to break the B-ring of 7-dehydrocholesterol (7-DHC) in the epidermis. Cutaneous synthesis of previtamin D3 occurs almost exclusively in the 290–315 nm UV-B band, with peak biological efficiency near 295–298 nm.

Because the ozone layer strongly absorbs UV-B, the ratio of UV-B to UV-A changes dramatically with the height of the sun (Solar Zenith Angle, SZA). Early in the morning, late in the afternoon, or during winter at mid-to-high latitudes, sunlight passes through a much thicker atmospheric path. Most UV-B is filtered out while UV-A still passes through — meaning you can experience photoaging and oxidative DNA damage from UV-A even when the UV Index is too low for meaningful vitamin D production.

2. UV Index Reference Table: Dose Rate & Estimated Minutes

Over 60 seconds, each point of UV Index delivers 1.5 J/m² of ambient erythemal UV dose (0.025 W/m² × 60 s). Accounting for human 3D body geometry and one-sided illumination (combined factor 0.40), the table below shows the WHO category, ambient dose rate, and baseline minutes at constant UVI for a young adult (age ≤ 20, 100% baseline efficiency, normal BMI, 25% skin exposed; at age 30, times are ~15% longer). Note: At UVI 1–2, low solar elevation strongly depletes short-wave UV-B relative to UV-A, so prolonged exposure at very low UVI is inefficient in practice.

3. Practical Guidelines & the UVI 3 Threshold

The World Health Organization recommends active sun protection (shade, clothing, wide-brimmed hat, UV-blocking sunglasses, and broad-spectrum sunscreen) whenever the UV Index reaches 3 or higher. At very high or extreme UV levels (UVI 8 to 11+), unprotected fair skin (Type I–II) can reach the model's 0.80 MED safety limit in 10 to 15 minutes, so short, carefully timed sessions — or exposing a larger skin area for fewer minutes — are much safer than prolonged midday sunbathing.

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