Vitamin D and Shade
Shade isn't zero-UVB. Scattered UV from the sky reaches shaded skin, and dense tree canopy or a beach umbrella still transmits 30–50% of ambient UVB. This matters for both sunburn risk (more than you'd think) and vitamin D synthesis (still meaningful in shaded midday outdoor time).
The scatter physics
UV reaches skin via two paths: direct (from the solar disk) and diffuse (Rayleigh-scattered from the whole hemisphere of sky). Under shade, direct is blocked but diffuse continues. On a clear day, diffuse UV is roughly 40–60% of total ambient UV — that's what shade delivers to shielded skin.
Shade types and transmission
- Dense evergreen canopy — 5–15% UVB transmission.
- Deciduous canopy in full leaf — 10–20%.
- Dappled shade — 20–40% (highly variable; sunflecks can approach 100%).
- Beach umbrella — 40–60%. Direct blocked, diffuse mostly through.
- Pergola / gazebo — 30–50%.
- Solid roof / awning — 5–15% (direct blocked, side-scattered UV).
- Between buildings, urban shade — 30–60% depending on sky visibility.
Beach umbrella deception
Studies of beach umbrellas (McKenzie 2017) found that shaded sunbathers still accumulated meaningful UVB exposure — enough that after 3–4 hours under an umbrella at a sunny beach, some subjects had light sunburns. Shade reduces exposure; it doesn't eliminate it. Sunscreen remains recommended for extended shade-only time in tropical or high-UV conditions.
Vitamin D synthesis in shade
An hour under a beach umbrella at midday tropical sun delivers roughly the vitamin D equivalent of 20–30 minutes in direct sun — still substantial. Sitting in dappled shade for an afternoon walking in a park produces meaningful synthesis. Shade is not a vitamin D-blocking strategy; direct sun-avoidance under bright sky still permits synthesis.
Practical takeaway
- Shade is not an on/off switch — it's a partial reduction.
- You can be in the shade all day at a sunny beach and still get sunburned; use sunscreen.
- You can be in dappled shade in a park and still make useful vitamin D.
- Full indoor time is different — see our windows page.