Vitamin D Through a Window
Sunlight streaming through a window feels warm and looks bright — but it doesn't make vitamin D. Standard soda-lime silica glass blocks essentially all of the UVB wavelengths (290–315 nm) required for cholecalciferol synthesis, while transmitting most of the visible light and much of the UVA. This has real consequences for office workers, greenhouse dwellers, and people who spend most of their "sun time" behind glass.
The transmission physics
- Standard window glass — transmits 0–5% of UVB (280–315 nm). Effectively total block.
- UVA (315–400 nm) — transmits 60–70% through standard glass. Sunburns and tanning through windows can occur; skin cancer damage accumulates.
- Visible light (400–700 nm) — 80–90% transmission.
- Infrared / heat — mostly transmitted, hence the greenhouse effect inside cars.
Car windows
Modern car windshields are laminated glass — an inner plastic layer blocks nearly all UV including UVA. Side and rear windows are usually tempered glass and transmit UVA (people can tan through them; skin damage from cumulative side-window exposure is a real dermatology finding in long-distance drivers). Neither transmits UVB in usable quantity. Driving does not make vitamin D.
Office workers behind glass all day
The classic vitamin D deficiency scenario. Even a sunny corner office with floor-to-ceiling windows produces essentially zero vitamin D. Cross-sectional studies of office workers in sunny cities (San Francisco, Sydney, Athens) find surprisingly high winter deficiency rates because the "outdoor time" people report is often behind glass — car, office, home.
Greenhouses and conservatories
Same principle. Standard greenhouse glass blocks UVB. Older greenhouses used quartz glass panels that transmitted some UVB, useful for horticulture. Modern polycarbonate greenhouse panels are opaque to UVB. Sitting in a sun room is not a vitamin D strategy.
UVB-transmitting glass — the exception
Special quartz or specific fused-silica glass transmits UVB. Used in laboratory equipment, some phototherapy lamps, and (historically) certain reptile enclosures. Not used in residential or commercial windows because of cost and because most people don't want UVB in their homes.
Low-E coatings and tinted glass
Modern energy-efficient windows have low-emissivity (Low-E) coatings that further reduce UV transmission, including some UVA. Tinted or reflective commercial glass blocks even more. From a vitamin D perspective, the difference is academic — none of them let usable UVB through.
The bright-day-indoors trap
Cognitive point: sunlight looks the same to your eyes whether you're indoors or outdoors. This creates the illusion that a sunny day at your desk is "getting some sun." It isn't — from a vitamin D perspective. Skin needs direct outdoor exposure to UVB for cholecalciferol production, and no amount of bright indoor light substitutes.
Practical takeaway
- If most of your daytime is behind glass, you need vitamin D supplementation year-round regardless of your climate.
- Adults spending 8+ hours indoors most days: standard 1,000–2,000 IU/day cholecalciferol.
- Take breaks outside — even 15 minutes at lunch several times a week meaningfully affects summer vitamin D synthesis. It's also good for eyes, mood, and circadian rhythm.
- Don't try to solve indoor lifestyle with a UV lamp — see our UV lamps page; supplements are almost always the better answer.