Sunscreen, Clouds, Glass, and Clothing: What Blocks UV-B and What Doesn't
Content last reviewed: October 2026
Between the top of the atmosphere and the 7-dehydrocholesterol molecules in your epidermis, ultraviolet photons encounter clouds, glass, fabrics, and topical sunscreens. Because UV-B (280–315 nm) has shorter wavelengths than UV-A (315–400 nm), physical barriers filter the two bands very differently.
1. Window Glass and Car Windshields
Standard soda-lime window glass (both residential and office) and laminated automotive windshields absorb virtually 99% to 100% of UV-B radiation below 315 nm. As a result, sitting in a sunny room behind closed windows or driving with the windows rolled up produces zero cutaneous vitamin D. However, ordinary clear glass transmits 50% to 75% of UV-A rays (except laminated front windshields, which block most UV-A), so prolonged sun through a side or room window can still contribute to cumulative UV-A photoaging, oxidative DNA damage, pigment darkening, and photosensitivity reactions.
2. Summary Table: Environmental & Clothing Multipliers in the Model
| Condition / Factor | UV-B Multiplier | Effect on Required Time |
|---|---|---|
| 0–20% – Clear Sky | 1.00× | Baseline (1.0× time) |
| 30–60% – Partly Cloudy | 0.70× | ~1.4× longer exposure |
| 70–100% – Overcast (Heavy clouds) | 0.30× | ~3.3× longer exposure |
| Behind closed window glass | ~0.00× | No vitamin D synthesis |
| Clothing: 10% vs. 25% skin exposed | 0.40× | 10% skin takes 2.5× longer (higher burn risk) |
The Sunscreen Paradox
Experiments with artificial UV show that sunscreen can reduce vitamin D production. However, studies of real-life use and an international expert consensus conclude that regular use of broad-spectrum sunscreen does not compromise vitamin D status in healthy people. In practice: do not skip sun protection because of vitamin D. The WHO recommends sun protection when the UV Index is 3 or higher. If you are concerned about your vitamin D level, ask a healthcare professional about a blood test and supplements.
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