No, sunlight through windows won’t build vitamin D because glass blocks the UV-B your skin needs.
Most people have heard that a little sun helps the body make vitamin D. That part is true—your skin makes this nutrient when ultraviolet-B (UV-B) light hits it. The catch: standard window glass stops nearly all UV-B. So that glowing patch on the floor, the car seat bathed in light, or the home office with big panes won’t move your levels. Below is a practical, evidence-based guide to why glass changes the picture, how vitamin D production works, and smarter ways to meet your daily target without sunburn risks.
How Body-Made Vitamin D Works
Vitamin D starts in the skin as 7-dehydrocholesterol. When UV-B (roughly 290–315 nm) reaches exposed skin, it kicks off a reaction that forms previtamin D₃. Heat then converts it to vitamin D₃, which the liver and kidneys later process into the active form. Cloud cover, season, time of day, latitude, skin tone, and sunscreen all nudge this process up or down. That’s the outdoor story. Indoors behind panes, the story changes because the needed wavelengths don’t arrive.
UV-B, Glass, And Why Indoor Sunlight Falls Short
Regular soda-lime glass—the kind used in most homes and cars—absorbs UV-B. You still get UVA (the longer wavelengths linked with tanning and photo-aging), but the skin-making step for vitamin D doesn’t start without UV-B. That’s why an afternoon on a sunny couch may feel nice yet won’t raise your status.
| Wavelength Band | Passes Standard Window? | Vitamin D Impact |
|---|---|---|
| UV-B (≈290–315 nm) | No (absorbed by glass) | Skin cannot start production |
| UVA (≈315–400 nm) | Yes (mostly passes) | No production; can still age skin |
| Visible light (400–700 nm) | Yes | No production |
Does Sun Behind Glass Make Vitamin D Safely?
Indoors, the safety question misses the main point: with UV-B largely blocked, your skin won’t make vitamin D. Outdoors, brief, sensible exposure can help, but there’s a trade-off. The same UV-B that sparks production can also damage DNA. Many readers prefer food and supplements for predictability, then keep outdoor time for overall well-being, shade, and activity—without chasing a nutrient goal from a window seat.
How Much Do You Need Each Day?
General targets for most healthy adults are set at 600 IU (15 µg) through about age 70 and 800 IU (20 µg) past that range. Those numbers trace back to well-established nutrition guidance. Routine megadoses aren’t advised for healthy adults, and screening isn’t needed for everyone. Certain groups—older adults, some medical conditions, limited sun exposure, or covering practices—may need tailored advice from a clinician who knows your history.
Smart Ways To Reach Your Daily Target
Use a mix: fortified foods, oily fish, eggs, and an affordable daily supplement when diet alone falls short. A single daily capsule is easier to remember and keeps intake steady. Sun can contribute when you’re outdoors, but there’s no need to chase a tan or a “window fix.”
Food Sources That Help
Fortified milk or plant drinks, yogurt, cereals, and orange juice often carry added vitamin D. Fatty fish like salmon, trout, or mackerel provide natural D₃ along with omega-3s. Egg yolks add a little. Labels vary, so check the nutrition panel and serving size. If intake swings from day to day, a modest supplement smooths things out.
Supplement Basics Without Guesswork
Pick a reputable brand, aim near your age-based target, and avoid high intermittent doses unless your clinician set them. Oil-based softgels or drops absorb well. Take D with a meal that includes some fat. If you’re already on a multivitamin, note the built-in amount and avoid stacking.
When Sun Can Still Contribute Outdoors
On clear days when the sun is higher in the sky, brief time outdoors with arms and lower legs uncovered can add some vitamin D. Midday tends to offer more UV-B, though exact timing shifts by latitude and season. People with deeper skin tones need longer exposure for the same amount of production. If you choose to count on outdoor time, keep it brief and avoid any pinkness. Broad-spectrum sunscreen protects against UVA and UV-B; real-world use may lower UV-B dose only modestly, but the priority is skin health over a nutrient that’s easy to get from food and a small supplement.
Common Myths, Clean Facts
“A Bright Window Is Enough.”
It’s not. Light and warmth do not equal UV-B. Glass is the barrier, which is why indoor tans fade and why drivers can pick up UVA-related skin changes on the window side of the face without making vitamin D.
“More Sun Means More Benefit.”
Skin self-limits production; once you’ve made enough, extra UV-B doesn’t stack a bigger supply. Past that point, damage only climbs. A food-plus-pill plan is steadier, with no burn risk.
“UV Through Panes Is Harmless.”
UVA passes through panes and reaches deeper skin layers. You may not burn behind glass, yet long-term exposure still ages skin. If you spend hours beside a window or drive often, a daily broad-spectrum SPF and window films that block UVA can help.
Practical Takeaways For Home And Work
- That sun patch on the sofa won’t change vitamin D status.
- If you sit near big panes, wear broad-spectrum SPF daily and consider UVA-blocking films for added protection.
- Build vitamin D with food plus a small daily supplement; keep doses near age-based targets unless your clinician says otherwise.
A Quick Plan You Can Use This Week
Pick one fortified staple you’ll actually consume daily—milk, a preferred plant drink, or yogurt. Add two fish meals this week, like salmon or trout. If your usual intake runs light, add a 600–800 IU daily D₃ capsule. Keep outdoor time pleasant and short; no chasing rays through glass.
Age-Based Targets And Simple Sources
| Source | Typical Amount | Notes |
|---|---|---|
| Daily supplement (D₃) | 600–800 IU | Match age target; steady intake is the goal |
| Fortified milk/plant drink | 100–150 IU per cup | Check label; brands vary |
| Salmon, cooked | 300–600 IU per serving | Varies by species and portion size |
| Fortified cereal | Up to 100 IU per serving | Adds a small boost with breakfast |
| Egg yolk | ~40–50 IU each | Natural source; smaller contribution |
| Outdoor midday sun | Highly variable | Depends on season, skin tone, latitude |
Window Films, Cars, And Skin Safety
Standard panes screen out most UV-B but let a lot of UVA in. Cars add laminated windshields that cut UVA better than untinted side windows. Clear or tinted films can block nearly all UV across both bands while keeping rooms bright. If you work by a window or drive for long stretches, films plus a broad-spectrum SPF are a smart combo. None of these steps will change vitamin D levels indoors, which is why diet and supplements carry the load.
When To Talk To Your Clinician
Some people do need tailored intake: older adults, those with limited sun exposure by choice or climate, people with deeper skin tones living at high latitudes, people with malabsorption, and anyone on medicines that affect vitamin D metabolism. If you’ve had a level drawn and it’s low, your clinician may set a short-term repletion plan followed by a steady daily dose.
Bottom Line For Daily Life
Indoor rays through panes feel pleasant, but they won’t move your vitamin D status. Keep glass for views and warmth. Build your intake with real food and a small daily capsule, use sun time for exercise and joy, and protect your skin year-round.
Learn more from the NIH vitamin D fact sheet and the Skin Cancer Foundation on UV through windows. Guidance on daily intake is also summarized by the Endocrine Society.