How does the sun make vitamin D?

UVB rays between 290 and 315 nanometers convert a cholesterol-related molecule in the skin into vitamin D, which the liver and kidneys then activate. UVA does not make vitamin D.

Does sunscreen block vitamin D?

In the lab SPF 15 can cut production by about 95%, but real-world use lets enough UVB through that studies rarely link sunscreen to deficiency. The AAD advises not skipping protection for vitamin D.

Can I get vitamin D through a window?

No. Ordinary window glass blocks the UVB rays that make vitamin D, so sitting in sun indoors does not help. You would still get UVA, which ages skin without producing vitamin D.

Should I tan to boost vitamin D?

No. The skin's vitamin D production plateaus after a few minutes, so longer or more intense exposure adds skin-cancer risk without adding vitamin D. Diet and supplements are safer sources.

How much vitamin D do adults need?

The NIH sets 600 IU (15 mcg) per day for most adults and 800 IU (20 mcg) after age 70. Fatty fish, egg yolks, fortified foods, and supplements all count.

Sun, UV, and Vitamin D: How Much You Actually Need

Sunlight is the body's main natural source of vitamin D. When UVB rays in the 290-to-315-nanometer range hit the skin, they convert a cholesterol-related molecule into vitamin D, which the liver and kidneys then activate. This is the one clear benefit of UV exposure, and it has to be weighed against UV's role as the leading preventable cause of skin cancer.

You need less sun than most people assume. Research often cites 5 to 30 minutes of midday sun on the face, arms, and legs about 2 times a week for lighter skin, with more needed for darker skin or in winter. The NIH sets the dietary target at 600 IU (15 micrograms) a day for most adults and 800 IU after age 70.

How sunlight makes vitamin D

Only the UVB slice of sunlight makes vitamin D, and only within roughly 290 to 315 nanometers. UVB is also the band most responsible for sunburn, so the same rays that build vitamin D damage skin in excess. The reaction happens in minutes of exposure and then plateaus, so longer sessions add burn risk without adding much vitamin D.

How much sun you actually need

For light skin, brief midday exposure of 5 to 30 minutes a few times a week is often enough in the warmer months. Darker skin has more melanin, which filters UVB, so it needs longer to make the same amount. Season and latitude matter too: in winter at higher latitudes the sun sits too low for much UVB to reach the ground, so little vitamin D is made outdoors no matter how long you stay out.

Because the skin's production plateaus quickly, deliberately baking in the sun does not bank extra vitamin D — it mostly adds UV damage. The NIH notes that many people meet their needs through a mix of short incidental sun, diet, and supplements.

Why sunscreen barely changes it

In the lab, SPF 15 sunscreen can cut skin's vitamin D production by around 95%. In real life the effect is much smaller, because most people apply far less sunscreen than the test amount and miss spots, letting enough UVB through. Studies generally find that routine sunscreen use does not cause vitamin D deficiency.

The AAD's position is that you should not skip sun protection to raise vitamin D. Diet and supplements are a safer source, since they avoid the cancer risk that comes with chasing vitamin D through extra UV.

Balancing vitamin D and skin cancer

UV is the main preventable cause of skin cancer, so the goal is enough vitamin D without the sunburns that drive that risk. Food sources include fatty fish, egg yolks, and fortified milk and cereal, and a daily supplement reliably hits the 600 IU adult target or the 800 IU target after 70. Window glass blocks UVB, so you cannot make vitamin D sitting inside in the sun.

Check your local UV index

UV is driven by latitude, elevation, and clear skies, so it runs hardest in the Sun Belt and the high mountain West. These metros see some of the country's strongest UV — check the current forecast for each, or look up any US city on the UV index hub:

Honolulu, HI UV index — A tropical latitude keeps the UV Index in the Very High to Extreme range for much of the year.
Miami, FL UV index — Low latitude and strong sun push midday UV to Extreme through the long summer.
Phoenix, AZ UV index — Desert sun and reliably clear skies produce some of the highest UV readings in the country.
Las Vegas, NV UV index — Clear desert skies keep midday UV very high from spring through fall.
El Paso, TX UV index — High-desert elevation and clear air raise UV well above the national average.
Albuquerque, NM UV index — Mile-high elevation and clear skies make for intense midday UV.

Frequently asked

How does the sun make vitamin D?: UVB rays between 290 and 315 nanometers convert a cholesterol-related molecule in the skin into vitamin D, which the liver and kidneys then activate. UVA does not make vitamin D.
How much sun do I need for vitamin D?: Research often cites 5 to 30 minutes of midday sun on the face, arms, and legs about 2 times a week for lighter skin. Darker skin and winter at high latitudes need more.
Does sunscreen block vitamin D?: In the lab SPF 15 can cut production by about 95%, but real-world use lets enough UVB through that studies rarely link sunscreen to deficiency. The AAD advises not skipping protection for vitamin D.
Can I get vitamin D through a window?: No. Ordinary window glass blocks the UVB rays that make vitamin D, so sitting in sun indoors does not help. You would still get UVA, which ages skin without producing vitamin D.
Should I tan to boost vitamin D?: No. The skin's vitamin D production plateaus after a few minutes, so longer or more intense exposure adds skin-cancer risk without adding vitamin D. Diet and supplements are safer sources.
How much vitamin D do adults need?: The NIH sets 600 IU (15 mcg) per day for most adults and 800 IU (20 mcg) after age 70. Fatty fish, egg yolks, fortified foods, and supplements all count.