Vitamin D from sunlight in Chicago
Chicago sits at 41.85°N, 179 m above sea level. Between 26 October and 22 February the midday sun never climbs high enough for your skin to make vitamin D at all — 120 days of the year.
How long do you need outside?
Month by month in Chicago
At solar noon, for skin type III with arms and legs exposed.
| Month | Sun height | UV index | 1000 IU in | Burns after |
|---|---|---|---|---|
| January | 27.1° | 1.6 | no synthesis | 143 min |
| February | 35.7° | 2.8 | no synthesis | 83 min |
| March | 46.2° | 4.5 | 24 min | 51 min |
| April | 58.1° | 6.6 | 16 min | 35 min |
| May | 67.1° | 8.0 | 13 min | 29 min |
| June | 71.5° | 8.8 | 11 min | 27 min |
| July | 69.6° | 8.9 | 11 min | 26 min |
| August | 62.0° | 8.1 | 12 min | 29 min |
| September | 51.0° | 6.2 | 17 min | 38 min |
| October | 39.4° | 3.9 | 29 min | 60 min |
| November | 29.5° | 2.1 | no synthesis | 111 min |
| December | 24.9° | 1.4 | no synthesis | 167 min |
When during the day?
Vitamin D-effective UV by month and hour, in America/Chicago local time (daylight saving included). Darker cells mean faster synthesis.
| Month | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Jan | 1 | 1 | 1 | 1 | ||||||||||||||
| Feb | 1 | 2 | 2 | 2 | 2 | 1 | ||||||||||||
| Mar | 1 | 2 | 3 | 4 | 4 | 3 | 2 | 1 | ||||||||||
| Apr | 1 | 2 | 4 | 5 | 6 | 6 | 5 | 3 | 2 | |||||||||
| May | 1 | 3 | 5 | 7 | 7 | 7 | 6 | 4 | 2 | 1 | ||||||||
| Jun | 1 | 2 | 4 | 6 | 7 | 8 | 8 | 7 | 5 | 3 | 1 | |||||||
| Jul | 2 | 3 | 5 | 7 | 8 | 8 | 7 | 5 | 3 | 1 | ||||||||
| Aug | 1 | 3 | 5 | 6 | 7 | 7 | 6 | 4 | 2 | 1 | ||||||||
| Sep | 1 | 2 | 3 | 5 | 6 | 5 | 4 | 3 | 1 | |||||||||
| Oct | 1 | 2 | 3 | 3 | 3 | 2 | 1 | |||||||||||
| Nov | 1 | 1 | 2 | 1 | 1 | |||||||||||||
| Dec | 1 | 1 | 1 | 1 |
none strongest
Best time in July: around 12:30 local time.
How much skin matters more than how much time
In July, at solar noon, for skin type III:
| Exposed | Body surface | 1000 IU in | 30 minutes gives |
|---|---|---|---|
| Face and hands | 5% | not reachable | 532 IU |
| Face, hands and forearms | 10% | 28 min | 1,064 IU |
| Face, hands, arms and legs | 25% | 11 min | 2,659 IU |
| Swimwear | 80% | 4 min | 8,510 IU |
Exposing more skin gets you there faster, but it also burns faster. See how long before you burn in Chicago →
Note the first row. With only face and hands exposed, 1000 IU is not reachable at any duration: vitamin D synthesis saturates, so the previtamin D3 in your skin converts to inactive products as fast as it forms. Past that ceiling, extra time adds sunburn risk and no more vitamin D. Exposing more skin briefly beats exposing less skin for longer.
Other cities in United States
Assumptions
- Clear sky. Every figure is an upper bound. Cloud can cut UVB by more than half, and these numbers do not model it.
- Direct exposure. Glass blocks UVB almost completely, so sunlight through a window produces no vitamin D. Sunscreen and clothing block it too.
- A healthy adult. Age, body composition, medication and existing vitamin D status all change the real answer.
- Ozone is modelled, not measured. Column ozone comes from a smooth climatology, so day-to-day variation is not captured.