Calculate the amount of sun you need using the UV index

Find out about how the UV index, time of day, skin exposure and time in the sun impact the amount of time you need to spend in the sun to achieve adequate vitamin D levels.

The sun's UVB rays help the skin synthesize vitamin D, which is essential for human health, and especially for people with MS. Vitamin D dampens the inflammatory immune responses and protects brain cells.


Considerations:

  • UV Index - which is dependent on time of year 
  • Time of day - the UV index varies throughout the day, climbing to a peak at midday and then dropping. A UV index of 14 means it's 14 at midday and around half that at 10 am and 2 pm.
  • Weather 
  • Skin complexion 
  • Clothing / amount of skin exposed 
  • BMI

Index Exposure

1-2  Low
3-5  Moderate
6-7  High
8-10  Very high
11  Extreme

It is best to get your sun when the UV index is high, because you need to spend less time in the sun. UVB is the form of ultraviolet light that produces vitamin D in the skin. All UV penetrates water, so swimming won't reduce the amount of vitamin D you make.

Can you get vitamin D through glass?

No, because vitamin D is only produced by the body when UVB rays touch the skin. The resulting chemical is called 7-dehydrocholesterol. Glass windows block this wavelength which means that your skin won’t be making vitamin D through glass. You can’t benefit from the vitamin D by staying inside, even if you are sitting in the sun by a window or in a conservatory or ‘sunroom’. If you don’t go outside enough, you can become deficient in vitamin D.

UV index time of day

The UV index identifies the strength of the sun (UV radiation) from a specific location on a particular day, at a particular time. 

The strength of UV varies, depending on the location, the time of year and other factors such as cloud cover. UV exposure is essential for everyone’s health, particularly PwMS, for the production of vitamin D, but overexposure to UV and sunburn can be detrimental to health, so it is important to get the balance right.

The UV index offers a scale which allows you to measure the intensity of the UV rays. The highest UV intensity is reached when the sun is at its highest point in the day, halfway between sunrise and sunset — called solar noon. This is usually between 11.30 and 12.30 (or 12.30 and 1.30 in locations where daylight saving is being observed).

UV levels are readily available online for all countries.

Here are a few useful pages:

  • In the US, see here
  • In Australia, generally between 0-12, can reach 17, see here
  • In the UK, maximum 8, see here

For many people living in cooler climates, you may not be able to get any vitamin D from the sun in the winter months. 

Sunbeds and vitamin D

If you want to use a sun-bed to increase your vitamin D production on cloudy days, be aware that most sun-beds do provide a mix of UVA and UVB rays, but check to be sure before you use or buy one.

If you do use a sun-bed, limit your time and UVB intensity to about a third of what you would spend in the sun to avoid the risk of unnecessary skin damage and possible melanoma later in life.

Examples for UV exposure

15 - 20 mins UV index 5 - midday, uncovered
10 - 15 mins UV index 7 - midday, uncovered
5- 7 mins UV index 14 - midday, uncovered

 

If you are going out at 10.00 am for example, you can spend a third longer in the sun.


The UV Index Worldwide

Maximal UV Index values are given for a range of cities in different countries, calculated for the 21st of each month. (From the WHO website)

  Country (City) . J F M A M J J A S O N D
  Argentine (Buenos Aires) 35°S  9 9 7 4 3 2 2 4 5 7 9 10
  Australia (Darwin) 13°S 12 13 12 10 8 8 8 10 11 13 12 12
  Australia (Melbourne) 37°S 8 8 6 4 2 2 2 3 5 6 8 9
  Australia (Sydney) 34°S 9 9 7 5 3 2 3 4 6 7 9 10
  Brazil (Rio de Janeiro) 23°S 12 11 9 7 5 5 5 7 9 10 12 12
  Canada (Vancouver) 49°N 1 1 3 4 6 7 7 6 4 2 1 1
  Cuba (Havana) 23°N 6 8 9 10 10 11 12 11 10 8 6 5
  Falkland-Islands (Port Stanley) 58°S 5 4 2 1 0 0 0 1 2 3 5 5
  France (Paris) 49°N 1 1 3 4 6 7 7 6 4 2 1 0
  Germany (Berlin) 52°N 1 1 2 4 5 7 7 5 3 1 1 0
  Greece (Iraklion) 35°N 3 4 5 8 9 9 10 9 7 4 3 2
  Japan (TokYo) 36°N 2 4 5 8 9 9 10 9 7 4 2 2
  Kenya (Nairobi) 1°S 12 13 13 12 11 10 11 11 12 12 12 11
  Madagascar (Tananarive) 19°S 12 12 11 9 7 6 6 8 11 11 12 12
  Mozambique (Maputo) 26°S 11 11 9 7 5 4 4 6 8 10 11 11
  Mongolia (Ulan Bator) 48°N 1 2 3 5 6 7 8 6 4 2 1 1
  New Zealand (Wellington) 42°S 7 7 5 3 1 1 1 2 4 6 7 8
  Panama (Panama) 9°N 9 11 12 12 11 11 12 12 12 11 9 9
  Russia (St Petersburg) 60°N 0 0 1 3 4 5 5 4 2 1 0 0
  Singapore (Singapore) 1°N 11 12 13 13 11 11 11 11 12 12 11 10
  South Africa (Cape Town) 34°S 9 9 7 5 3 2 3 4 6 7 9 10
  Spain (Palma de Mallorca) 39°N 2 3 4 6 8 9 9 8 6 4 2 1
  Sri Lanka (Colombo) 13°N 8 10 12 12 11 11 12 12 12 10 8 8
  Thailand (Bangkok) 14°N 8 10 12 12 11 12 12 12 11 10 8 8
  USA (Los Angeles) 34°N 3 4 6 8 9 10 10 9 7 5 3 2
  USA (New York) 41°N 2 3 4 6 7 8 9 8 6 3 2 1
  Vietnam (Hanoi) 21°N 6 8 10 11 11 11 12 12 10 8 6 6
Vitamin D