Weighing the Evidence on Sunshine in the Landscape

Shady park in Chicago.  Photo credit Victoria Cox.

Shady park in Chicago.  Photo credit Victoria Cox.

Two things landscape architects can do to make outdoor environments healthier for people

You’ve probably heard the statistics about how sunburn can lead to skin cancer. The Skin Cancer Foundation reports on their website skincancer.org that: “One blistering sunburn in childhood or adolescence more than doubles a person's chances of developing melanoma later in life.”  

That sounds terrible, but it’s important to keep in mind that those are relative risks.  What are the actual chances of getting melanoma?  According to Cancer Research UK (cancerresearchuk.org) the absolute risk of getting malignant melanoma each year in the UK is 17 per 100,000 people, or 0.00017.  And the risk of dying from it is 2.5 per 100,000 people (0.000025) or 1 chance in 40,000.  Double that because of sunburn and the number is still very small – 1 in 20,000.

Is the risk real?  Absolutely.  Is it worth being careful not to get a sunburn?  Definitely.  Is it worth staying completely out of the sun at all times?  Maybe not.   

On the other side of the sunshine ledger there is growing evidence of the risks of staying out of the sun.  A person’s body produces vitamin D when exposed to ultraviolet B (UVB) radiation – a part of the solar spectrum.  Vitamin D deficiency has been linked to a wide range of ailments and diseases from depression to hypertension.  The most certain link is with rickets.  It was the search for a cure for rickets that led to the discovery of vitamin D in the 1920s.  Some of the health benefits of having an adequate amount of vitamin D in your body are healthy bones, reduced risk of influenza, reduced risk of diabetes, and reduced risk of several kinds of cancer (prostate, colon, and breast among others).  Other possible health benefits being studied include cardiovascular disease, Alzheimer’s disease, autism, and multiple sclerosis.

Which is more important - staying out of the sun and reducing your changes of developing melanoma, or spending time in the sun generating vitamin D and avoiding a wide range of afflictions?  We don’t yet know, but one study in the USA (Grant et al 2005) estimated that the economic cost of people receiving too much UVB radiation was US$ 6 to 7 billion, while the cost of people not receiving enough UVB and/or vitamin D was US$ 40 to 56 billion per year. 

In the end it’s a personal decision but the issue has important implications for landscape architects.  Outdoor environments should provide people with the opportunity to get enough UVB to produce a healthy amount of vitamin D without getting so much that it increases their risk of developing melanoma. 

Is there a way to quantify the minimum amount of UVB required and a safe maximum amount of UVB?   One of my MLA advisees, Victoria Cox, decided to investigate (Cox 2013).  For her thesis she conducted a pilot study that measured UVB radiation in a school yard in southern Ontario in mid-winter.  We used some tiny but very sophisticated instruments called dosimeters (do sim’ i ters) to measure the amount of UVB radiation children received during typical winter school days.  The children wore the dosimeters on their arms and all day long the instruments tallied up how much UVB they were receiving.  At the end of each test day Victoria collected the dosimeters and downloaded the data. 

What did she find?  The children received only small amounts of UVB and didn’t come close to receiving enough to cause sunburn, even on sunny winter days.  These results were based on a single location and a small number of students so are not universally applicable, but they suggest that further research should be done.

The second part of Victoria’s study was to use the computer model fastRT (Ola Engelson, Norwegian Institute for Air Research) that estimates amount of UVB received under different environmental conditions.  We used fastRT to estimate how long a student would have to spend outdoors on a February day to generate 200 I.U. of vitamin D (considered adequate intake).  On a sunny day it would take 26 minutes and on an overcast day a student could spend the whole day outdoors and not produce 200 I.U.

Next we ran the model in springtime conditions and found that a person could obtain 1000 I.U.  (the recommended daily dose) of vitamin D in only 7 minutes on clear days and 48 minutes under overcast skies.  Then we calculated how long it would take to produce a sunburn.  Throughout the year it would take from 3 to 5 times as long in the sun to get a sunburn as it would to produce 1000 I.U. of vitamin D.

But UVB isn’t the only part of the solar spectrum that affects people.  How do the recommendations that would result from this study compare with advice about solar radiation and thermal comfort (e.g. Brown 2010)?  During summer heatwaves solar radiation can add so much energy to a person’s body that it can lead to heat stroke.  The time of year when we need shade to reduce the possibility of heat stroke is also the time of year when the UVB radiation levels are so high that people can get sufficient vitamin D even in the shade.  During the winter the solar radiation we receive has a lot less UVB in it and a person needs to spend quite a bit more time outdoors to get sufficient vitamin D.  But it also typically takes an even longer time to get a sunburn. 

What’s the bottom line?  There are two simple, straightforward actions that landscape architects can take to both allow people the opportunity to get vitamin D and also maintain thermal comfort and safety.

1. Outdoor environments that are going to be used in summer should provide lots of shade.  This will reduce the amount of UVB that people receive while also helping to keep people cool. 

2. Outdoor areas to be used in the winter should provide lots of opportunity for people to get into the sun.  This can provide them with some UVB to produce vitamin D and will also add some heat to their shivering bodies. 


Brown, R.D. 2010.  Design with Microclimate: The secret to comfortable outdoor space.  Island Press.  Washington, D.C.

Cox. V. S. K. 2013. Exploring Ultraviolet B Radiation in the Landscape.  Master of Landscape Architecture thesis.  University of Guelph.

Grant, W.B., C.F. Garland, and M.F. Holick. 2005. Comparisons of estimated economic burdens due to insufficient solar ultraviolet irradiance and vitamin D and excess solar UV irradiance for the United States. Photochem Photobiol.  81: 1276-86.