Do not assume that standing under a tree affords complete protection from the sun, say researchers at Hopkins University. When the sun comes out, it's pleasant to sit under a tree and assume that the foliage must be protecting you from the sun. But don't take the shade for granted, say researchers at Purdue University in the US.
They've built a mathematical model which reveals just how much protecting from harmful ultraviolet B light a tree can provide. It all depends on how much sky you can see through the tree canopy and the latitude you're at.
At a latitude between 30 and 60 degrees - Hawaii to Juneau, Alaska - you can have double the exposure to sunlight if the canopy shows 40 per cent of the sky. So if you can stay in the sun for 20 minutes without burning without tree cover, it would be 30 minutes under this kind of foliage. Where canopy coverage is 90 per cent, you can expect sun protection ten times normal - the equivalent of wearing a sunblock of Sun Protection Factor 10.
The greatest benefit of tree cover is in more northern latitudes, between 45 and 60 degrees. Urban and residential planners should take the tree cover and sun exposure link into account, say the Purdue researchers. Many apartments and offices have no tree cover close by - and could really benefit from the addition of some greenery to the surrounding environment.