![]() If you’re standing in the middle of a field, you’re exposed to the inside of a continuous dome of sky. We can use something called Sky View to estimate how much UV is bounced off the sky. If you’re outside, the main thing diffusing UV onto you is probably the sky. Working out diffuse UV is trickier – we need to think about what’s bouncing UV onto us. Working out how much direct UV you’re getting is straightforward: if there’s sun shining directly on you, you’re getting that 40% direct UV exposure. The key takeaway is that UV shining directly on you is often less than half of the UV you could be getting. The actual percentages change with lots of factors such as how high the sun is in the sky, the wavelength of UV you’re talking about, and where you are in the world. If you’re standing in the middle of a field, you’re getting 60% diffuse UV and 40% direct UV as a very rough estimate. Shorter wavelengths get diffused more, so there’s more diffuse UV around than diffuse visible light, and more diffuse UVB than UVA. The “something else” is mostly air molecules up in the sky, but UV can also be reflected by other objects onto you.ĭiffusion also happens with visible light, and it’s why you can still see things that are in the shade. The other type is indirect or diffuse UV exposure. Basically, the sun’s UV bounces off something else onto you. Related post: Should You Wear Sunscreen Indoors? The Science (with video)ĭirect UV exposure is when the sun is directly shining on you. When you’re in full sun you’re getting direct exposure, but when you’re in the shade you have no direct exposure. It’s the sun, obviously! But there are two types of UV exposure, which you might remember from my post about whether or not you need to wear sunscreen indoors. Let’s start by talking about where UV comes from. In Australia, we have the slogan Slip Slop Slap Seek Slide: Slip on a shirt, slop on sunscreen, slap on a hat, seek shade, and slide on sunglasses.īut how well do each of these work? Why do we still get burnt in the shade? Can you just wear a hat instead of sunscreen? UV exposure The idea is that although each layer of protection has shortcomings, enough layers of imperfect protection will give you pretty decent protection overall.Īnd this is how we should think about sun protection too – sunscreen is an important layer, but it’s not the only layer. ![]() I think most of us have seen this Swiss cheese diagram, with respect to protecting ourselves from COVID-19: Credit: Ian M Mackay ![]() 6 Takeaways Swiss cheese and sun protection
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